Complete developmental cycle of Leishmania mexicana in axenic culture
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Complete developmental cycle of Leishmania mexicana in
axenic culture
P. A. BATES*
Laboratory for Biochemical Parasitology, Department of Zoology, University of Glasgow, Glasgow G12 8QQ
(Received 24 March 1993; revised 4 June 1993; accepted 23 June 1993)
SUMMARY
A complete developmental sequence of Leishmania mexicana has been produced in axenic culture for the first time. This
was achieved by manipulation of media, pH and temperature conditions over a period of 16 days. All experiments were
initiated with lesion amastigotes that were transformed to multiplicative promastigotes by culture in HOMEM, 10%
foetal calf serum, pH 75, at 25 °C. Metacyclogenesis was induced by subpassage in Schneider's Drosophila medium, 20%
foetal calf serum, pH 5-5, and the resulting forms transformed to axenically growing amastigotes by subpassage in the
same medium and raising the temperature to 32 °C. Parasites from each day were characterized with respect to their
general morphology using light microscopy of Giemsa-stained smears, and biochemically by analysis of total protein
content, proteinases, nucleases and secretory acid phosphatase. The results demonstrated that the three main stages
identified - amastigotes, multiplicative promastigotes and metacyclic promastigotes - each exhibited the expected suite of
biochemical properties. Further, the changes in morphology observed as the developmental sequence proceeded from
stage to stage were accompanied by appropriate changes in biochemical properties. These results provide both useful
biochemical markers and a culture system in which to examine the regulation of differentiation and transformation during
the Leishmania life-cycle.
Key words: Leishmania, axenic culture, amastigote, promastigote, metacyclic, proteinase, nuclease, acid phosphatase.
INTRODUCTION
There is evidence that biochemical differences
exist between multiplicative promastigotes, meta-
Leishmania parasites exist in two hosts: as flagellated cyclic promastigotes and amastigotes (see, for ex-
extracellular promastigotes in the alimentary canal of ample, Mallinson & Coombs, 1989; Rainey &
female phlebotomine sandflies and as intracellular MacKenzie, 1991; Robertson & Coombs, 1992;
amastigotes within phagolysosomes of mammalian Schneider et al. 1992). These differences are likely to
macrophages. Transmission from sandfly to mam- reflect the biochemical adaptations of each stage to
malian host is achieved by inoculation of a distinct its immediate environment and its particular role in
mammal-infective form termed the metacyclic pro- the life-cycle. One of the major aims of current
mastigote (Sacks, 1989; Killick-Kendrick, 1990). research is to further understand the molecular basis
Metacyclics are phagocytosed by tissue macrophages of adaptation and survival of each parasite stage.
and subsequently transform into intracellular amasti- Determination of the conditions required for axenic
gotes. These multiply within the phagolysosomal culture has yielded much useful information on
system, eventually rupturing the host cell, and the parasite biochemistry and cell biology. Promastigote
liberated amastigotes are taken up by other macro- cultures are relatively easy to obtain. They can be
phages. Infection of a female sandfly is initiated if initiated by transformation of amastigotes in vitro
infected macrophages are ingested when taking a and the resulting promastigotes maintained in long-
bloodmeal. Within the midgut freed amastigotes term culture (reviewed by Evans, 1987). Axenic
transform into promastigotes which multiply and culture of amastigotes has proved more difficult (as
establish an initial midgut infection. Subsequently, these are an intracellular stage) but has been achieved
various morphological types of promastigotes have for some species (reviewed by Bates, 1993P. A. Bates
Tetley, 1993). In this report these culture methods ical appearance which had not undergone any
have been further developed to reproduce, for the apparent increase in size were also classified as lesion
first time, a complete developmental sequence of L. amastigotes. (ii) Type I transformation inter-
mexicana. The resulting parasites have been charac- mediates. After inoculation of promastigote culture
terized with respect to their morphology and various medium with lesion amastigotes various intermedi-
biochemical properties: protein content, nucleases ate forms in the process of transformation from
and secretory acid phosphatase. amastigotes to promastigotes were observed (see
Hart et al. 1981 for a fuller description). This
category included enlarged amastigotes, ellipsoid
forms and promastigote-like cells but with short
MATERIALS AND METHODS flagella equivalent to less than one cell body length,
(iii) Multiplicative promastigotes, which were de-
Cell culture fined as promastigote-like cells with aflagellumof
Leishmania mexicana (MNYC/BZ/62/M379) was approximately one body length (thus distinguishing
maintained in rump lesions of female CBA mice. them from transformation intermediates) and not
Amastigotes were isolated and transformed to pro- possessing metacyclic-like morphology, (iv) Meta-
mastigotes as described (Hart et al. 1981; Bates et al. cyclic promastigotes, defined as short (Leishmania development in axenic culture
Samples were heated in a boiling water bath for
2 min but were not reduced prior to electrophoresis.
