Online

Neal, Allison T.

Biology

2014

PhD

Sex ratio theory is a focus in evolutionary biology that explores how natural selection shapes investment in males and females. It has provided some of the best quantitative evidence of evolution and could find utility in public health efforts through its application to malaria parasites. These parasites have distinct male and female forms that are produced following massive asexual replication, and they mate within the blood-feeding insects that transmit them between vertebrate hosts. A very similar population structure is assumed by local mate competition (LMC), a model from sex ratio theory that predicts female-biased sex ratios dependent on the degree of selfing within a mating patch. In this dissertation, I test a series of predictions from LMC for the lizard malaria parasite Plasmodium mexicanum. These include: (i) sex ratios have heritable variation that is not constrained by other life history traits; (ii) single-genotype infections have female-biased sex ratios that are determined by male fecundity; (iii) multiple-genotype infections have less biased sex ratios than single genotype infections; (iv) if males are limiting, sex ratios may be less biased when there are fewer parasites present (an extension of LMC called fertility insurance); and (v) less biased sex ratios may also be favored if increased female production yields diminishing returns on transmission to a new vertebrate host. To test these predictions, I combined the study of natural and experimental infections, microscopy (parasite density and sex ratio), molecular genetics (infection genetic diversity), and mathematical modeling (of how transmission patterns might affect sex ratio evolution). Overall, the results were qualitatively consistent with both LMC and my new model predictions. Sex ratios showed evidence of heritable variation that was unlinked to other life history traits measured. Sex ratios in single-genotype infections were female biased and consistent with the male fecundity observed, and were lower than sex ratios in experimental multiple-genotype infections, as predicted. Sex ratios were not less biased with lower sexual cell density, suggesting that males were not limiting. In fact, the opposite trend was sometimes observed: sex ratios were less biased with more sexual cells. This pattern has been observed previously in this and other species, and the only model that currently predicts such a trend is the new transmission model I outline. This dissertation contributes to our understanding of sex ratio evolution for malaria parasites in a number of ways. First, it adds evidence to the idea that the selective forces implicated in LMC are at work in malaria parasites and that malaria parasites are able to detect and respond to relevant cues. Second, it helps account for discrepancies in existing data, which have often reached conflicting conclusions. Third, it offers one of the first detailed studies of malaria parasite male fecundity, an essential piece of the sex ratio puzzle. Finally, it outlines a new theoretical extension of LMC that provides novel predictions and highlights areas of study that may be fruitful for future work on malaria parasites and other organisms.