CHROMOSOMES AND NUCLEOLI OF THE AXOLOTL, AMBYSTOMA MEXICANUM

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J. Cell Sci. i, 85-108 (1966J                                                                     85
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          CHROMOSOMES AND NUCLEOLI OF THE
          AXOLOTL, AMBYSTOMA                               MEXICANUM

          H. G. CALLAN
          Department of Zoology, The University, St Andrews, Fife

SUMMARY
   Amongst the axolotl's haploid complement of fourteen mitotic chromosomes, one of the four
largest, with a greater arm asymmetry than the other three, shows a nucleolar constriction
subterminally in its shorter arm. Low-temperature treatment causes further secondary con-
strictions to appear; these constrictions enable most of the mitotic chromosomes to be identified;
the constrictions occur at similar sites in the chromosomes of tail-fin epithelial cells, hepatocytes,
and brain cells.
   Homology between the mitotic and oocyte (lampbrush) nucleolar organizers has been
established, and thus the several hundred free nucleoli in oocytes are genetically related to the
two nucleoli of diploid somatic interphases. During oocyte development the free nucleoli
transform from solid structures to rings and back to solid structures again without detectable
increase in number. During the contraction and aggregation of the lampbrush chromosomes
within the oocyte nucleus as maturity approaches, in most axolotls the free ring-shaped nucleoli
become stretched between the nuclear periphery and central chromosome group, and take on
a characteristic beaded appearance. These transformations of the free nucleoli are largely
paralleled by forms which nucleoli attached subterminally to the shorter arm of lampbrush
chromosome III concurrently assume.
   The question as to whether fully developed nucleoli detach from the organizer loci and add
to the population of free nucleoli in oocytes remains undecided. It may well be that virtually
all the DNA-generators of free nucleoli detach from the organizer loci before starting to carry
out nucleolar functions, and before there is any significant accumulation of protein and
RNA around them. If so, the variability in quantity of attached nucleolar material may not
reflect different states in a nucleolar synthesis and detachment cycle, but rather variation in the
number of nucleolar DNA Anlagen which happen to remain attached to the organizer loci
after the synthesis and detachment of the great majority of the Anlagen has ceased.
   In occasional oocytes the only chromosomal continuity maintained across the organizer locus
consists of a nucleolar' double bridge'; this indicates that the genetically persistent (i.e. chromo-
somal) organizer DNA bears the same structural relationship to neighbouring parts of a lamp-
brush chromosome as any other chromomere with its attendant pair of lateral loops.
   The lampbrush chromosomes of the axolotl have been provisionally mapped. The centro-
meres are represented by short portions of chromosome axis without lateral loops, and there
are two spheres close to the centromeres of both chromosome VI and chromosome XIII.
Other recognition characters are inconspicuous or not very reliable, and features of the lamp-
brush chromosomes related to the low-temperature induced secondary constrictions of mitotic
chromosomes have not been identified.

INTRODUCTION
   There are two particular reasons why axolotl chromosomes warrant detailed study.
First, this is one of the very few urodeles about which there is any genetic information
(see Signoret, Briggs & Humphrey, 1962; Humphrey, 1962, 1964). If one hopes to be
able to correlate some particular genetic trait with a lateral loop characteristic on the
86                                   H. G. Callan
oocyte lampbrush chromosomes, both the genetical and cytological attributes of a
species, for preference a urodele, need to be known. Secondly, the axolotl, like its near
relative Ambystoma tigrinum, is unusual among urodeles in having a simple somatic
nucleolar condition. There is one nucleolar organizer per haploid chromosome set.
Having failed to determine whether or not there is a genetic relationship between
somatic nucleolar organizers and free oocyte nucleoli in Triturus cristatus, where the
somatic nucleolar condition is complex, there seemed better hope of deciding this
question by working with the axolotl. Moreover, there is an established nucleolar
mutant in the axolotl (Humphrey, 1961) and hence the possibility that this might be
recognized in the lampbrush chromosome complement.
   These two considerations were put to me by Dr J. G. Gall of Yale University, early
in 1964. An opportunity to take up the study was presented soon afterwards when
I was invited by Dr T. M. Sonneborn to spend a year at the Zoology Department of
Indiana University, where Dr R. R. Humphrey maintains an axolotl colony.
   In order to gain some general idea of the relative lengths of the various chromosomes
making up the axolotl complement, the positions of their centromeres and of the
nucleolar organizer constriction, mitotic preparations were first studied. Thereafter,
in the hope that the mitotic chromosomes might be further characterized, prepara-
tions from cold-treated animals were examined in order to learn the distribution of
several more secondary constrictions which become apparent after appropriate ex-
posure to low temperature.
   With this information as a guide, the lampbrush chromosomes were examined. In
the course of this study I was struck by the remarkable variability in form of the free
nucleoli, and this led me to investigate the developmental history of the free nucleoli
during the growth of axolotl oocytes.
   The various topics are discussed section by section in this paper.

THE NORMAL MITOTIC CHROMOSOMES
   The axolotl's diploid chromosome number of 28 was established by Fankhauser &
Humphrey in 1942. In order to study details of the normal karyotype, axolotl larvae
about 15 mm long, raised at room temperature (21 °C), and which had just started to
feed, were placed for 12 h in 0-5 % colchicine in pond water, and then fixed entire for
12 h in Clarke's (1851) 3:1 ethyl alcohol/acetic acid fixative. From these fixed larvae
the tail fins and livers were excised and placed on separate slides. To each preparation
a drop or two of 0-5 % aceto-orcein (G. T. Gurr, synthetic) was added, a small watch
glass was set over each preparation to reduce evaporation, and 30 min of staining
followed. Each preparation was then tapped out to dissociate the cells from one another,
and covered with a long, siliconed cover glass. The preparations were squashed by
finger-pressure between folded filter paper, the cover glasses removed by the ' dri-ice'
method, and the slides dehydrated and mounted in' Permount' (Fisher Scientific Co.).
   Better chromosome spreads were obtained from hepatocytes than from tail-fin
epithelial cells; an example is shown in Figs. 1 and 7. The range in chromosome size
from largest to smallest is without well-marked discontinuities, and I have found
Chromosomes and nucleoli of the axolotl                            87
individual identification difficult. In practice one can readily subdivide the haploid
complement into eight larger chromosomes with median or submedian centromeres,
in no case with arm ratios approaching 2:1, and six smaller chromosomes with
marked arm asymmetry, all with ratios greater than 2:1.

    Fig. 1. Camera lucida drawing of axolotl hepatocyte mitotic chromosomes from a
    normal larva. A photograph of this same complement is shown in Fig. 7; in the drawing
    the chromosomes have been spaced out to avoid overlaps, n, nucleolar organizer
    constriction.

