Evolution of alternative mating tactics and sex ratios in the Southern African Otiteselline fig wasps

Abstract

This thesis considers the evolution of alternative reproductive tactics and sex allocation in the non-pollinating Otitesella fig wasp genus. Fig wasps provide an excellent model system for testing hypotheses of how selection could act on the above-mentioned traits as it is relatively easy to obtain accurate data for both sex and morph ratios. The male dimorphisms exhibited by many fig wasps are often seen as a genetic polymorphism. If this is the case, morph ratios in one generation depend on the mating success of the alternative morphs in previous generations. The match between the alternative morph proportions of Otitesella pseudoserrata and the proportion of females each morph can mate across generations however is shown to be too tight to be produced by a genetic polymorphism mechanism. Rather, morph ratio data from three Otitesella species are in strong quantitative agreement with a model that allows ovipositing females to determine both the sex and male morph of their offspring, based on an assessment of arrival order at a fig and the average mating population densities. Early arrivals were predicted to lay females or dispersing morphs and later arrivals females or a non-dispersing morph. Given that they lay one egg per fig, it can be seen that fitness pay-offs to non-dispersing males increases when more females utilize a fig as the number of mating opportunities within that fig increases. Statistical analysis reveals that the non-dispersing males of a number of Otitesella species are themselves dimorphic. The smaller morph, a result of a condition-dependent decision, is shown to have a fitness advantage in the specific case where there are two larger males in the same fig. Hence the lengthy fights between the larger males give the smaller male an opportunity to sneak matings in the resulting confusion. A comparative analysis demonstrates that the specific body size at which males switch tactics is set by frequency dependent selection. An interesting result of the maternal control model was the prediction of female biased sex ratios in certain situations. Further theoretical analysis reveals that when females have knowledge of their arrival orders, they are able to take advantage of an asymmetry in mating opportunities that arises because dispersing males are also able to mate within their natal figs. As they only laid one egg per fig, local mate competition does not occur and as such the mechanism, termed “mating opportunity distribution” (MOD), is a new addition to sex ratio theory. Sex ratio data from O. pseudoserrata conform to MOD predictions.