Female mate choice and male ornamentation in the stalk-eyed fly, Diasemopsis meigenii

Female mate choice is a crucial driver of sexual selection, leading to the evolution of the diverse male sexual ornaments seen in nature. Yet little is known about the factors that cause variation in different components of female preference, or how these components interact to influence the series of choices that exert selection on male ornaments. Likewise, it is not known how the signals of genetic condition revealed by male ornaments, or reproductive capacity, vary across environments. But this variation is important for both the direct and indirect genetic benefits of female mate choice, as well as for the male-driven effects of reproductive investment on ejaculate allocation and sexual selection. Here, I use the sexually dimorphic African stalk-eyed fly species Diasemopsis meigenii to conduct empirical studies to address these issues. First, I manipulate female mating status (virgin versus mated) and use mathematical and statistical techniques to decompose mate choices made in a repeated sequential no-choice design into estimates of preference, and selection on the male ornament. I show that choosiness and selection, but not the preference function, are elevated in mated females. Second, I use larval diet manipulation and a series of crosses within and between a suite of inbred lines to investigate the across-environmental genetic condition dependence of male sexual ornaments relative to nonsexual traits. I find evidence for the heightened condition dependence of the male sexual trait (male eyespan), and for a novel gene-by-environment interaction in which the effects of genetic stress on sexual trait expression are masked in both high and low but not intermediate environments. Finally, I measure the effects of environmental (E) and genetic (G) stress on reproductive, fertility and attractiveness traits, and find evidence for a qualitative alignment of trait responses (all tend to be positive), but a negative integration across traits (traits that respond most to E respond least to G). The results have important implications for the operation of sexual selection in nature.