   Of the eight larger chromosomes, two with virtually median centromeres (VII and
VIII) are substantially smaller than the rest; they are indistinguishable from one
another. Among the remaining six larger chromosomes there is one with a greater
arm asymmetry than the others (it has a ratio of about 5:3), and subterminally in its
short arm there is a secondary constriction. This is the only axolotl chromosome which
normally shows such a constriction; the constriction is more evident in hepatocyte
than in tail-fin epithelial chromosomes. In the light of Dearing's (1934) classic study
on the related A. tigrinum, where a subterminal constriction in the short arm of a
large chromosome of similar arm ratio was shown to be the site of production of the
nucleolus, this secondary constriction in the axolotl can be assumed to mark the
nucleolar organizer. The nucleolar chromosome of the axolotl can be further ranked
as one of the four largest chromosomes in the complement, but I have not found it
possible, from a study of mitotic plates alone, to rank it more precisely than this.
   The three smallest chromosomes XII, XIII and XIV can be recognized with ease
and arranged in length order without ambiguity. However, I have been unable to
differentiate with certainty between chromosomes IX, X and XL
H. G. Callan

THE MITOTIC CHROMOSOMES IN LARVAE EXPOSED TO L O W TEMPERATURE
   When larvae of three species of Triturus were subjected to low temperature for a
few days prior to fixation, their mitotic chromosomes showed many secondary con-
strictions which are not apparent in normal mitoses (Callan, 1942). The same is true
of the axolotl.

    Fig. 2. Camera lucida drawing of axolotl hepatocyte mitotic chromosomes from a low-
    temperature treated larva. A photograph of this same complement is shown in Fig. 8 ;
    in the drawing the chromosomes have been spaced out to avoid overlaps. Identified
    chromosomes (all but I, II and IV) are marked by Roman numerals, n, nucleolar
    organizer constriction.

   Axolotl larvae which had just started to feed were placed in a refrigerator adjusted
so that the water temperature fell to 2-5 °C. A 0-5% solution of colchicine in pond
water was also brought to this temperature, and after 3 days in plain cold water the
larvae were transferred to the cold colchicine solution, and left in the cold for a further
12 h. They were then fixed and preparations made as already described. From a few
of the larvae samples of brain tissue as well as liver and tail-fin epithelium were
excised to make preparations.
   The mitotic chromosomes in these preparations show many secondary constrictions.
The constrictions are rather more sharply defined in tail-fin epithelial cells (Figs. 3, 9)
Chromosomes and nucleoli of the axolotl                            89
than in hepatocytes, but the latter spread much better, and provided more fully
analysable complements (Figs. 2, 8).
   No attempt was made to determine the minimum duration of cold treatment
necessary to make these constrictions evident, but a point concerning temperature
deserves mention. Larvae treated at o to 0-5 °C show constricted chromosomes, but
in tail-fin preparations in particular the chromosomes are poorly spread and have fuzzy
outlines, which I think is connected with a failure of the nuclear membrane to break
down. Larvae treated at 3-5-4-0 °C have chromosomes with less-pronounced con-
strictions, inadequate for consistent identification.

    Fig. 3. Camera lucida drawing of axolotl tail-fin epithelial cell chromosomes from
    a low-temperature treated larva. A photograph of this same complement is shown in
    Fig. 9; in the drawing the chromosomes have been spaced out to avoid overlaps.
    Identified chromosomes (all but I, II and IV) are marked by Roman numerals.
    n, nucleolar organizer constriction.

   The cold-induced secondary constrictions in axolotl chromosomes occupy well-
defined and constant positions, and these positions are the same in hepatocytes as in
tail-fin epithelial cells. I have classified the constrictions into two groups, A and B.
Group A consists of those which form more readily, in the sense that if a particular
chromosome set shows any cold-induced constrictions, one may expect to find all of
those in group A. The group B constrictions are apparent in fewer cells, they are more
evident in tail fin than in liver, and when they are present one can be sure to find all
the group A constrictions too.
   As I had established from measurements on the lampbrush chromosomes a reason-
ably accurate rank order for the complement, with centromere positions, I did not
90                                       H. G. Callan
attempt'to make systematic length measurements of the mitotic chromosomes. For
this reason in Fig. 4 the positions of the cold-induced secondary constrictions have
been superimposed on a lampbrush karyotype. It will be apparent that the cold-
induced secondary constrictions aid considerably in the identification of the axolotl's

                                               r-h
                                               B                A
                                                        I
     "                                          1                1
                                                B       ^        B
                                                            tt               tn
                                                            BA
     IV
                                                t                t
                                                B            B
                                                        \
                                               t
                                               A
     VI

     VII
                                                    t        tt
                                                A            AB
                                                        I
     VIII
                                                t                    t   '
                                               A                 A
     IX
                                                        i
                                                                     t
                                                                     A
                                                        I
     X
                                                                 t
                                                                 A
     XI
                                                        I
                                                             t t
                                                             A B
                                                        \
     XII
                                                    t        tt
                                                A            AB
     XIII
                                                        \
                                                            t
                                                        1A
                                                            ff
                                                            BA
     Fig. 4. Axolotl karyotype based on relative lengths of the lampbrush chromosomes.
     The positions of centromeres are indicated by arrows pointing down. The arrows
     pointing up mark the sites of mitotic secondary constrictions, n, nucleolar organizer;
     A and B, first- and second-order low-temperature induced constrictions.

mitotic chromosomes, and the only troublesome ambiguity which remains concerns
three of the four largest members of the complement.
   I was unable to make any well-spread preparations from brain tissue. However, the
chromosomes show constrictions much as in hepatocytes, and the ability to recognize
several individual chromosomes by the familiar distribution of their constrictions,
Chromosomes and nucleoli of the axolotl                           91
notably III, XII, XIII and XIV, leads me to suppose that the constriction pattern is
the same in brain cells as in liver or tail fin.
  The origin and significance of these cold-induced constrictions is obscure. In 1942,
when Darlington's nucleic acid 'starvation' theory was being energetically propounded,

                                                I ft

   IV

   V

   VI

   VII

   VIII

   IX

   XI

   XII

   XIII

   XIV

    Fig. 5. Working map of the axolotl's lampbrush chromosomes. The positions of
         centromeres are indicated by arrows pointing down, n, nucleolar organizer.

I accepted the possibility that the cold-induced constrictions in Triturus might
represent 'under-charged' regions, possibly even regions devoid of nucleic acid. This
whole concept has now rightly fallen into disrepute, and another explanation is called
for. By analogy with McClintock's (1934) description of the origin of the nucleolar
constriction in metaphase chromosomes of maize, where the nucleolus at prophase
remains for a time attached to the nucleolar chromosome and interferes with its
92                                    H. G. Callan
condensation, one might suppose that the action of an abnormally low temperature
is to cause certain other gene products to remain for an abnormally long time associ-
ated with their sites of production on the chromosomes, and likewise interfere with
condensation during prophase.
   Whether or not this explanation is valid, it is clear that the cold-induced constric-
tions are not related to tissue-specific gene products, for otherwise one would
expect quite different constriction patterns in tissues as unlike as liver and ectodermal
epithelium.

