Male genital spines harm females as a pleiotropic side effect of promoting mating success

What is it about?

Previously, we demonstrated that the male genital spines of the fruit fly D. ananassae function prior to copulation to promote male mating success by grasping onto the female genitalia in the context of male-male competition for mates. That study did not investigate the potential for those genital spines to function in postcopulatory sexual selection (e.g. sperm competition or cryptic female choice). Here we investigated just that. We found no evidence for surgical reduction of male genital spines reducing any measure/component of male or female postcopulatory fitness. Instead, females that were mated by cut males actually laid more eggs than those mated by control males, suggesting that the natural form of the trait (which comes to a sharp point) is harmful to females, and that by cutting (and also dulling) males' spines we enabled females to lay more eggs. This result was consistent across experiments.

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Why is it important?

First, this study solidifies the notion that the genital spines of D. ananassae do not function during or after copulation (i.e. do not function in postcopulatory sexual selection)--they only function prior to copulation to promote competitive mating success. Second, as a pleiotropic side effect of evolving to promote male mating success, the spines actually consequently harm female fecundity. The harm itself is not adaptive, because it does not cause females to do anything that would promote the fitness of the male that harmed her relative to other previous or prospective males. This is congruent with findings in other insects (e.g. C. maculatus), in that regardless of whether the trait functions before, during or after copulation to promote male mating success or sperm competition, male genital traits can evolve to become consequently harmful to females, providing selection pressure on females to counter adapt to male harm as part of a coevolutionary "arms race" between males and females known as sexually antagonistic coevolution.