Activity against poly(A) was revealed by staining
with Toluidine Blue, which indicated the positions
of enzymes as clear bands within a blue-staining gel.
_ 10 7
Acid phosphatase
Extracellular acid phosphatase (EC 3.1.3.2) activity
was assayed using the substrate £-nitrophenyl phos-
106 .
phate as described (Gottlieb & Dwyer, 1982). One
unit of enzyme activity is equivalent to the hydrolysis
of 1 /tmole of />-nitrophenyl phosphate/min at 37 °C.
Cell-free culture supernatants were prepared by 1 2 3 4 5 6 7 8 9 10 1112 13 1415 16
centrifugation of cultures at 2000 g and 4 °C for Day
10 min and filtration of the resulting supernatant 100 w- * • • * - • - • - •
medium through 022 /tm Millex GV niters (Milli- 90
pore). 80
70
60
RESULTS 50
40
To reproduce an entire developmental sequence for o 30
L. mexicana in axenic culture previous methods 20
(Bates et al. 1992; Bates & Tetley, 1993) were 10
combined and the following protocol developed 0
(Fig. 1 A). On day 0, lesion amastigotes were isolated 12 3 4 5 6 7 8 9 1011 12 13141516
Day
from infected CBA mice, inoculated at 5 x 105/ml
into HOMEM supplemented with 10% (v/v) foetal Fig. 1. Sequential axenic culture of Leishmania mexicana
multiplicative promastigotes, metacyclic promastigotes
calf serum (FCS) at pH 7-5, and incubated at 25 °C.
and axenic amastigotes. (A) Cell density: the vertical
Under these conditions the amastigotes transformed lines indicate subpassage on days 3 and 9. (B) Percentage
into promastigote forms within 24-^-8 h and began to cell types: (O) lesion amastigotes; ( # ) multiplicative
multiply. At the late logarithmic phase of growth on promastigotes; (V) metacyclics; ( • ) axenic amastigotes.
day 3, the resulting promastigotes were subpassaged For the sake of clarity, type I and type II transformation
into Schneider's Drosophila medium supplemented intermediates, present on days 1 and 10, respectively,
with 20% (v/v) FCS at pH 5-5 and incubated at are not shown.
25 °C. This medium induced metacyclogenesis as
previously described (Bates & Tetley, 1993) and the cultures upon which all the different types of analysis
resulting stationary phase population on day 9 were performed.
contained a homogeneous population of metacyclic The relative percentages of amastigotes, multi-
forms. These were then subpassaged into the same plicative promastigotes and metacyclic promasti-
medium, but the temperature of incubation was gotes were determinecd from Giemsa-stained smears
increased to 32 °C. These are conditions which and the results are shown in Fig. 1 B. By day 1 the
support long-term axenic growth of amastigote-like majority of the initial population of lesion amasti-
forms (Bates et al. 1992). This subpassage resulted in gotes had transformed to type I transformation
transformation of the metacyclics to amastigote-like intermediates (65 %): a mixture of elongated amasti-
forms which subsequently multiplied in axenic gotes and promastigotes with short flagella. This was
culture until they reached stationary phase at day 16. accompanied by an increase in cell density (cf. Fig.
Thus a complete developmental sequence, initiated 1 A) and intermediate forms were noted in the
with lesion amastigotes, proceeding through multi- process of division (see Fig. 2). Amastigote to
plicative promastigotes, metacyclic promastigotes promastigote transformation was essentially com-
and finally to amastigote-like forms, was obtained. plete by day 2 and resulted in a log phase population
This experiment has been performed on 6 separate of multiplicative promastigotes which exhibited a
occasions with essentially identical results. Each of variety of morphologies (see Fig. 2C-F). These were
the analyses of the resulting parasites described the predominant cell population from day 2 to day 7.