THE LAMPBRUSH CHROMOSOMES
   Axolotl lampbrush chromosomes were studied by the techniques devised for
Triturus viridescens (Gall, 1954) and T. cristatus (Callan & Lloyd, i960). Three
particular technical problems presented by axolotl oocytes were overcome, and these
deserve attention.
   The nuclear sap within axolotl germinal vesicles is a stiff jelly, stiffer than I have
ever encountered in T. cristatus, and very much stiffer than the nuclear sap of T. viri-
descens. This sap must be made to disperse if one wishes to study the lampbrush
chromosomes in detail. The standard saline in which germinal vesicle nuclei are
isolated, o-1M KC1 or NaCl or a mixture of the two, can be modified in two ways to
achieve sap dispersal within the observation chamber. It may be diluted, though only
to a limited extent, for if the saline is too dilute the lateral loop ribonucleoproteins
(matrix) dissolve. CaCl2 may be added to the saline, though again only to a limited
extent, for at concentrations in excess of IO~*M the chromosomes contract and stiffen
as though fixed.
   A saline suitable for dispersing the axolotl oocyte's nuclear sap, and which leaves
the chromosomes in good condition, consists of o-i M K/NaCl containing 0-5 x I O ^ M
CaCl diluted to three-fifths of its original concentration with distilled water. A four-
fifth dilution of this saline fails to disperse the sap completely, and a two-fifth dilution
causes appreciable loss of matrix from the loops. Furthermore, a three-fifth dilution
of saline containing 0-25 x I O ^ M CaCl2 fails to disperse the sap completely.
   If one wishes to make long-lasting preparations of lampbrush chromosomes it is
common practice to expose the nuclear contents, in dispersing saline, to the fumes of
concentrated, neutralized formalin for a minute or two before the preparation is
covered. This treatment kills some bacteria which might otherwise contaminate the
preparation, and apparently stabilizes the lateral loop ribonucleoproteins against •
autolysis. With oocytes of several species of urodeles this short exposure to formalin
fumes has no detectable influence on nuclear sap dispersal, i.e. dispersal in an appro-
priately chosen saline occurs whether or not the formalin treatment has been given. In
axolotl oocytes, on the contrary, a short exposure to formalin vapour is necessary; with-
out such treatment the nuclear sap forms a coarse meshwork of fibres as it dilutes with
dispersing saline, and these fibres anchor the lampbrush chromosomes to one another.
   The membrane surrounding the axolotl's germinal vesicle nucleus adheres to any
clean glass surface on which it comes to rest. If a considerable area of adhesion occurs
Chromosomes and nucleoli of the axolotl                     93
it is difficult to remove the membrane without damage to the chromosomes, for when
the membrane is seized by forceps and the nucleus lifted, a hole is torn around the
area of adhesion and some of the nuclear contents ooze out through this hole before
a tungsten needle can be manipulated to tear the membrane clear away. ' Strangled'
preparations result. This trouble can be avoided by keeping the nucleus continually
on the move while the cytoplasm is pumped away with a pipette, and by manipulating
forceps and needle without delay once the cleaned nucleus has been placed in its
observation chamber.
   Until the developmental stage is reached when the chromosomes start to aggregate
in the middle of the germinal vesicle nucleus (oocyte diameter of about 1-5 mm)
axolotl chromosomes are long objects, the longest being 1-5 mm or thereabouts;
furthermore, the lateral loops which they bear are especially numerous, and also very
long. Thus there is a great congestion of chromosomal material within the nuclei of
smaller oocytes, and I have been unable to obtain fully analysable preparations from
oocytes below 1-4 mm diameter; even after the membrane has been removed without
damage to the nuclear contents, the chromosomes become entangled with one another,
stretched and broken, while the sap disperses.
   The oocyte size-range from within which analysable preparations can be obtained
is therefore limited, much more restricted than is the case with T. viridescens or
T. cristatus. The lower limit is 1-4 mm, the upper 1-7 mm. In oocytes larger than
1-7 mm in diameter retraction of the lateral loops is well under way, and most of the
landmark structures, including the centromeres, are difficult or impossible to identify.
In the ovaries of mature axolotls, oocytes within the range 1-4-1 -7 mm are not
numerous; it is rare for such oocytes to exceed 5 % of the pigmented oocytes present.
   A provisional working map of the axolotl's lampbrush chromosomes has been con-
structed (Fig. 5) based on detailed drawings offivecomplete undamaged complements,

              Table 1. Relative lengths of axolotl lampbrush chromosomes

                   Chromosome          Longer arm Shorter arm        Overall
                       I                  64-4        52-6            117
                      II                  60-3        55-7            116
                     III (nucleolar)      65-3       367              102
                     IV                   S4-o       46-0             100
                      V                   50-2       37-8              88
                     VI (bearing          5O-S       36-S              87
                         spheres)
                    VII                   33-3        30-7             64
                   VIII                   3i-6        3°-4             62
                     IX                   42-0        18-0             60
                      X                   41-0        16-0             57
                     XI                   39-o        13-0             52
                    XII                   32-3         97              42
                   XIII (bearing          21-9         8-i             30
                         spheres)
                   XIV                    17-8         62              24
                                                             Total   1001
94                                  H. G. Callan
with subsidiary data taken from over ioo other preparations. By convention, the longer
chromosome arms are called ' left' arms. Relative lengths of the fourteen chromosomes
(Table i), and the positions of landmarks, were worked out in the manner described
by Callan & Lloyd (i960). In order that the data should be roughly comparable to the
corresponding data for T. cristatus, an overall map length of 1000 units was allocated
to the haploid complement.

Centromeres
   The positions of the centromeres in the mitotic chromosomes were already known,
and the corresponding loci in the lampbrush chromosomes were identified without
difficulty. The centric regions are short lengths (10/iora little less) of chromosome
axis devoid of lateral loops. These stretches of axis are of irregular width, in general
somewhat wider than chromomeres bearing loops but nowhere thicker than 1 fi
(Figs. 10, 13 and 14). Unlike the centric regions of T. cristatus karelinii, there is no
sign of a discrete granule flanked by axial bars in the centric regions of axolotl lamp-
brush chromosomes.

