Genetic variance and genotype reaction norms in response to larval food manipulation for a trait important in scorpionfly sperm competition

1. Sperm competition is an important attribute of many mating systems. Examining the genetic and environmental factors influencing male sperm competition success is essential in order to understand variation in reproductive success. 2. In the scorpionfly Panorpa cognata , male success in sperm competition is influenced by the number of sperm transferred during copulation. This will be determined by copulation duration and the sperm transfer rate of males. Sperm transfer rate is a trait which shows considerable phenotypic variance. 3. Here, I use a full-sib split-brood design in order to investigate both to what extent this trait is heritable and the influence of larval food availability on male sperm transfer rate. 4. The results demonstrate considerable genetic variance underlying the phenotypic expression of sperm transfer rate. Heritability estimates were slightly larger, but not significantly so, for offspring reared at low food availability. 5. In contrast, there was no straightforward evidence that larval food availability had an effect on the sperm transfer rate of males. However, a significant family × treatment interaction provided evidence of a genotype × environment effect on male sperm competitive ability. These results demonstrate different reaction norms for sperm transfer rate in response to larval treatment for individuals with different genetic background.