Indirect parental effects on offspring fitness by egg-derived fluids in an external fertiliser

The capacity for parents to influence offspring phenotypes via nongenetic inheritance is currently a major area of focus in evolutionary biology. Intriguing recent evidence suggests that sexual interactions among males and females, both before and during mating, are important mediators of such effects. Sexual interactions typically extend beyond gamete release, involving both sperm and eggs, and their associated fluids. However, the potential for gamete-level interactions to induce transgenerational parental effects remains under-investigated. Here, we test for such effects using an emerging model system for studying gamete interactions, the external fertiliser Mytilus galloprovincialis. We employed a split-ejaculate design to test whether exposing sperm to egg-derived chemicals (ECs) from one female would affect fertilisation rate and offspring survival when those sperm were used to fertilise a different female's eggs. We found significant and separate effects of ECs from non-fertilising females on both fertilisation rate and offspring survival. The offspring survival effect indicates that EC-driven interactions can have transgenerational implications for offspring fitness independent of the genotypes inherited by those offspring. These findings provide a rare test of indirect parental effects driven exclusively by gamete-level interactions, and to our knowledge the first evidence that such effects occur via the gametic fluids of females.