Other landmarks
   In contradistinction to the lampbrush chromosomes of T. cristatus, axolotl lamp-
brush chromosomes do not possess many strikingly distinctive landmarks apart from
the centromeres. Nevertheless, there are objects which permit the identification of
various chromosomes, and these will be described.
   Spheres. There are two spheres just to the left of the centromeres of chromosomes VI
and XIII (Figs. 10, 14 and 16). These objects closely resemble the structures also
called spheres on chromosomes V and VIII of T. cristatus. They are directly attached
to the chromosome axis. Homologous sphere-generating loci are often jointly involved
in the production of single spheres, i.e. sphere fusions are common. Attached spheres
vary in size up to a maximum diameter of about 10 /*, and free spheres of similar
dimensions and refractility are frequently encountered in oocytes within the size
range which I have studied. Just as in T. cristatus it is probable that detachment of the
spheres from their generating loci occurs, and is followed by the production of more
spheres; a further comment on this point is made in the section concerned with the
nucleolar organizer.
   The disposition of the spheres with respect to one another, and to the centromeres,
is so similar in chromosomes VI and XIII that these two chromosome regions may
well be duplicate.
   Suspended granules. This term has been introduced to define spherical objects
which hang on short stalks from the chromosome axes (Fig. 11). Except for one sus-
pended granule on chromosome III, the maximum size reached by these objects
 (about 5 /i) is decidedly smaller than the maximum size of spheres.
   I am not sure whether all the terminal regions of axolotl lampbrush chromosomes
 can carry suspended granules (e.g. Figs. 15, 34 and 35), but this is certainly true of
 most. When a suspended granule is present it is often possible to see that its attach-
 ment is not strictly to the telomere (which in axolotl chromosomes are small structures
Chromosomes and nucleoli of the axolotl                       95
no larger than nearby chromomeres) but to an immediately adjacent chromomere.
These near-terminal suspended granules are of no value as landmarks, for they are
quite variable in their occurrence. They are not indicated on the working map.
   On the other hand, the distribution of interstitial suspended granules, even though
individually these too may be present or absent in a given preparation, are of great
help in identifying particular chromosomes and parts of chromosomes. Thus, for
example, chromosomes I and II are much of a muchness for length and centromere
position, but the places where suspended granules are likely to be found are distinctive
for the two chromosomes, and the right arm of chromosome I bears none. The posi-
tions occupied by suspended granules also help to discriminate between chromo-
somes IV and V, VII and VIII, and they are the only sure means I have found to
discriminate between chromosomes IX, X and XL
   Suspended granules are composed of somewhat more refractile material than spheres
of similar size. As with spheres, though less frequently, suspended-granule fusions
occur. Moreover, the variable presence or absence of granules at particular loci, and
the presence of granules of comparable size and refractility free in the nuclear sap
(especially abundant in larger oocytes) suggest that cycles of formation and detach-
ment exist.
   In the right arm of chromosome III, about one-third the way out from the centro-
mere, there is a suspended granule which is often as large (up to IO/J diameter) as
the spheres (Fig. 13).
   Axial granules. These are defined as dense, swollen regions of the chromosome axis,
about 3 /i wide, and they are regularly present at two sites. One lies about one-eighth
the way in from the telomere in the left arm of chromosome VI (Fig. 12), while the
other lies very close to the telomere of the right arm of chromosome VII. Neither of
these axial granules bears lateral loops; either may fuse with its homologue.
   Fluffy loops. These are more bulky structures than the generally long and slender
lateral loops of axolotl lampbrush chromosomes, and their matrix has afluffytexture.
They are often so contorted and compacted around the chromosome axes that their
sites of attachment appear as fuzzy, ill-defined regions. They are of more diagnostic
value in oocytes at the lower end of the size range which I have studied, for in larger
 oocytes, where some degree of loop retraction has set in, they may no longer show
 their distinctive character.
    It is only in the case of chromosome III that I have made much use of fluffy loops
 for chromosome identification. The nucleolar organizer locus lies on the right arm of
 chromosome III, but in some oocytes there is little or no nucleolar material present at
 the organizer locus (Fig. 38), and when the long left arm of this chromosome happens
 to be broken in a preparation the position of the centromere no longer serves in
 diagnosis. Provided, however, that the chromosome can be followed through from its
 right end to just beyond the centromere, the distribution of suspended granules, and
 still more of the fluffy loops relative to the centromere (Fig. 13), allow of certain
 identification.
    Stiff loops on chromosome XIII. Chromosome XIII can be readily identified on
 account of its small size and two spheres located near its centromere. In addition it
96                                    H. G. Callan
bears two adjacent pairs of loops subterminally in the left arm which, unusually for
the axolotl, are of a character found nowhere else in the complement (si in Fig. 16).
These loops are strikingly asymmetric (in itself unusual for axolotl lateral loops) and
their matrix is stiff and dense.

