Genotype-by-environment interactions for seminal fluid expression and sperm competitive ability

Abstract

AbstractSperm competition commonly occurs whenever females mate multiply, leading to variation in male paternity success. This can be due to variation in the various traits that might affect sperm competitive ability, which itself depends on both genetic and environmental factors, as well as on genotype-by-environment interactions (GEI). Seminal fluid is a major component of the male ejaculate that is often expected to mediate sperm competition, where different genotypes can differ in their seminal fluid expression as a response to different level of sperm competition (i.e., exhibit GEI). We therefore here focussed on testing for GEI in expression of two recently identified seminal fluid transcripts,suckless-1andsuckless-2, which potentially modulate sperm competitive ability in the simultaneously hermaphroditic flatwormMacrostomum lignanovia their effects on manipulating post-mating partner behaviour and ultimately the fate of transferred ejaculates. In addition, we sought to test for GEI in sperm competitive ability, to investigate the relationship between natural variation in the expression of these seminal fluid transcripts generated through GEI and relative paternity success. To do so, we manipulated social group size, which has been shown to successfully alter sperm competition level inM. lignano, in a set of inbred lines (genotypes) and then measured both the expression level ofsuckless-1andsuckless-2in focal worms together with their relative paternity success in a standardised sperm competition (P1&P2) assay. We found GEI for the expression level ofsuckless-1andsuckless-2, as well as for sperm competitive ability. Moreover, we found a positive relation between the expression ofsuckless-1and paternity success. This suggests that natural variation in the expression of this seminal fluid transcript indeed can influence sperm competition outcomes inM. lignano.