Local mate competition, and extraordinary and ordinary blood parasite sex ratios

Blood parasites of the protozoan suborder Haemosporina include Plasmodium, Leucocytozoon, and Haemoproteus. Because life cycles of these parasites often lead to matings between individuals of the same genetic lineage (clones), selection for female-biased sex ratios should occur. Accordingly, sex ratios of populations of Plasmodium and Leucocytozoon closely matched expectations for probable rates of outbreeding. However, sex ratios of Haemoproteus populations were similar across a wide range of inferred outbreeding rates. Of several hypotheses we consider to explain this disparity, there are three we favour. The first is that the interaction among host immunity, parasite pathogenicity, and clone longevity differs among genera, so that on average more Haemoproteus clones coexist within hosts, thus reducing selection for female-biased sex ratios in that genus. The second hypothesis posits that hosts of the Haemoproteus populations we analysed had greater dispersal than did hosts of Plasmodium and Leucocytozoon. A consequence of this might be greater temporal and spatial variability in Haemoproteus outbreeding rates, so that Haemoproteus populations were prevented from achieving ideal sex ratios for the situation in which they found themselves. The third hypothesis is that Haemoproteus is vectored by relatively small midges so that biased parasite sex ratios in blood can result in too few gametes of one sex in a vector. Each hypothesis has its merits, and each could contribute to the patterns we observed. Our objective here is to stimulate further interest in the disparity.