THE FREE NUCLEOLI IN OOCYTES
   In the smallest oocytes which I have examined, of diameter about 0-25 mm, the
free nucleoli are spherical objects of uniform refractility and they all lie immediately
adjacent to the smoothly rounded nuclear membrane. Oocytes of this size lack yolk,
and the nucleus can be observed directly within the oocyte. The several hundred
nucleoli are not flattened in the plane of the nuclear membrane, so the area of contact
between each nucleolus and the membrane is slight. Nevertheless, when nuclei from
these small oocytes are isolated and ruptured, all the nucleoli remain attached to the
membrane. The nucleoli range from minute dots near the limit of resolution to a
maximum diameter of about 6 [i.
   Axolotl oocytes become fully opaque and white at a diameter of about o-6 mm.
By the time this stage is reached the free nucleoli are noticeably larger, are of even
texture and refractility, and are much more uniform in size (Fig. 17). Most nucleoli
remain round objects ranging in diameter from about 7 to 10 fi, but a small proportion
are of irregular shape. In some oocytes there are occasional nucleoli 2 or more times as
large as the general run (upper right in Fig. 18); these in all probability arise by fusion
between normal-sized nucleoli.
   The largest all-white oocytes are of about I-I mm diameter. By this stage all the
free nucleoli are of irregular shape (Figs. 18 and 26). Slightly larger oocytes develop
a uniform pigmentation over the surface, and oocytes of 1-25 mm diameter are intense
dark brown, almost black, all over. In oocytes of this size the nucleoli of most irregular
shape are clearly in the process of transforming into rings. The nucleolar substance
first appears as though it were perforated by an eccentric hole. Later it becomes con-
stricted into a few lumps, usually between 5 and 10, which remain strung together
in a ring (Figs. 19, 27). Increase in volume of nucleolar substance accompanies this
transformation.
    Oocytes of diameter 1 -\ mm are dark brown all over except for a small whitish area;
as growth continues the white area extends until in the mature oocyte there are roughly
equal hemispheres of brown and white. In young axolotls about a year old the oocytes
mature at i-8 mm diameter, but in older axolotls they reach a larger diameter, about
 2 mm.
    The transformation of free nucleoli into lumpy rings is complete in an oocyte of
 1-5 mm diameter. In some axolotls, but not all, this transformation is accompanied
 by release of granular material from the nucleoli, and in such animals the edges of the
nucleoli appear very ragged at this and the immediately subsequent stage.
    In oocytes up to a diameter of 1-5 mm the lampbrush chromosomes extend through-
 out the nuclear volume and bear long lateral loops. Further increase in oocyte size is
accompanied by extensive movements of materials within the nuclei. In the axolotl
Chromosomes and nucleoli of the axolotl                           97
 oocyte as it approaches maturity, and as in other urodeles, the lampbrush chromosomes
 start withdrawing their lateral loops, shorten, and retire towards the middle of the
 nucleus. This contraction and aggregation of the chromosomes is a well-known though
 by no means well-understood phenomenon. In many axolotls (23 out of the 27 which
 I have examined) pointers to the mechanism involved are provided by the free nucleoli,
whose appearance during the aggregation process is bizarre in the extreme. I will first
 describe the more common situation, and will leave consideration of the exceptional
until later.
    Each nucleolus, which starts out as a ring of interconnected lumps, extends inwards
at one or a few places and each extension takes the form of a loop (Figs. 20, 21). Along
these loops, which all point towards the middle of the nucleus, the nucleolar substance
is distributed as a series of beads of rather uniform size (Figs. 23-25), giving very much
the appearance of droplets of dew on a spider's web. I deliberately draw the comparison
to a spider's web, for it is conceivable that the texture of the nucleolar substance and
the boundary it makes with the nuclear sap are such that surface tension determines the
form assumed. Be this as it may, the nucleoli are evidently responding to strains set up
in the nuclear sap, which in the axolotl, just as in T. cristatus and Xenopus laevis
oocytes, consists of two colloid phases, one rigid and one fluid, co-extensive within the
nuclear volume (Callan, 1952). The initially variable number of stretched loops per
nucleolus (as stretching proceeds these merge into a single, radially extended ring)
suggests that each nucleolus is responding passively to strains in its general neigh-
bourhood rather than that it is being pulled towards the middle of the nucleus at one
special point, like a chromosome at anaphase.
    The rigid colloid of the nuclear sap impedes free movement of the ring nucleoli,
and for a time at least tethers some of the lumpy components of individual rings at
the nuclear periphery. With continuing aggregation of the chromosomes in the middle
of the nucleus, more and more nucleolar material is drawn inwards and, in the case
of most of the nucleoli, the peripheral residues ultimately break loose from their
anchorage. Those nucleoli which succeed in passing across the nuclear sap come to
form a layer investing the chromosome group. Here they are no longer subject to the
radially arranged stresses of the contraction phase, and they assume the form of short
rings with rough contours (Fig. 29). However, some nucleoli (and it may be up to a
hundred or so; the number varies from oocyte to oocyte) fail to make the passage; at
the end of the contraction phase these remain peripheral, and continue so until
immediately prior to ovulation.
   Each ring nucleolus, whether it be centrally or peripherally located, finally amalga-
mates its substance to form a solid, roughly spherical object of somewhere between
10 and 20 ju, diameter (Figs. 22 and 30). In oocytes of i-8 mm and above all the nucleoli
are round and solid.
   The nucleolar transformations just described are illustrated diagrammatically in
Fig. 6. In order to take the series of photographs of whole isolated nuclei shown in
Figs. 17-22, nuclei were isolated and cleaned of cytoplasm in o-i M K/NaCl, and then
transferred to an observation chamber containing the same saline. Optical sections of
the nuclei were photographed using an ordinary (not phase) objective, with the con-
   7                                                                        Cell Sci. 1
98                                      H. G. Callan
denser iris stopped down sufficiently to enhance contrast. Ino-iM K/NaCl the rigid
colloid of the nuclear sap maintains its form for several minutes but the fluid colloid
exerts osmotic pressure and rapidly distends the membrane. This accounts for the
gap between the membrane and the peripheral nucleoli in Figs. 18-22; it is an artifact.
There is no such gap in Fig. 17; in oocytes of this stage (and younger ones) the nucleoli
are attached to the membrane, and follow the membrane as it distends.

                                                                Nuclear membrane

     Fig. 6. A sketch to show the transformations of axolotl nucleoli in oocytes developing
             from about 1 mm (extreme left) to 1-9 mm (extreme right) diameter.

   During the contraction phase some nucleoli move towards the middle of the nucleus
early, others late or not at all. Those which move early suffer less deformation at the
hands of the contraction mechanism than those which move late. Altogether there is
a substantial lack of synchrony in the migration of the nucleoli during the contraction
phase, and this accounts for the at first sight perplexing range of nucleolar form in
preparations of dispersed nuclear contents from oocytes of between i-6 and i-8 mm
diameter. To take an extreme example, a single preparation may contain short lumpy
rings (nucleoli which have not yet started their migration inward, or which never will
(Fig. 27)), multiple beaded loops arising from partly unstretched lumpy rings (nucleoli
at the beginning of migration (Fig. 28)), single beaded rings ranging up to 300 /i long
(nucleoli at full stretch between the nuclear periphery and central chromosome group
(Fig. 25)), short rings of irregular outline, often markedly distorted at one point
(nucleoli which have been subjected to stretching, but which have now contracted on
reaching the chromosome group (Fig. 29)), and various condensed forms representing
Chromosomes and nucleoli of the axolotl                        99
the final stages in the amalgamation of nucleolar substance to form solid rounded
objects (Fig. 30).
   The appearance of those nucleoli which consist of several beaded loops attached
together (Fig. 28) suggests at first sight that a process of nucleolar self-replication
occurs in axolotl oocytes, and this is made the more plausible because Dr J. Kezer
(personal communication) and Miller (1964) have recently established that a thread
of DNA forms an axis within ring-shaped nucleoli in Plethodon cinereus and Triturus
pyrrhogaster. However counts of total nucleoli in single oocytes do not support such
an interpretation.
   Accurate counts of nucleoli cannot be obtained from oocytes containing ring
nucleoli, because in the early stages of transformation it is often impossible to decide
whether two or three neighbouring objects are separate, individual nucleoli; further-
more, during the contraction phase it is often impossible to decide whether several
beaded loops lying jumbled together are parts of one nucleolus, or of more than one.
Though laborious, it is possible to make accurate counts of nucleoli from oocytes at
the stages just before transformation to rings (diameter 0-9 mm) and just after ring
nucleoli have reverted to solid round objects (diameter i-8 mm).
   From five oocytes of each of these two sizes preparations were made in the usual
way in dispersing saline, except that to guard against the possibility of nucleoli
becoming trapped in the nuclear membrane at the time of its removal, and not counted,
the membrane was laid down elsewhere on the floor of the observation chamber
instead of being discarded. This precaution proved necessary for the smaller oocytes,
but not for the large ones. The preparations were then photographed, and the nucleoli
in each preparation were counted directly at the microscope, marking off areas on the
photographic prints as these were covered. Such a process is necessary if accurate
counts are to be made, for the nucleoli are scattered over an area of about one square
millimetre and there is insufficient resolution in single photographs of such a large
area to permit direct counting from the print.
   The nucleolar counts are given in Table 2. There is a wide variation in nucleolar
number from oocyte to oocyte, but no evidence that large oocytes, after the ring

         Table 2. Counts of total nucleoli in preparations from axolotl 1671-2

                                       Oocyte
                      Preparation     diameter       Number of
                        number          (mm)         free nuclei
                           1            0-9             415
                          3             0-9             523
                          4             0-9             S°i
                          9             09              667
                         10             09              493
                          2              i-8            417
                          5              i-8            484
                          6              i-8            724
                          7              i-8             S°7
                          8              i-8             607

