Seminal Fluid-Mediated Manipulation of Post-mating Behavior in a Simultaneous Hermaphrodite

Seminal fluid proteins (SFPs) are uniquely positioned to mediate post-mating sexual selection and sexual conflict [1-3]. This role may be especially important in simultaneous hermaphrodites, in which individuals will often agree to receive sperm in order to be able to donate it, shifting the arena of sexual selection to post-mating reproductive interactions [4-7]. Nevertheless, as in separate-sexed organisms, identifying individual SFPs responsible for specific post-mating effects is difficult, owing to the complexity, rapid evolution, and functional redundancy of seminal fluid [8-11]. Here, we sought to identify SFPs that influence one striking post-mating behavior of the simultaneously hermaphroditic flatworm Macrostomum lignano, the so-called "suck behavior," in which worms respond to ejaculate receipt by placing their pharynx over their female genital opening and seemingly attempt to remove sperm and/or other ejaculate components [12-14]. We hypothesized that sucking is counter to the sperm donor's interests, potentially selecting for SFPs that reduce the suck propensity of mating partners. We tested this using a combination of quantitative genetics and RNA interference (RNAi) knockdown. As predicted, we found negative genetic correlations between the expression levels of six (out of 58) seminal fluid transcripts and partner suck propensity. RNAi knockdown confirmed that two of these transcripts, designated suckless-1 and suckless-2, indeed caused mating partners to suck less often. We suggest that these proteins are male counter-adaptations to recipient suck behavior, which itself is likely a female counter-adaptation in the ongoing evolutionary conflict to (re)gain control over ejaculate fate after mating in this hermaphroditic organism.