Mem Inst Oswaldo Cruz, Rio de Janeiro, Vol. 91(5): 519-541, Sep./Oct. 1996 519 The Biology of Malarial Parasite in the Mosquito - A Review Amauri Braga Simonetti Departamento de Microbiologia, Instituto de Biociência Universidade Federal do Rio Grande do Sul, Rua Sarmento Leite 500, 90050-170 Porto Alegre, RS, Brasil The purpose of this review is to summarize the biology of Plasmodium in the mosquito including recent data to contribute to better understanding of the developmental interaction between mosquito and malarial parasite. The entire sporogonic cycle is discussed taking into consideration different para- site/vector interactions and factors affecting parasite development to the mosquito. Key words: malaria - mosquito - sporogonic cycle - cell biology The control and prevention of malaria has been et al. 1996). This review foccuses on the cell biol- pursued for a long time. Although campaigns ogy of malarial parasites at different stages of its against malaria were initially successful in some development in the mosquito and also includes areas, the emergence of resistance of the parasite factors that may affect the parasite infectivity. to drugs and of the mosquito vector to insecticides, The sporogonic cycle combined with the difficulties in implementing and Some erythrocytic parasites differentiate into maintaining effective control schemes have led to sexual forms called gametocytes. Mature and func- a resurgence of the disease in many parts of the tional gametocytes ingested by an appropriate spe- world (Wernsdorfer 1991, Schapira et al. 1993, cies of mosquito in a bloodmeal are stimulated to Roush 1993). Much of the current work on ma- transform into the stages which establish the para- laria focuses on immunological aspects of the dis- sites in their vector (Garnham 1966). Under the in- ease and there has been remarkable progress in the fluence of changes in the mosquito midgut environ- identification of a variety of parasite antigens in ment the gametocytes become extracellular within different stages of the parasite development, some 8-15 min of ingestion. After emergence from the of which may be included in a future vaccine red blood cell (exflagellation) the male gametes fer- (Nussenzweig & Nussenzweig 1989, Mitchel 1989, tilize the female gametes within 60 min of ingestion Targett 1989, Nussenzweig 1990, Hommel 1991, of blood. The fertilized macrogamete (zygote) dif- Good 1991a). However, experimental and field ferentiates into a single motile ookinete over the next studies have shown the complexity of the immune 10-25 hr, which migrates from the bloodmeal response to parasites, indicating that an efficient through the midgut wall to form an oocyst under- malaria vaccine is difficult to achieve (Cattani neath the basal lamina of the midgut. Each oocyst 1989, Good 1991b, 1992, Philips 1992). There- produces many thousands of invasive sporozoites fore, for control of the disease greater understand- over a period of 7-12 days. The sporozoites escape ing of the biological mechanisms involved in host/ from the oocyst and then invade the salivary glands, parasite/vector interactions is essential. here they stay for possibly very long periods until In recent years an increasing number of stud- injected into another vertebrate host when the next ies have concentrated on the mosquito stages of bloodmeal is taken (Sinden 1984, Carter & Graves the parasite development. Comprehensive reviews 1988). A diagram of the sporogonic cycle in the on biological, ultrastructural, biochemical, molecu- mosquito is shown in Figure. lar and immunological aspects of the parasite and the vector can be obtained in the literature (Sinden Development of gametocytes (gametocytogen- 1978, 1983, 1984, Carter & Graves 1988, Carter esis) et al. 1988, Alano & Carter 1990, Billingsley 1990, As with other members of the order Crampton et al. 1990, 1994, Alano 1991, Brey Haemosporina, gametocytes of Plasmodium can 1991, Coluzzi 1992, Kaslow et al. 1994a, Sinden arise from merozoites from pre-erythrocytic para- sites. Nevertheless, during natural infections, most gametocytes arise from merozoites of blood-stage origin (Alano & Carter 1990). Two alternative possibilities by which malaria parasites could be- Fax: 55-51-3362779. E.mail: [email protected] come committed to either sexual or asexual devel- Received 20 December 1995 opment have been proposed (Inselburg 1983, Mons Accepted 13 May 1996 1985, 1986, Bruce et al. 1990): first, the merozoite 520 Biology of Malarial Parasites in Mosquito • AB Simonetti EXOERYTHROCYTIC CYCLE ERYTHROCYTIC IN LIVER CYCLE IN BLOOD Penetrates red cell Merozoites red cells reinvade Schizont Merozoites Sporozoites Macro- Micro- penetrates gametocyte gametocyte liver cell IN MAN Gametocytes taken Sporozoites injected into mosquito IN MOSQUITO into man with saliva stomach with (ANOPHELES) of mosquito bloo meal Sporozoites in Microgametes salivary gland FERTILISATION GAMETOGENESIS Ookinete Ookinete penetrates midgut wall of mosquito to develop into oocyst SPOROGONY Oocyst ruptures to liberate sporozoites which penetrate