Analysis of correlated responses in key ejaculatory traits to artificial selection on a diversifying secondary sexual trait.

Positive genetic covariance between male sexual display traits and fertilizing capacity can arise through different mechanisms and has important implications for sexual trait evolution. Evidence for such genetic covariance is rare, and when it has been found, specific physiological traits underlying variation in fertilization success linked to trait expression have not been identified. A previous study of correlated responses to bidirectional artificial selection on the male sex comb, a secondary sexual trait, in Drosophila bipectinata Duda documented a positive genetic correlation between sexual trait size and competitive fertilization success, and found that transcript levels of multiple seminal fluid proteins (SFPs) were significantly increased in the large sex comb (high) genetic lines. These results suggest that changes in SFP activity may be a causal factor underlying the increased fertilizing capacity of high line males. Here, we tested for correlated responses to this selection in a suite of additional reproductive traits, measured in the context of variation in male age and exposure to rivals. Whereas several traits including sperm length, number and viability, and accessory gland size, increased with age, only sperm viability was influenced by selection treatment, but in complex fashion. Sperm viability of high line males surpassed that of their smaller-combed counterparts when they had been housed with rivals and were 5-6 days old or older. Interestingly, this interaction effect was evident for sperm sampled from the female seminal receptacle, but not from the male seminal vesicles (where sperm have yet to be combined with accessory gland products), consistent with the differential SFP activity between the lines previously found. Our results suggest that differences in sperm quality (as viability) may be a contributing factor to the positive genetic correlation between sexual trait size and competitive fertilization capacity in D. bipectinata.