                                                                                 7-2
ioo                                  H. G. Callan
transformations have occurred, have more nucleoli than smaller ones. It is arguable,
though improbable, that an increase in nucleolar number does occur, but that it is
compensated by nucleolar disintegration. This would need to be on a substantial scale,
for during the early part of the contraction phase fully 50% of the nucleoli, indeed
often more, take the form of multiple loops. There is no evidence for any such dis-
integration except in over-ripe oocytes,, and in normal oocytes at the time of ovulation;
consequently, in my opinion, the counts establish that the multiple loop forms are
nucleoli in the early stages of their stretching and migration across the nuclear sap,
not nucleoli in the process of replication.
   The mechanism responsible during the contraction phase for the aggregation of the
chromosomes and nucleoli at the middle of the oocyte nucleus is obscure. Like the
anaphase movement of chromosomes on a cell division spindle, it operates within an
overall volume which does not alter substantially. There is, of course, a progressive
increase in nuclear volume which keeps pace with increasing oocyte size, but no
untoward disturbance of this progression occurs before, during or after the contraction
phase. One is tempted to envisage a series of radially arranged canals within the
nuclear colloid, with fluid streams moving outward in the canals compensated by a
return flow inward, between canals, which would exert a generally distributed centri-
petal pressure on chromosomes and nucleoli. However, this is merely one of several
possible explanations, and further study will be necessary to resolve the problem.
   The degree to which the free nucleoli are stretched and deformed during the aggrega-
tion process varies between one axolotl and another, and in Table 3 I have indicated
this variation by a scale ranging from o to + + + . The variation may be connected
with nuclear sap consistency; certainly those axolotls which showed greatest nucleolar
stretch had particularly stiff sap.
   Four relatively young axolotls were exceptional in showing no stretching of free
nucleoli. In these animals, in oocytes of between 1-2 and 1-4 mm diameter, the trans-
formation to rough rings took place as usual, but re-amalgamation of the nucleolar
substance to form smooth spheres had already occurred in oocytes of 1 -5 mm diameter,
i.e. before the beginning of the movement of chromosomes and nucleoli towards the
middle of the germinal vesicle nucleus.

THE NUCLEOLAR ORGANIZER IN OOCYTES
   The locus responsible for the production of free nucleoli in axolotl oocytes has been
identified. It lies subterminally in the shorter (right) arm of chromosome III. This
chromosome has a much greater arm asymmetry (roughly 5:3) than any other of the
six largest chromosomes of the axolotl. The ratio of lengths: centromere to nucleolar
organizer, and nucleolar organizer to telomere, in the right arm of chromosome III,
is nearly 9:1. These characteristics are in good accord with those which define the
locus of the nucleolar constriction in mitotic chromosomes, so with a fair measure of
assurance, just as Gall (1954) has maintained for A. tigrinum, homology between the
nucleolar organizer in mitotic and lampbrush chromosomes can be assumed.
   The objects attached laterally to the nucleolar organizing locus of lampbrush
Chromosomes and nucleoli of the axolotl                         101
chromosome III exceed all other chromosomal materials in their refractility. Even if
small, they appear brightly illuminated when viewed with Zeiss Neofluar phase
objectives, whereas all other chromosomal components appear darker than background.
   The nucleolar organizing region is immensely variable in form in oocytes within the
size range on which I have concentrated, and this variability was at first bewildering.
However it turned out to provide particularly strong evidence for homology between
the free nucleoli and objects attached at the organizer locus.
   In preparations from the smallest oocytes in which I have been able to identify the
nucleolar organizer (oocyte diameter of about i mm) the one or a few (up to six)
objects attached to the chromosome axis are irregular and jagged in outline, and of
any size up to, but not greater than, that of the free nucleoli. The free nucleoli in these
smaller oocytes are likewise irregular rough objects and they are of nearly uniform
size. This description holds good up to an oocyte diameter of about 1-25 mm, and in
occasional oocytes of slightly larger size (Fig. 31).
   In oocytes within the size range 1-4-1-7 mm the nucleolar organizer locus may be
occupied (and the chromosome axis obscured) by a cluster of rounded objects (Figs. 32,
and 39-43) or by an array of long drawn-out beaded loops (Figs. 33-36). Exceptionally
there are no, or quite minute, objects attached at the organizer locus, which is then
visible as a cylindrical, not very refractile chromomere about 4 fi long by 2 fi wide
(Figs. 38, 39). The cluster of round objects at the organizer locus is precisely com-
parable in morphology to free nucleoli which have transformed into lumpy rings, and
the arrays of beaded loops similarly correspond to free nucleoli which have been
stretched and deformed during the contraction phase.
   There is a considerable range in the number of round objects or beaded loops
attached to the organizer locus, and this is apparently due to variation in the original
number of attached nucleoli. If several nucleoli happen to be attached, each transforms
into a lumpy ring, with the rings stacked in a row along the chromosome. This causes
the chromosome axis to stretch, and in extreme cases nucleolar material may occupy
a length of 50/t or so (Figs. 41, 43).
   There is great variation between axolotls, and between oocytes from a single axolotl,
in the degree to which the attached lumpy-ring nucleoli become drawn out into
beaded loops during the contraction phase. This variation has been scored on a o to
 + + + scale and is indicated in Table 3. There is a rough correspondence between the
degree of deformation of free and attached nucleoli, but even in those axolotls in
which the stretching of the free nucleoli is extreme, there are always some oocytes
in the appropriate size range whose attached nucleoli are not deformed. It seems
probable that in such oocytes the nucleolar organizing loci happen to lie centrally
within the germinal vesicle; the attached nucleoli would then be less exposed to the
stresses of the contraction phase than they are when the organizers lie nearer the
nuclear periphery.
   The morphological similarity between attached and free nucleoli, and the very
comparable transformations which they undergo, is evidence that the free and attached
nucleoli are homologous to one another in the sense that they share a common genetic
origin. The variable number (o to about 6) of attached nucleoli, be they solid or rings,
102                                  H. G. Callan
may further suggest that nucleoli periodically detach from the organizer locus of the
axolotl and add to the population of free nucleoli. Such an assertion could be made
with some assurance regarding the generation and release of spheres from the lamp-
brush chromosomes of T. cristatus (Callan & Lloyd, i960) for in small oocytes of
T. cristatus there are no free spheres, and it is only in oocytes above a certain size,
and where large attached spheres occur on the chromosomes, that free spheres make
an appearance. In that instance the evidence based on sequence was compelling. But
regarding the free nucleoli of T. cristatus the situation is quite otherwise, as Mac-
gregor (1965) has pointed out. Similar considerations apply to the axolotl.
   The overwhelming majority of nucleolar 'Anlagen' in axolotl oocytes have already
taken up position at the nuclear membrane in oocytes of 0-25 mm diameter. The origin
of these nucleolar Anlagen by shedding from the organizer locus can only be inferred;
it has not been demonstrated. The crux of the problem of deciding whether in older
oocytes well-developed nucleoli are shed lies in the fact that if such a process occurs,
as well it may, free nucleoli are being added to a number that is already in the hundreds.
That any such addition must be marginal only can be inferred from the total counts
of nucleoli in five smaller (0-9 mm) and five larger (i-8 mm) oocytes given in Table 2.
   Therefore there are two alternative ways to explain the observed variation in bulk
of nucleolar material attached to the organizer loci. Either there is a continuing low
rate of production at, and detachment from, the organizer locus, or for some unknown
reason a variable number of the last nucleolar DNA Anlagen synthesized at the
organizer do not detach, but grow, transform and develop, and carry out the charac-
teristic nucleolar functions (such as RNA-synthesis in connexion with ribosome forma-
tion (Brown & Gurdon, 1964)) in much the same fashion as their earlier synthesized
and detached sisters.
   In the axolotl, Humphrey (1961) has described a chromosomal variant n which
determines the production of a smaller nucleolus than that determined by its normal
allelic alternative JV. Somatic interphase nuclei of Nn heterozygotes, provided the
nucleoli are not fused, generally contain two nucleoli of markedly different size, and
the heterozygotes can be readily distinguished from either homozygote by this
character. Fankhauser & Humphrey (1959) had earlier shown that the white mutant d,
whose dominant normal allele D determines dark skin colour, lies on the nucleolar-
organizing chromosome, and they gave evidence supporting the view that the locus
for D (or d) lies close to the nucleolar organizer.
   By studying the lampbrush chromosomes of axolotls of various genetic constitu-
tions from Dr Humphrey's colony I hoped to obtain critical evidence for the genetic
identity of oocyte and somatic nucleolar organizers. I also hoped that I might be able
to locate the gene d on the lampbrush chromosomes.
   Twenty-six axolotls of the constitutions shown in Table 3 were studied. The right
arm of bivalent III was identified in a convenient number of preparations (or as many
as it proved possible to obtain from a fragment of ovary about 0-5 to 1 ml in volume)
and sketches were made of the region extending from the nearest chiasma to the left
of the nucleolar organizer out to the end of the right arm. The preparations were
scored for symmetry or otherwise of the nucleolar material attached at the two homo-
Table 3. Genetic constitution and nucleolar condition in axolotls
                                                                        Status of nucleoli
                                                                         attachea at t he
                                                                         organizer locus
                                                                                    A.
                                                                   (

