SEX ALLOCATION AND POPULATION-STRUCTURE IN MALARIA AND RELATED PARASITIC PROTOZOA

Here we demonstrate how sex allocation theory, one of the best verifie d areas of metazoan evolutionary biology, can be successfully applied to microparasitic organisms, by relating parasite prevalence and sex r atio in the Haemosporina. Members of this taxon, which includes Plasmo dium, are parasitic protozoa with obligate sexual cycles in which dioe cious haploid gametes drawn from the peripheral blood of a vertebrate host fuse within a dipteran vector. Consequently mating takes place wi thin a highly subdivided population, a condition known to promote loca l mate competition and inbreeding and hence the evolution of female-bi ased sex ratios. We used an epidemiological framework to investigate m ating patterns and sex ratio evolution within natural populations of t hese parasites. This phenotypic approach compliments more conventional biochemical approaches to the population genetics of parasitic protoz oa. Data are presented which support a theoretical relation between tr ansmission-stage sex ratio and prevalence across parasite populations. These results are consistent with a large inter-population variation in genetic structure and argue against sweeping generalizations about the clonality or otherwise of populations of these parasitic protozoa.