Abstract
Sperm viability is a major male fitness component, with higher sperm viability associated with enhanced sperm competitiveness. While many studies have focussed on sperm viability from the male fitness standpoint, its impact on female fitness is less clear. Here we used a panel of 32 isogenic Drosophila simulans lines to test for genetic variation in sperm viability (percentage of viable cells). We then tested whether sperm viability affected female fitness by mating females to males from low or high sperm viability genotypes. We found significant variation in sperm viability among genotypes, and consistent with this, sperm viability was highly repeatable within genotypes. Additionally, females mated to high sperm viability males laid more eggs in the first seven hours after mating, and produced more offspring in total. However, the early increase in oviposition did not result in more offspring in the 8 hours following mating, suggesting that mating with high sperm-viability genotypes leads to egg wastage for females shortly after copulation. Although mating with high sperm-viability males resulted in higher female fitness in the long term, high quality ejaculates would result in a short-term female fitness penalty, or at least lower realised fitness, potentially generating sexual conflict over optimal sperm viability.
Highlights
Sperm viability is a major male fitness component, with higher sperm viability associated with enhanced sperm competitiveness
In order to capture a significant portion of the genetic variation of a natural population of D. simulans, gravid females collected from Greece in 2010 were used to establish 32 isolines via fullsib mating
Sperm viability varied significantly among D. simulans genotypes, and its repeatability was consistent with the heritability of sperm competitiveness previously reported in this taxon[59]
Summary
Sperm viability is a major male fitness component, with higher sperm viability associated with enhanced sperm competitiveness. The number of eggs a female Drosophila melanogaster can produce over a lifetime and the number of sperm stored after a single mating are of similar magnitude[34,35], variation in fertilization efficiency could mean that more than one sperm is required per egg fertilized[36] This suggests that, with limited mating opportunities, low sperm viability might result in unfulfilled reproductive potential for females. We used a panel of D. simulans genotypes to test for the presence of genetic variation in sperm viability, and to test how ejaculate quality (measured as sperm viability) affected female fitness by mating females to genotypes that had low- or high-viability sperm
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