                                                                            Large
                                                                              A
                    Age                                                             SmallS             Degree of         Degree of
                                                                   r
Axolotl     r        -*       v             Genetic               Sym- Asym- or                     stretching of      stretching of
number     Years Months                   constitution           metrical metrical absent          attached nucleoli   free nucleoli               Remarks
2171-3          1         0                                             0           0         6           0                ++
2145-4          1         1                                             0           0         7           0                 0
1793-11                   0                                             0           0        10           0
                3                                                                                                           +          Recent importation from Mexico
                                                                                                                                                                         s"*
1690-5
                                           DN/DN                                    0         0
                3         5                                            IS                         Variable to + + +       +++
1647-2          3         9                                            10           0         0   Variable to + +         +++
1518-2                                                                              0         0
                4         3                                             7                         Variable to + +         + ++
                                                                                                                                                                         s3
2165-14         1         1                                             0           0         7           0                 0          Pure Wistar white
2111-130        1         2                                             2           0         5           0                 0          Remote dark ancestor
1988-3          2         1                                             0           0         6           0                 0          Pure Wistar white
                2         2                                                         0         0
                                                                                                                                                                         i
1968-1                                                                  8                         Variable to + + +       +++ |
                                      •     dN/dN        •                                                                             From mating Dd x Dd
1968-3          2         2                                                         6         1   Variable to + +         + + +1
                                                                                                                                                                        cleoh

                                                                        4
1737-2          3         2                                             4           0        10           0
                                                                                                                           ++ I        Pure Wistar white
                3         9                                             1           1        10           0
1660-4                                                                                                                     ++ )
1230-1          5         6                                             3           6         1           0                 +          Remote dark ancestor
1919-1          2
                          ^                                            10           2        0    Variable to + +        +++           Several dark ancestors            r
                2           •                            f                          1        0
1919-2                    4
                              1 dn/dn                                  13                         Variable to +           ++                                             S"
1671-21         3                                                       8           2         0   Variable to + +        + ++
                          6 J
1898-1          2         6                                            9            5         0   Variable to + +        +++
1708-8          3         4
                                                             [         7            8         0   Variable to +           ++
                                           DN/dN                                    8         1
1708-37         3         4                                            6                          Variable to +           ++
1540-24         4         2                                  1         0            5         1   Variable to +          +++
1498-4          4         4                                             4         12          2   Variable to +          +++
H98-5           4         4       J   1DN/dn             (I            0          14         0    Variable to + +        +++
1671-7          3
                          6                                             2           5         3           0                ++                                            w
                                                                                                                                                                         0
                                                         (I
1671-16         3         6                                             3           0         6           0
                                  1-       dN/dn                                                                           ++
1625-2          3                                                       6           3         6           0                ++
104                                  H. G. Callan
logous organizer loci, an assessed volume ratio of 2: i or more being arbitrarily
designated asymmetric, of less than 2:1 symmetric. When both organizer loci had
attached nucleolar material as small or smaller in volume than is to be seen in Figs. 31
and 39 the nucleolar status was classed separately as 'small or absent', without regard
to symmetry. I came to adopt this method of classification because of the great vari-
ability in size of attached nucleolar material that is encountered in most axolotis, and
because it seemed to me that more significance should be attributed to symmetry or
asymmetry of large masses of nucleolar material than to small. Whether this method
of scoring is justifiable or not (and it was adopted before I had properly appreciated
the metamorphoses characteristic of free nucleoli) it has had the effect of discriminating
between bivalents where both nucleolar organizers carry a single nucleolus (or its
single derivative ring), and bivalents where one or both nucleolar organizers carry
several nucleoli (or their derivative rings).
   The data are set out in Table 3. The observations are not conclusive. There are
indications that nucleolar organizers in the oocytes of young axolotis tend to carry
small (i.e. single) nucleoli, and that in regard to this character the ' pure Wistar white'
strain may remain juvenile longer than dark axolotis. As regards the symmetry or
asymmetry of larger nucleolar masses, the uniform symmetry of three older DN/DN
animals 1690-5, 1947-2, and 1518-2 (Figs. 32, 34 and 35), the preponderant symmetry
of the three dnjdn animals (Fig. 33) and the preponderant asymmetry of the two
DN/dn animals (Figs. 41, 42) certainly suggest that the different potentialities of
N and n are expressed in oocytes. Further, Dr Humphrey has told me that, based on
the relative sizes of somatic interphase nucleoli, he suspects there may be a difference
in nucleolar organizing capacity between N in the pure Wistar white strain and N in
dark axolotis; if so this might at least in part account for the extensive occurrence of
nucleolar asymmetry in the four DN/dN animals (Fig. 36). However, it must be
clearly stated that animals which are genuinely homozygous AW may show in Some
oocytes quite as striking asymmetry of attached nucleoli as heterozygotes, and vice
versa animals which are known to be Nn heterozygotes may in some oocytes show
symmetry (Fig. 43).
   The n variant was first detected by Humphrey in heterozygous combination in an
exceptional diploid white axolotl among offspring from a mating DD x dd. Dr Humphrey
has told me that another exceptional diploid white offspring from an earlier mating of
the same kind was also heterozygous for somatic nucleolar determinants, though he
did not detect it as such until later. This led Humphrey to propose (1961) that these
exceptional diploid white animals owe their origin to a deletion including the locus D
and part of N. I have examined with particular care the regions immediately to left
and right of the nucleolar organizers in the lampbrush chromosomes of two animals
of the constitution DN/dn, but have not found evidence of heterozygosity for a dele-
tion. Often enough there are chiasmata close to the organizers, and any gross asym-
metry in the chromosome axes between the two organizers and a nearby chiasma ought
to be apparent; but such asymmetry has not been observed.
   At nucleolar organizing loci where the attached ring nucleoli were drawn out into
beaded loops I have occasionally noticed a pair of beaded strands extended in the long
Chromosomes and nucleoli of the axolotl                       105
axis of the chromosome and forming the only connexion between the chromosome
regions to left and right of the organizer (Figs. 34, 35). Now this is exactly comparable
to the characteristic 'double bridges' formed by the lateral loops of lampbrush
chromosomes when their parent chromomere has cleaved transverse to the chromo-
some axis. The observation is significant because it indicates that the nucleolar
organizer locus bears exactly the same structural relationship to the rest of the chromo-
some as any other chromomere with its attendant pair of lateral loops. Moreover, it
must also mean that those DNA strands of the nucleolar organizer which remain as
persisting components of the chromosome are capable, once the cycles of replication
of DNA for the free nucleoli are over, of themselves becoming involved in the syn-
thesis of nucleolar RNA and protein. This being so, the length relationship between
the persisting (chromosomal) nucleolar DNA and the detached nucleolar DNA is of
interest. Is the detached DNA of the same length as the attached? If it is, and if N is
longer than n, one would expect to find nucleoli with two different DNA lengths in
Nn heterozygotes. This question deserves further investigation.
   At the end of the contraction phase there can be no doubt that attached nucleoli are
shed from the organizers. However, if at the stage when the free nucleoli have rounded
up, there remain any attached nucleoli, these too are no longer rings but instead solid
round objects (see Fig. 37).