salivary gland Life cycle of the malaria parasite in mammals (from Vickerman & Cox 1972). is not committed at the time of invasion of a red ronmental factors influence it so that at maturity blood cell. During early growth (as a ring form), the schizont produces merozoites that are the parasite is suceptible to factors that will com- precommitted to form asexual parasites or com- mit it to either sexual or asexual development. Sec- mitted to form gametocytes upon subsequent in- ond, during growth of an asexual parasite, envi- vasion of a red blood cell. Mem Inst Oswaldo Cruz, Rio de Janeiro, Vol. 91(5), Sep./Oct. 1996 521 A number of studies have attempted to iden- showed in vitro that induction of exflagellation of tify factors that may influence the differentiation P. berghei is triggered by a rise in the intracellular ++ and production of sexual stage parasites including pH (pHi) which is mediated by Ca and cGMP cAMP, presence of antimalarial drugs and other regulation. pHi can be modulated by alkaline me- environmental factors associated to the host such dia and is controlled by a complex series of inter- as the presence of serum undetermined components dependent ion pumps and channels controlling Na+, + - - (reviewed by Sinden 1983, Mons 1986, Carter & K , Cl and HCO3 transport between the parasite Graves 1988, Dearsly et al. 1990). In a recent re- and the environment. Other influential factors de- port (Schneweis et al. 1991) it was claimed that scribed include cAMP analogues and inhibitors of the production of infective gametocytes in vitro phosphodiesterase (Martin et al. 1978). The dura- can be enhanced by products of haemolysis but tion of microgametogenesis is both temperature and the nature of the erythrocytic factor was not iden- species dependent, e.g. at 20-22°C it may take 7- tified. 15 min for P. falciparum in vitro, although Gametocyte development takes longer than exflagellation may be detected after shorter peri- asexual schizogony, e.g. 26 hr as opposed to 22.5 ods in the fluid excreted by feeding Anopheles hr in P. berghei (Mons et al. 1985) or in the more (Sinden 1983). There is no evidence that extreme case of P. falciparum 8-12 days as op- exflagellation is influenced by factors released by posed to 48 hr (Sinden 1983). The proportion of digestion of the blood meal since digestion nor- parasites that develop into gametocytes varies mally begins several hours later (Graf et al. 1986). greatly during the course of natural infections even Microgamete formation involves three mitotic di- at its peak but is very low in relation to the total visions with a rapid assembly of eight axonemes parasitaemia (Smalley et al. 1981). The growth and on the single microtubule organizing centre that differentiation of gametocytes of P. falciparum has divides and passes to the spindle poles. This divi- been divided into five stages (I to V) covering about sion simultaneously segregates the genome and the eight days from merozoite invasion to mature ga- axoneme so that each of the eight emergent ga- metocyte, each stage being distinguished by suc- metes receives a single axoneme and haploid ge- cessive changes in the organization of the cell nome, both being connected to a common micro- (Hawking et al. 1971). tubule organizing centre. After exflagellation the Little is known about commitment of gameto- microgametes, normally bearing a single axoneme, cytes to be male or female. The fact that both fe- a single condensed nucleus and a single kineto- male and male can be produced by haploid blood- some with its sphere and granule at the distal end, stage parasite, the sex of a gametocyte does not are torn from the microgametocyte surface and result from chromosomal differences between both rapidly move away into the blood meal (Sinden & types of cell but their development must be due to Croll 1975). selective gene expression. In general, the number Macrogametogenesis at the morphological of female gametocytes predominates over the num- level involves little more than escape from the host ber of male, but this predominance may vary at cell (Sinden 1984). At the cellular level there is de different times between cloned infections novo synthesis of the proteins which are expressed (Cornelissen 1988, Schall 1989). on the surface of macrogamete (Kumar & Carter 1984). It was recently identified a gametocyte spe- Gametogenesis Mature and functional gametocytes ingested by cific protein of P. falciparum called Pf11-1 and there is some evidence that this protein might be the mosquito in a bloodmeal are stimulated by the involved in the emergence of gametes from the midgut environment to transform into gametes. infected erythrocyte (Scherf et al. 1992, 1993). Studies indicate that various triggers induce game- Interactions between the vertebrate host and the tocytes to undergo differentiation. Microgameto- parasite do not cease when the blood meal is taken. genesis in vitro is, optimally, dependent upon a rise in pH (Nijhout & Carter 1978), a rise in pCO and Following induction of gametogenesis the parasite 2 is liberated into the blood meal.