below in Figs 2-6 have also been performed on at Metacyclic-like promastigotes were first noted on
least 2 separate occasions. However, for the sake of day 4 but only at very low prevalence. By day 9,
comparison, the actual data shown in Figs 1-6, however, they represented 95 % of the cell popu-
which are representative of this larger number of lation. These were then transformed to amastigote-
experiments, were obtained from a single series of like forms by incubation at 32 °C. The metacyclicsP. A. Bates
10/im
•• •
4
Fig. 2. Morphology of parasites in Giemsa-stained smears from various days of the developmental cycle in vitro. (A),
day 0; (B) day 1; (C) day 2; (D) day 3; (E) day 4; (F)day 5; (G)day 7; (H) day 9; (I) day 10; (J) day 11; (K) day
13; (L) day 16. All pictures shown are at the same magnification.
resorbed their flagella and adopted a rounded present on day 10 were type II transformation
morphology such that by day 10, 85% of the intermediates. Amastigote-like forms accounted for
population were rounded aflagellates. However, 100% of the parasites observed from day 11
from their morphology and biochemistry (see below) onwards.
fully transformed amastigote-like forms were not The morphology of the parasites at various points
present until day 11. The remaining 15 % of cells in the developmental sequence is illustrated in Fig.Leishmania development in axenic culture
cells in the mid log phase population on day 2. This
20
18- correlated with an increase in size of the parasites as
16- described above. Interestingly, by day 3 at the late
14 log phase of growth the protein content had
12 decreased somewhat coincident with the appearance
c 10
of the elongated promastigotes, and this process
(D 8
C
o 6 continued upon subpassage at pH 5-5. T h e meta-
o cyclics on day 9 had an estimated protein content of
c 4
CD 2 3-5 mg/10 9 cells, the lowest observed. Incubation at
s 0 2 4 6 8 10 12 14 16
32 °C resulted in a dramatic increase in protein
0-
Day content to approximately 10 mg/10 9 cells. On sub-
sequent days this was observed to decrease such that
Fig. 3. Protein content of parasites through the the axenically growing amastigotes remained at
developmental sequence. Estimates are the means from approximately 5 mg/10 9 cells.
4 determinations, error bars are 1 standard deviation.
The proteinases expressed by parasites on each
day of the developmental sequence were analysed by
2. The lesion amastigotes were small ovoid cells of gelatin SDS-PAGE and the results shown in Fig. 4.
mean body length 3-86 + 0-40/wn on the major axis, The lesion amastigotes possessed a prominent group
and which lacked emergent flagella (Fig. 2 A). Within of bands of apparent molecular mass 22—24 kDa
24 h most of these had elongated, developed short (arrowed, Fig. 4) which have been extensively
flagella and dividing forms were observed (arrowed, characterized and previously shown to represent
Fig. 2B). The resulting promastigotes continued to cysteine proteinases (Robertson & Coombs, 1990).
multiply and grow in size. The mid-log phase They are also expressed by axenically cultured
population on day 2 consisted of large broad amastigotes of L. mexicana (Bates et al. 1992). These
promastigotes which stained very intensely with became undetectable in cell lysates of parasites
Giemsa's stain (Fig. 2C). By day 3 at the late log harvested from days 2 to 5 as they transformed and
phase of growth the population consisted largely of grew as multiplicative promastigotes. However, in
elongated promastigotes with a mean body length of day 6 samples 2 bands were observed which
15-22 + 3-49 lira. (Fig. 2D). These were then sub- increased in intensity until day 9, when the parasites
inoculated into medium at pH 5-5 with the tem- were predominantly metacyclic promastigotes. A
perature remaining at 25 °C. The mid-log phase band equivalent to the faster migrating of these two
population on day 4 resembled those on day 2 in bands was not detected in amastigotes. Upon
general appearance and intensity of staining (cf. Fig. transformation to axenically growing amastigotes
2C and 2E). However, the late log phase population this pattern was replaced by that observed in lesion
on day 5, in comparison with those from day 3, were amastigotes. This process appeared to be essentially
much shorter and fatter promastigotes with a mean complete by day 11 and the pattern remained
body length of 808 + 1-39 /an (cf. Fig. 2D and 2F). constant thereafter in lysates of axenically cultured
By day 7 the promastigotes had further reduced in amastigotes.
size becoming both shorter and narrower as dif- The nucleases expressed by parasites harvested
ferentiation to metacyclic forms occurred (Fig. 2G). from each day were also analysed by substrate
By day 9 a relatively homogeneous stationary phase SDS-PAGE using poly(A) and the results are shown
population of metacyclic forms was produced with a in Fig. 5. The lesion amastigotes possessed two
mean body length of 7-59 +1-15 jim (Fig. 2H). When prominent bands of apparent molecular masses 29
the temperature was raised to 32 °C these had and 31 kDa as previously shown (Bates, 19936).