MITOTIC SECONDARY CONSTRICTIONS VIS-A-VIS                LAMPBRUSH CHROMOSOME
  LANDMARKS
   The evidence for a straightforward genetic relationship between the nucleolar
organizer constriction in mitotic chromosomes and the nucleolar locus in lampbrush
chromosomes has already been presented. However I have been unable to detect any
outstanding landmarks on the lampbrush chromosomes corresponding in distribution
to those secondary constrictions in mitotic chromosomes which appear when the somatic
tissues are exposed to low temperature (compare Figs. 4 and 5). As earlier mentioned,
one might suppose that these constrictions owe their origin to a mechanism analogous
to that responsible for the nucleolar constriction, i.e. exceptional accumulation and
tardy detachment of certain gene products such as to interfere with prophase spiraliza-
tion. These secondary constrictions are constant and identical in their distribution
over the chromosome complements of three different tissues; moreover, similar
constrictions appear in the male meiotic metaphase chromosomes of Triturus helveticus,
T. vulgaris and T. cristatus when the diplotene stage (corresponding to the lampbrush
phase of female meiosis) is subjected to low temperature (Callan, 1942).
   The short arm of the XlVth mitotic chromosome of the axolotl is divided by two
cold-induced secondary constrictions into three regions of roughly equal length, but
there are no landmarks along this arm of the lampbrush bivalent (Fig. 15). The short
arm of the Xlllth mitotic chromosome has a nearly median secondary constriction,
but again there is no landmark along this arm of the lampbrush bivalent (Fig. 16);
contrariwise in the left arm of the lampbrush bivalent there are two characteristic
spheres near the centromere (Figs. 14, 16), yet there are no cold-induced secondary
106                                     H. G. Callan
constrictions anywhere in the left arm of mitotic chromosome XIII. A similar state-
ment holds for all the other chromosomes, and a general negative conclusion is
unavoidable. This is to say that loci which one may presume to carry out some special
function in somatic interphase nuclei are represented by quite unremarkable lateral
loops in oocytes.

CHIASMATA IN OOCYTES
   Chiasmata are formed at a very high frequency in axolotl oocytes, and as few other
fusions occur between the lampbrush chromosomes, and as most of these can be
discriminated from chiasmata, accurate determination of chiasma frequency is possible.
Centromere fusions such as occur abundantly in T. cristatus karelinii (Callan & Lloyd,
i960; Watson & Callan, 1963) and occasionally in T. viridescens (Gall, 1954) have not
been observed in the axolotl, and scarcely any terminal fusions have been noticed.
   Total chiasmata per nucleus in the five lampbrush chromosome complements which
were fully analysed are: 114, 126, 101, 101, 123, the mean of these figures being 113.
The distribution of chiasmata looks to be substantially at random, unlike the situation in
T. cristatus lampbrush chromosomes. If it were truly random, taking the mean chiasma
frequency at 113 per nucleus and the total haploid chromosome length at 1000 arbi-
trary units, this would amount on the average to 0-34 chiasmata per 3 units of length.
The one chromosome region where I have systematically scored chiasmata in many
preparations is the 3 units between the nucleolar organizer and the end of the right
arm of chromosome III. The recorded chiasma frequency in this region (based on
eighty-one observations) is o-88, so in detail the distribution is probably not random.
   Female heterogamety in the axolotl has been firmly established (Humphrey, 1945).
I have found no cytological evidence for an extensive sex-differential chromosome
segment, devoid of crossing-over, in this animal, and this is in accord with Humphrey's
(1948) conclusion, based on extensive breeding experiments with sex-reversed
axolotls, that: ' the W chromosome probably lacks nothing found in the Z except the
gene or genes of the differential segment responsible for stimulating gonad develop-
ment in the male direction'.
   Brunst & Hauschka (1963) and Hauschka & Brunst (1965), from studies of axolotl
mitotic chromosomes, have claimed that in the female complement the XHIth pair
of chromosomes are heteromorphic, the longer arm of one being substantially longer
than that of the other. My studies of the lampbrush chromosomes indicate that no
such differences exist between the XHIth or any other chromosome pair. An example
of bivalent XIII is shown in Fig. 16.

   I am greatly indebted to Dr R. R. Humphrey for providing me with axolotls from his colony,
to Dr R. Briggs for supplying several laboratory materials and facilities, and to Mrs L. Lloyd
for help with the illustrations accompanying this paper.
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