Different sexual traits show covariation among genotypes: implications for sexual selection

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.