TRENDS in Parasitology Vol.18 No. 1 January 2002 http://parasites.trends.com 1471-4922/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S1471-4922(01)02122-5 32 Review Review Richard Paul* Paul Brey Unité de Biochimie et Biologie Moléculaire des Insectes, Institut Pasteur, 28 rue du Dr Roux, 75724 Paris Cedex 15, France. *e-mail: topotito@pasteur.fr Vincent Robert Unité de Recherche Paludologie Afro-tropicale de l’IRD and Institut Pasteur de Madagascar, BP 1274, Antananarivo 101, Madagascar. The transmission of Plasmodium spp. from the vertebrate host to the insect vector is accomplished solely by the sexual stages, the male or female gametocytes (gamete precursors) [1]. Mature gametocytes are arrested in G 0 of the cell cycle (cells in G 1 phase that have not yet committed to DNA replication) [2] in the vertebrate blood until they are taken up in the blood meal by a female mosquito, where they transform into gametes. Each male gametocyte undergoes exflagellation and produces up to eight male gametes, although this number is variable and could be as low as four [3,4] or two gametes [5]. By contrast, each female gametocyte produces only one female gamete [1,2]. Gametogenesis occurs within 10–15 min following uptake in the blood meal in response to the drop in temperature, pH and mosquito factors [1,2,6,7]. Within 30 min, the male gametes swim to find and fertilize the female gamete, then the male gamete motility decreases as the digestion of the blood meal creates a biochemically hostile environment. The subsequent zygote transforms into a mobile ookinete, which penetrates the stomach wall of the mosquito, and encysts. Eight to 15 days later (depending on the Plasmodium spp.), the mature oocyst releases several thousand sporozoites which invade the salivary glands of the mosquito and then injected into the vertebrate host during bloodfeeding. Gametocytes develop from proliferating asexual stage parasites within the vertebrate host. Although the majority of gametocytes emanate from the asexual blood stage parasites, direct production of gametocytes from exo-erythrocytic stages has been observed in some Plasmodium spp. [1] but not P. falciparum. Sexual stage differentiation from asexual blood-stage parasites has been the subject of considerable interest and many factors, notably those which slow or inhibit parasite asexual proliferation (e.g. immunological stress and chemotherapy), can induce gametocytogenesis [1,2,8]. However, the mechanism of gametocyte sex determination has been largely ignored. Plasmodium is haploid in the vertebrate host – there are no sex chromosomes [1] and a single clone of P. falciparum can produce males and females [4] (and can self-fertilize). The merozoites released from a single sexually committed schizont can become either all male or all female gametocytes [9]. There is considerable variation in the gametocyte sex ratio (the proportion of gametocytes that are male) among clones [10,11]. Classically, there are more female gametocytes than males, although there are very few studies where actual sex ratio counts have been performed [12]. Parasitologists have simply interpreted this female gametocyte numerical superiority with respect to the imbalance in numbers of gametes produced by a gametocyte of each sex. Each male gametocyte can potentially fertilize up to eight female gametocytes (each of which produces one gamete) because one male gametocyte can produce up to eight male gametes. Therefore, the female-biased gametocyte sex ratio is balanced by the greater number of male gametes per gametocyte, which results in an equal number of gametes from each sex. Thus, a clone could maximize the number of zygotes produced for a given investment in gametocytes. The flaw in this interpretation of Plasmodium female-biased sex ratio has, however, been recently and extensively highlighted by evolutionary biologists [12–14]: if there is heritable variation in sex ratio (sex ratio has a genetic basis and is variable), natural selection will result in an optimization of this phenotype [15]. Although maximizing the zygote number through female- biased sex allocation is optimal when infections are monoclonal, such sex allocation is not optimal when there are several co-infecting clones. Here, competition between clones will favour a more even sex ratio. Indeed, producing equal numbers of males and females is the optimal sex allocation strategy in both hermaphrodites (which must partition resources between male and female function) and dioecious species in randomly mating populations (i.e. an equal sex ratio is the norm) [15]. However, biased sex ratios are expected under certain conditions when mating is non-random in a population [15]. For example, when one clone In order to be transmitted by their mosquito vector, malaria parasites undergo sexual reproduction, which occurs between specialized male and female parasites (gametes) w ithin the blood meal in the mosquito. Nothing was know n about how Plasmodium determines the sex of its gametocytes (gamete precursors), which are produced in the vertebrate host. Recently, erythropoietin, the vertebrate hormone controlling erythropoiesis in response to anaemia, was implicated in Plasmodium sex determination in animal models of malaria. This review examines the available information and addresses the relevance of such a sex determining mechanism for Plasmodium falciparum transmission to mosquitoes, with special reference to low gametocytaemias. Plasmodium sex determination and transmission to mosquitoes Richard E.L. Paul, Paul T. Brey and Vincent Robert