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An unresolved question in sexual selection research is whether different secondary sexual traits are developmentally independent or instead whether their degree of expression is a manifestation of a general resource pool (i.e., condition) within the organism. If degree of expression of different sexual traits reflects ability to accumulate condition, then covariation should exist across genotypes in the expression of these traits, even if they are very different in kind. Here we present evidence for predicted covariation between morphological (sex comb size) and behavioral (courtship song) sexual traits among genetic lines of Drosophila bipectinata Duda extracted from a natural population. There is evidence that both these traits in Drosophila are condition dependent and subject to sexual selection. We detected significant body size–independent differences in comb size among 32 lines. Replicate lines exhibiting relatively high and low values of comb size were then subjected to analyses of courtship song. High sex comb lines exhibited shorter mean burst period and shorter mean burst duration than low sex comb lines. These song differences occurred only during the distant pursuit phase of male courtship and existed despite factoring out individual variations in sex comb size, the trait on the basis of which test lines were originally chosen. The results verify the prediction of an association between condition-dependent secondary sexual traits across genotypes and, therefore, support the existence of an overall genetic quality related to condition acquisition. [ABSTRACT FROM AUTHOR]

The evolution of male secondary sexual traits traditionally has been ascribed to precopulatory sexual selection. In contrast, the importance of postcopulatory sexual selection for the evolution of secondary sexual traits is uncertain, and what little evidence exists for this process to contribute to the evolution of such traits is mixed. Here we test the hypothesis in Drosophila bipectinata Duda that the male sex comb, a rapidly evolving secondary sexual trait, is under positive postcopulatory sexual selection. We extracted replicate genetic lines exhibiting relatively large and small sex comb size from a natural population. Males from these lines were subjected to an assay of competitive fertilization ability, measured as P2, the proportion of a females clutch of eggs fertilized by the second male to mate. Males with the largest sex combs sired more offspring than less ornamented individuals, demonstrating for the first time in any Drosophila species that postcopulatory sexual selection favors increasing sex comb size. This study identifies a postcopulatory selective mechanism that may be contributing to the evolutionary diversification of a secondary sexual trait. [ABSTRACT FROM AUTHOR]

Research on the role of parasites in sexual selection has focused mainly on host mate choice favoring relatively unparasitized males. But parasites can also generate variance in host reproductive success by influencing the ability of individual hosts to directly compete among themselves for mates or fertilizations, a subject area that has received far less attention. We demonstrate experimentally that parasitism by mites can drive sexual selection by way of a novel mechanism involving male competition: physical inhibition of host copulation. Mite resistance in natural populations is heritable, emphasizing the evolutionary potential of parasite-mediated sexual selection in this system and indicating that females should be receiving indirect fitness benefits as a result of this process. We show that parasitism by mites, Macrocheles subbadius, reduces mating success of male Drosophila nigrospiracula. Smaller males were more strongly compromised, identifying host body size as a tolerance trait. As parasite load increased, the rate at which males attempted to copulate but failed because of obstruction by mites increased. When mites were removed from infested males, host mating success was restored. Thus, the physical presence of the mites per se generates differential mating success, in this case by interrupting the normal flow of mating behaviors. This study elucidates a potent mechanism of parasite-mediated sexual selection in a system wherein parasite resistance is demonstrably heritable, and as such expands our understanding of the evolutionary potential of sexual selection. [ABSTRACT FROM AUTHOR]