transformed into aflagellates by day 10, that appeared Upon transformation to promastigotes these de-
slightly larger than amastigotes and were often creased in intensity but did not disappear com-
ellipsoid in shape (Fig. 21). Subsequently the pletely. However, an additional promastigote-
parasites became amastigote-like (Fig. 2J) and grew specific band of 40 kDa was induced. This was
axenically (Fig. 2K, cells 3-99 + 0-54 /im) until detectable from day 1 through to day 9 b u t appeared
stationary phase was reached (Fig. 2L, cells 3-59 + to be expressed at the greatest specific activity on day
0-42 fim). 3, judging from the relative intensity of the banding
To determine whether the different cell popula- pattern (Fig. 5). This band was lost upon trans-
tions were biochemically distinct various properties formation to amastigotes (day 10), coincident with a
of parasites harvested on each day of the devel- marked increase in the intensity of the 29/31 kDa
opmental sequence were analysed. The total protein doublet, thus restoring an amastigote-like pattern of
content/cell is shown in Fig. 3. As lesion amastigotes enzymes from day 11 to day 16.
transformed and grew as promastigotes an increase Secretory acid phosphatase in the supernatant
in protein content was observed, which rose from culture medium of the parasites was assayed and the
approximately 7 mg/10 9 cells on day 0 to 16 mg/10 9 results are shown in Fig. 6. Extracellular enzymeP. A. Bates
10 11 12 13 14 15 16
Fig. 4. Gelatin SDS-PAGE of parasite lysates from cells harvested through the developmental sequence (days 0-16).
Molecular mass markers are: pyruvate kinase 58 kDa; glyceraldehyde-3-phosphate dehydrogenase 36 kDa;
trypsinogen 24 kDa; soybean trypsin inhibitor 20 kDa.
12 13 14 15 16
Fig. 5. Nuclease SDS—PAGE of parasite lysates from cells harvested through the developmental sequence (days
0—16). Molecular mass markers are as in Fig. 4.
DISCUSSION
The current report is the first to demonstrate a
complete developmental sequence for Leishmania in
axenic culture, initiated with amastigotes and pro-
ceeding through multiplicative promastigotes, meta-
cyclic promastigotes and returning to amastigotes.
This was achieved by manipulation of culture
conditions using changes in both pH and tem-
perature and clearly demonstrated the importance of
these environmental cues in regulating parasite
Fig. 6. Secretory acid phosphatase in cell culture growth and development. The resulting sequence of
supernatants through the developmental sequence. morphological forms resembled that observed in the
Estimates are the means from 3 determinations, error natural life-cycle of the parasite. The experiments
bars are 1 standard deviation. described were all initiated with lesion amastigotes,
i.e. those derived from an infected animal, rather
activity rapidly increased as lesion amastigotes than from long-term cultures, because culture-
transformed and grew as promastigotes on days 1—3. adapted parasites are likely to differ significantly
This process continued upon subpassage at day 3 from their in vivo counterparts. For example, it is
until day 6 but halted as the cultures entered known that the long-term culture of promastigotes
stationary phase concomitant with the production of can result in a decreased capacity to produce
metacyclics. When the metacyclics transformed and metacyclic forms (Da Silva & Sacks, 1987).
