Figure 1.
Effects of immunity on gametogenesis and fertility of malaria parasites.
The effects of transmission-blocking immune factors on the sexual development of malaria parasites investigated in our model. Female and male gametocytes circulating in the host (white background) undergo gametogenesis when taken up by a mosquito vector (blue background). Each male gametocyte differentiates into χ gametes (χ≤8) and each female gametocyte produces one gamete. Male gametes locate and fertilise female gametes, and the resulting zygotes develop into ookinetes. Immune factors circulating in the host can act on males and females throughout their sexual development, from gametocytes to zygotes. The developmental stages of females are shown above the stages of males and each individual gametocyte/gamete is shown in the same relative position throughout development. The effects of immune factors (lighting) on sexual stages can either be cryptic (i.e. render gametocytes/gametes dysfunctional; green), or fatal (i.e. gametocytes/gametes die; black). Healthy, unaffected, parasites are represented in yellow, dysfunctional parasites in green, and dead parasites in black. Immune factors kill female gametocytes with probability dF and male gametocytes or gametes with probabilities dM or δM, respectively. Dead sexual stages do not participate further in the mating pool. Immune factors render female gametocytes and gametes dysfunctional with probabilities ΩF and ϖF respectively, and male gametocytes and gametes with probabilities ΩM and ϖM, respectively. Dysfunctional gametocytes/gametes participate in the mating pool and can be fertilized as for healthy gametes, however zygotes are unviable and die before reaching the ookinete stage. Immune factors can also directly lead to zygote death with probability ΩZ. All possible fertilization scenarios are represented: mating between two healthy gametes, mating between one healthy and one dysfunctional gamete and mating between two dysfunctional gametes.
Figure 2.
Ability of gametocytes to undergo gametogenesis after exposure to RNS and TNF-α.
Mean (± S.E.) proportion (n = 20) of emerged female gametes (A), emerged male gametocytes (B), and exflagellating male gametes (C), relative to the total number of male or female gametocytes/gametes observed, when gametocytes are exposed to immune factors during incubation in ‘host conditions’ and then activated in un-manipulated ‘vector conditions’ media.
Figure 3.
Exflagellation rates and ookinete production after exposure to RNS and TNF-α during gametogenesis.
Mean (± S.E.) proportion of exflagellating male gametes (A; n = 16) or ookinetes (B; n = 9) produced when parasites are exposed to RNS and TNF-α during gametogenesis (in-vector conditions media). Proportions are relative to the total number of exflagellating male gametes or ookinetes produced from each infection, across treatments.
Figure 4.
Ookinete production after exposure of males or females to RNS, before or during gametogenesis.
Mean (± S.E.) proportion (n = 19) of ookinetes produced, when females (A) or males (B) are exposed to RNS as gametocytes (in-host conditions media) or during gametogenesis (in-vector conditions media). Proportions are relative to the total number of ookinetes produced by the focal sex from each pair of infections.
Figure 5.
Evolutionarily stable sex allocation strategies when sex- and stage-specific mortality rates vary.
Effect of male and female gametocyte mortality and male gamete mortality on the ES gametocyte sex ratio (z*), for a clonal population, when the number of gametes produced per male gametocyte (χ) is 2 (this fecundity has been estimated for this system by other studies; see ref. [19]). Figures S1, S2, S3 show similar patterns to Figure 5A for χ = 1; 4; 8, respectively. (A) For each plot within the panel, z* varies with male gamete mortality rate (δM). The coloured lines represent different gametocyte group sizes (q): 2 (grey), 5 (blue), 10 (red), 20 (green) and ∞ (yellow). Each plot depicts different parameter combinations of male gametocyte (dM = 0.1; 0.5; 0.9) and female mortality rate (dF = 0.1; 0.5; 0.9), with dM increasing left to right and dF increasing bottom to top. (B) Cartoon summarizing the effects observed in Figures 5A and S1, S2, S3. The set of possible values for z* is strongly influenced by q. The number of gametes of each sex reaching the mating pool (which depends on the mortality parameters and on χ) influences z* within the constraints determined by q. Within each plot, the effects of δM and q on z* can be clearly observed: the magnitude of sex ratio change increases with q and z* increases to compensate for higher δM. The effects of dM and dF can be observed by comparing the points where the lines cross the y axes (i.e. δM = 0) across the plots: z* increases along rows with increasing dM and decreases up the columns with increasing dF. The effect of χ on z* can be observed by comparing plots that are in the same position in different figures: sex ratio becomes more female biased as χ increases.