Abstracts (first author)
Phenotypic engineering of testicular and ovarian function reveals a sex allocation trade-off in a simultaneously hermaphroditic flatworm
A resource allocation trade-off between investment towards the male and the female function is a fundamental assumption of sex allocation theory for simultaneous hermaphrodites, but there is currently limited empirical support for it, especially among animals. Here we experimentally manipulated testicular and ovarian function in the free-living flatworm, Macrostomum lignano, and subsequently tested for the occurrence of the sex allocation trade-off. Specifically, we used RNA interference—by soaking worms in a solution of double-stranded RNA—to knock down the expression of specific candidate genes, which we identified in two ways. First, we screened 11 gonad-specific candidate genes obtained from a published microarray study in the distantly related planarian flatworm, Schmidtea mediterranea. Seven of these candidate genes had testis-specific expression in M. lignano, and the knockdown of five of these had evident effects on testicular function. The most prevalent effect was a disruption of sperm production, which for one candidate gene was accompanied by an increase in ovary size. Second, using an RNA-Seq screen in M. lignano we identified a candidate gene with ovary-specific expression, knockdown of which prevented the formation of yolk in developing oocytes and also resulted in larger testes (and a trend for a higher sperm production rate). Both of our approaches thus yielded candidate genes whose knockdown uncovered sex allocation trade-offs (notably via manipulation of both the male and the female function). This, to our knowledge, is the first study to use phenotypic engineering of testicular and ovarian function to empirically test the trade-off assumption of sex allocation theory. Furthermore, our results suggest that disrupting the function of one gonad does not necessarily lead to a trade-off, possibly because some types of disruption do not liberate resources that become available to the opposite function.