Artificial selection and heritability of sperm length in Gryllus bimaculatus

The adaptive significance of variation in sperm size remains poorly understood but there has been even less attention focused on the genetic mechanisms controlling spermatozoal traits (only three species have been studied). Here we explore heritability and artificial selection of sperm length in the cricket Gryllus bimaculatus. Using a within-family selection design we selected sperm with increasing and decreasing sperm size and bred divergence across five generations. Although we recorded no significant parent-offspring heritability, realized heritability from a within-family analysis was significant at 0.52 +/- 0.06 SE (P = 0.01) and we successfully achieved significant divergence. However, we only achieved a response when the maternal line was incorporated into selection (from the F1 onwards) and our findings therefore suggest that sperm length in G. bimaculatus is a sex-linked trait that is influenced by genes which are active on the female chromosome. Accordingly, sperm length heritability in G. bimaculatus can only be measured using a within-family design because the spermatozoal phenotype is sex-limited (to males) while the genotype is sex-linked (to females). The evolutionary significance of the heritability of sperm length is discussed with reference to sex-linkage of this important sex-limited trait.