grew as amastigote-like forms very little activity Three main stages can be defined in the Leishmania
could be detected in the supernatant medium of life-cycle: multiplicative promastigotes, metacyclic
axenically cultured amastigotes (days 10-16). promastigotes and amastigotes. In the current studyLeishmania development in axenic culture these correspond to L. mexicana populations from 1992) and suggests that metacyclics, the mammal- days 2-5, day 9 and days 0/11-16, respectively. infective form, have adopted a similar strategy. Multiplicative promastigotes showed little detectable However, the lack of secretion cannot be simply cysteine proteinase activity (Robertson & Coombs, explained as a non-specific blockage, as it is still 1992), exhibited a promastigote nuclease banding possible for various tracers to gain access to the pattern, including a promastigote-specific band flagellar pock"1' in amastigotes (Russell et al. 1992). (Bates, 19936), and also secreted acid phosphatase In addition to providing an opportunity to com- (Ilg et al. 1991). Metacyclic promastigotes were pare the main life-cycle stages, the present study cultured as previously described (Bates & Tetley, offers a unique system in which to examine the 1993) and in the present study showed two promi- transitions between these stages. However, it should nent bands in proteinase gels. These have been be emphasized that the morphological categories, observed previously and are only found in stationary- whilst useful to illustrate the sequential appearance phase populations of L. mexicana promastigotes of the major forms, are essentially arbitrary divisions (Robertson & Coombs, 1992). They appear to be in a continuous sequence. The multiplicative pro- metacyclic-specific as there was a strong correlation mastigotes, in particular, exhibited variable mor- between the prevalence of metacyclic promastigotes phology, both on a given day and between different in cultures and the detection of these bands in the days of culture. The functional significance of such corresponding lysates when examined by gelatin variation is unknown but is partly explained by the SDS-PAGE (Bates, unpublished observations). existence of promastigotes at various points in the Lesion amastigotes and axenically cultured amasti- cell cycle. However, changes in morphology with gotes both showed appropriate morphology, and time were also reported for development in vivo expressed amastigote-specific proteinases (Robert- within sandflies in studies of L. mexicana (Lawyer et son & Coombs, 1990; Bates et al. 1992) and nucleases al. 1987; Walters et al. 1987) and other species. For (Bates, 19936). There was no evidence for secretion example, in a study of L. major ten distinct of acid phosphatase by amastigotes, consistent with morphologies were identified (Lawyer et al. 1990). short-term incubations of amastigotes (Antoine et al. In this respect, the possible reasons for the change 1987; Stierhof et al. 1991) and long-term cultures of from the broad promastigotes observed on day 2 to amastigote-like forms (Russell, Xu & Chakraborty, the elongated forms observed on day 3 are of interest. 1992; Bates, unpublished observations). Thus, each These two populations bear close resemblance to the of the individual populations exhibited morpho- promastigotes which initially multiply in the blood- logical and biochemical properties consistent with meal and those which occur subsequently attached to previous studies. the midgut epithelium (Lawyer et al. 1987; Walters In contrast to multiplicative promastigotes, meta- et al. 1987). These have been termed procyclic and cyclics did not appear to secrete acid phosphatase, as nectomonad promastigotes, respectively (Lawyer et the extracellular activity showed a plateau coincident al. 1990). with metacyclogenesis and entry into stationary Interestingly, the protein content/cell showed a phase. Similar effects have been noted before in marked increase from day 9 to day 10, but then populations likely to contain few, if any, metacyclics decreased again. This temporary increase occurred (Gottlieb & Dwyer, 1982) and this feature may, even though the cells were not obviously larger, therefore, be a general property of stationary-phase although they were a different shape, but was also promastigotes. The alternative explanation, that the coincident with the increase in temperature of plateau resulted from a balance between synthesis incubation from 25 to 32 °C. Therefore, this effect is and degradation is less likely as there was no evidence probably explained by the induction of a protective of cell lysis here or previously (Gottlieb & Dwyer, heat shock response (reviewed by Maresca & 1982) and extracellular proteinase activity has not Carratu, 1992) in addition to the induction of been detected (Bates, unpublished observations). amastigote stage-specific proteins. The decrease Thus, secretion of acid phosphatase appears to be a observed from day 10 to day 11 may be explained by stage-specific feature in L. mexicana, restricted to cell division and the turnover of heat shock proteins multiplicative promastigotes. In this regard, it has or promastigote-specific proteins as full biochemical been previously shown that the flagellar pocket is the transformation was completed. route for secretion in multiplicative promastigotes of The current study is the first to investigate L. donovani (Bates, Hermes & Dwyer, 1989) and metacyclic to amastigote transformation in axenic L. mexicana (Stierhof et al. 1991). Monoclonal anti- culture and permits a comparison with amastigote to body staining indicated that although L. mexicana promastigote transformation (Hart et al. 1981) to be amastigotes do synthesize acid phosphatase, it made. In general outline the process of transform- is retained in the flagellar pocket (Stierhof et al. ation is initiated when appropriate environmental 1991). These observations support the idea that signals are received by the parasites (for example, amastigotes attempt to minimize the exposure of changes in temperature and/or pH) which, in turn, their antigens to the immune system (Russell et al. result in biochemical and morphological changes.
P. A. Bates
The main changes in morphology in the trans- BATES, P . A., HERMES, I. & DWYER, D. M. ( 1 9 8 9 ) .
formation of metacyclics to amastigote-like forms, Leishmania donovani: immunochemical localization
resorption of the flagellum and rounding of the cell and secretory mechanism of soluble acid phosphatase.
body, appeared to precede any significant increase in Experimental Parasitology 68, 335—46.
