Sex in the Apicomplexa
Sexual reproduction is a common phenomenon in natural populations; all apicomplexan have a sexual stage in the course of their life cycle [
127]. The advantages conferred by sex have intrigued evolutionary biologists for many years. The various theories [
128‐
132] fall into one of two types of models: the environmental model, which suggests that sex has evolved to allow adaptation to a changing environment by favouring recombination, and the mutation-based models, which assume that sex is advantageous to suppress deleterious mutations more rapidly. However, the explanation might be in a pluralistic approach [
133].
Aside from the nucleus,
P. falciparum has two organelles containing DNA: the mitochondria (linear, 8 kb) and the apicoplast (circular, 35 kb). The latter seems to be evolutionarily related to a structure present in red algae and chloroplast of plants. This plastid is essential to the parasite, but its comprehensive role is still uncertain [
134]. The exclusive maternal inheritance of this structure suggests the existence of a cytoplasmic incompatibility between male and female gametes, as previously demonstrated in plants [
135]. Such incompatibility had been strongly suspected in outcrossing experiments and could be linked to reproductive isolation of the parasite from different geographical areas [
136]. In the same way, research into evolutionary maintenance of sex can benefit from the large amount of work already done in plants and algae, of which the sex life seems quite related or may even be homologous to that of
P. falciparum.
Apicomplexan parasites supposedly benefit from genetic recombination following sexual achievement during meiosis. Recombination has been shown to occur in
P. falciparum [
137‐
140],
Plasmodium vivax [
141],
P. chabaudi [
105], and
Toxoplasma gondii [
142].
Mixed infections
Where there is recombination, there has to be at least two genetically distinct parasites; the interaction of several populations of different genotypic types remains elusive. The adaptive sex ratio function of complexity of the parasite population in the long term has already been discussed (see Sex allocation theory). It was postulated that the parasite could also have a fast adaptive system responding to the complexity of the infection [
143]. This was borne out by the fact that multiple-genotype infections are often more successful in terms of transmission than single-genotype infections [
105]. This might not reflect co-operation between conspecific clones, but it could be that competition between parasites increases the chances of transmission by creating an advantageous effect on the competiting clones. This may be confirmed by the work of Arez and colleagues [
144], who observed a lower proportion of mixed-genotype infections in the mosquito than in the human host. This may, however, only be due to the fact that mixed-infections are not necessarily synchronous, which lowers the chances of recombination. The dynamics of genotype interaction needs to be further characterized, to reveal how it might affect both the disease and the transmission.
Commitment to gametocytogenesis has also been shown to be altered when another species of human malaria parasite is present. For instance, the presence of
Plasmodium malariae may boost the gametocyte production of
P. falciparum [
145].
Why so few gametocytes?
Not only are gametocytes the first step to sexual genetic recombination, but they also are the transmission stage. A great selection pressure is, therefore, upon them. The transmission phenotypes observed in natural infections must be ones that optimize the greater chances of a successful shift of host. A positive correlation has been found between gametocyte density in the blood and infectiousness to mosquitoes [
94,
146‐
148]. However, this correlation must be qualified as loose and may be hampered by the low sensitivity of microscopy [
149] and by transmission-blocking immunity [
19].
Plasmodium and closely related apicomplexan have evolved an asexual erythrocytic cycle. This character is thought to have evolved several times in the Apicomplexa [
127]; an acceptable explanation is that erythrocytic proliferation allows them to generate more transmission stages than with tissue merogony alone. Furthermore, Dyer and Day [
79] proposed that indefinitive rounds of asexual proliferation would augment the length of time of circulation of gametocytes and therefore increase the chances for successful transfer to the mosquito. One cannot forget, however, that various related parasites, such as
Leucocytozoa and
Haemoproteus, release only sexual stages in the blood and are very successfully transmitted [
150].
More intriguing is the fact that very few of the erythrocytic stages actually commit to a differentiation to transmission stages in the blood [reviewed in [
151]]. It is paradoxical that, having the possibility to generate more gametocytes through asexual proliferation, the parasite actually makes less, especially since a higher density would increase transmission [
152]. Several hypotheses have been proposed to explain this restraint.
Taylor and Read [
151] have postulated that immune pressure through transmission-blocking activity could account for the low density of gametocytes. Transmission-blocking immunity has been demonstrated for several surface antigens of
P. falciparum. If this anti-gamete immunity is dependent on the density of gametocytes, it is obviously advantageous to reduce the number of gametocytes. Alternatively, it could be that the level of this immunity was dependent on the ratio of asexual: sexual parasites, the asexual parasites acting as a decoy for the immune system. Piper and colleagues [
63] proposed a role for a cross-stage immunity against PfEMP-1. An increase of anti-PfEMP-1 immunity with age was observed, which was accompanied by a decline in prevalence and density of asexual and sexual blood stages of
P. falciparum. PfEMP-1 being expressed on the surface of both asexual stages and young gametocytes (see Sequestration of gametocytes), regulation of the level of gametocytes could have evolved either by controlling asexual proliferation (the source of gametocytes) or directly by affecting maturing gametocytes.
It has also been proposed that frequencies of super-infection, modulated by cross-immunity between the phenotypes involved, may serve to constrain gametocyte production to levels less than would be optimal for transmission in a single-phenotype context [
153].
Another possible explanation is that the gametocyte number is kept low in order to reduce the damage mosquito stages are likely to inflict on their host [
151]. Evidence that natural selection drives the parasite towards lessening the number of mosquito stages in a mosquito to favour survival of the host and, therefore, successful completion of the life cycle, has only recently come to light. The ookinete, which is responsible for the penetration of the stomach wall, is the most harmful stage to the mosquito; this stage has been shown to be capable of apoptosis (programmed cell death) [
154], which results in a reduction in the number of ookinetes and, therefore, lessens the damage due to penetration and eventual opportunistic bacterial infection.
It has been demonstrated that the parasite has evolved several strategies to maximize transmission in spite of the low number of gametocytes: (a) aggregation of gametocytes to favour encounter of males and females in the bloodmeal [
155] (b) preferential localization of infectious rodent-malaria gametocytes in sub-dermal capillaries [
156] and possible sequestration of mature gametocytes in the derma [
157] and (c) active suppression of insect melanization by ookinetes [
158].
Malaria parasites may have evolved restraint in the production of transmission stages in order to avoid harmful consequences either directly for the parasite through the immune system of the definitive host or to spare the vector in order to maximize its success in transmitting the parasite. This restraint provides an evolutionary justification for the development of autocrine/exocrine mechanisms for means of communication. One could imagine that asexual parasites express a highly labile diffusible molecule [
108] that prevents other asexual parasites from undergoing gametocytogenesis or encourages them to pursue asexual proliferation. When a high level of "stress" (immunity) arises, the parasite population would respond by restricting the expression of the diffusible molecule and, therefore, would stimulate a higher level of asexual parasites to develop sexually.