We tackled objectives 1 and 2, a panel of Drosophila genotypes (isolines) that capture the genotypic variation of a population while facilitating the repeated measurement of the same genotype in a high number of individuals. We tested for the presence of genetic variation in sperm viability, i.e. the proportion of living sperm in the ejaculate, and its repeatability within the same genetic line (which sets the upper limit on the heritability of sperm viability). Furthermore, we mated tester females with genotypes from the extremes of the sperm viability range to assess how this trait affected female fitness. We discovered that while sperm viability is highly heritable and shows a high degree of variability between genotypes. In addition, we showed for the first time that high sperm viability increases female fitness by increasing the offspring numbers after single insemination (Tourmente et al., in revision). As noted above, there was also evidence of sexual conflict over sperm viability as genotypes producing more viable sperm caused females to waste more eggs despite generating higher female fitness overall.
To address objectives 3, 4 and 5 we performed a set of assays using males of 13 Drosophila isolines that were subjected to either, pre-meiotic sperm senescence (by aging the males), or post-meiotic sperm senescence (by manipulating the time since sperm depletion). In addition, some of the isolines were infected with Wolbachia which enabled testing of infection impacts on sperm viability. We discovered that genotype has a profound effect upon sperm both pre and post-meiotic sperm senescence. In other words, the genotype determines the direction and magnitude of the change in sperm viability with increased male aged and sperm storage time. Moreover, increased male age negatively impacts copulatory performance and male fertility, but results in higher quality daughters. Interestingly, however, neither sperm age nor parasite infection has an effect on these male reproductive traits. These findings are now in preparation for publication.
The knowledge gathered in the SPERMAGE project (the Fellow and his host group), reveal the main role of genotypic variance upon pre and post-meiotic sperm senescence. Since the mechanisms and phenomena studied are conserved across animals, this knowledge will inform further fundamental and applied research in human aging and reproductive health and animal breeding.
