House fly proto-Y chromosomes differentially affect courtship success regardless of social context and female type

Genetic variation exists for many male reproductive traits despite apparent strong sexual selection in favor of particular males. Understanding how that variation is maintained is an unresolved problem in evolutionary biology. One proposed solution to this problem is genotype-by-environment interactions (GEIs), where no male genotype is universally superior across environments. Another proposed answer is variation in female preference, such that different females prefer different male genotypes. Additionally, male genotypes may differ in their reproductive output as they age. The house fly Musca domestica is a well-suited system to evaluate these explanations for genetic variation in male reproductive traits because two male genotypes (YM and IIIM) differ in mating performance. Specifically, IIIM males outcompete YM males in mating assays because IIIM males have a shorter copulation latency. To determine the relative importance of GEIs, female variation, and male age on the maintenance of genetic variation in male reproductive behaviors, we performed non-competitive (one male to one female) and competitive (one IIIM male and one YM male to one female) mating assays while considering multiple ages and different female strains. IIIM males attempted more matings regardless of competition level, female genotype or age, but mated faster than YM males only with females of their own strain and only performed more matings in non-competitive trials. However, female strain did not improve the mating success of YM males. We also did not observe evidence for genotype-by-age interactions on male mating success. Importantly, we did not identify any conditions where YM males outcompeted IIIM males, and therefore there was no evidence for interactions or trade-offs that could explain the maintenance of genetic variation for male mating success. To explain the maintenance of IIIM and YM males, other factors such as additional social environments or varying temperature should be investigated, as the genotypes are found in natural environments with contrasting temperatures.