BATES, P . A., ROBERTSON, C. D., TETLEY, L. & COOMBS, G. H.
cell numbers. However, the day 10 aflagellates
(1992). Axenic cultivation and characterization of
differed morphologically from true amastigote-like
Leishmania mexicana amastigote-like forms.
forms and, also from the biochemical results, fully Parasitology 105, 193-202.
transformed amastigotes did not appear until day 11, BATES, P. A. & TETLEY, L. (1993). Leishmania mexicana:
by which point the cell density was already in- induction of metacyclogenesis by cultivation of
creasing. Amastigote to promastigote transformation promastigotes at acidic pH. Experimental Parasitology
exhibited similar features. Thus, cells were found in 76, 412-23.
the process of division before the morphological or BERENS, R. L., BRUN, R. & KRASSNER, S. M. ( 1 9 7 6 ) . A
biochemical changes were completed as shown here simple monophasic medium for axenic culture of
and previously (Hart et al. 1981). Therefore, it hemoflagellates. Journal of Parasitology 62, 360-5.
appears that morphological and biochemical changes DA SILVA, R. & SACKS, D. L. (1987). Metacyclogenesis is a
accompany division in both kinds of transformation. major determinant of Leishmania promastigote
Entry into the cell cycle is initiated from an arrested virulence and attenuation. Infection and Immunity 55,
state in the case of metacyclics, which are a non- 2802-6.
EVANS, D. A. (1987). Leishmania. In In Vitro Methods for
dividing cell population. The growth status of lesion
Parasite Cultivation (ed. Taylor, A. E. R. & Baker,
amastigotes is not well defined, although as an
J. R.), pp. 52-75. London: Academic Press.
overall population lesion amastigotes are not growing GOTTLIEB, M. & DWYER, D. M. (1982). Identification and
as fast as their in vitro axenic equivalents. Thus, partial characterization of an extracellular acid
whether lesion amastigotes are a mixture of arrested phosphatase activity of Leishmania donovani
and growing cells or a population with a much lower promastigotes. Molecular and Cellular Biology 2,
growth rate or some combination, is unknown, and 76-81.
may also vary with the age of and site within the HART, D. T., VICKERMAN, K. & COOMBS, G. H. ( 1 9 8 1 ) .
lesion. Also the possibility that a differentiated Transformation in vitro of Leishmania mexicana
amastigote transmission stage exists, although a amastigotes to promastigotes: nutritional requirements
matter for speculation, should not be dismissed. In and the effect of drugs. Parasitology 83, 529-41.
ILG, T., STIERHOF, Y . - D . , ETGES, R., ADRIAN, M., HARBECKE,
any event, for both amastigotes and metacyclics, cell
D. & OVERATH, p. (1991). Secreted acid phosphatase of
division occurs during transformation to the next
Leishmania mexicana: a filamentous
stage. It is an interesting possibility that cell division
phosphoglycoprotein polymer. Proceedings of the
may be obligatory to achieve complete transform- National Academy of Sciences, USA 88, 8774-8.
ation, or alternatively the apparent linkage of these KILLICK-KENDRICK, R. (1990). The life-cycle of
processes could simply result from a timing effect. Leishmania in the sandfly with special reference to the
Ultimately, the next stage is produced which then form infective to the vertebrate host. Annales de
undergoes repeated cycles of growth and division to Parasitologie Humaine et Comparee 65 (Suppl. 1),
establish an infection in the new host. More detailed 37^2.
investigation of the time-course of events at the LAWYER, P . G., YOUNG, D. G., BUTLER, J. F. & AKIN, D. E.
molecular level will be required to understand these (1987). Development of Leishmania mexicana in
processes more fully. Lutzomyia diabolica and Lutzomyia shannoni (Diptera:
Psychodidae). Journal of Medical Entomology 24,
347-55.
This work was supported by the Royal Society of LAWYER, P . G., NGUMBI, P . M., ANJILI, C. O., ODONGO,
Edinburgh and the Caledonian Research Foundation. The S. O., MEBRAHTU, Y. B., GITHURE, J. I., KOECH, D. K. &
technical assistance of G. Tobasnick is gratefully acknow-
ROBERTS, c. R. (1990). Development of Leishmania
ledged. I thank K. Vickerman for comments on the
preparation of this manuscript. major in Phlebotomus duboscqi and Sergentomyia
schwetzi (Diptera: Psychodidae). American Journal of
Tropical Medicine and Hygiene 43, 31—43.
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