Digest: The evolution of sex‐specific mating preferences in <i>Drosophila</i> *

Do mating preferences evolve at the same rate in males and females? Yukilevich and Peterson (2019) found that among Drosophila species, male and female mate preferences evolved at different rates via distinct mechanisms. Rates of evolution were also affected by geographical proximity of species. In allopatric species, male and female mate preferences evolved at similar rates, while in sympatric species, female mate preferences developed more rapidly than that of males. Two important phenomena influencing speciation are the emergence of behavioral and anatomical phenotypes that prevent incipient populations from mating to produce viable, fertile offspring. For example, incompatible genital morphology can act as aversive stimuli, or even make mating impossible (Isaacson 2017; Masly 2012), whereas behavioral factors such as differing sexual cues and mating preferences often make heterospecifics appear less attractive than members of the same species. Traditionally, because females are the “choosier” sex, researchers assumed divergence in females’ mating preferences was the sole driver of sexual isolation (Kirkpatrick 1987). However, a growing body of work argues that male mating preferences also play a role in the sexual isolation of species as diverse as garter snakes (Thamnophis spp.; Shine et al. 2004), mosquito fish (Gambusia spp; Espinedo et al. 2010), and leaf beetles (Chrysochus spp; Peterson et al. 2007). While both male and female mate choice play a role in sexual isolation of species, the extent of each sex's contributions and the relative rates of evolution of mating preferences are less understood, particularly in the contexts of sympatry and allopatry. In this issue, Yukilevich and Peterson (2019) used pairs of sixteen species groups within the subgenera of Drosophila and Sophophora to identify sex-specific rates of divergence in mating preferences and sex-specific contributions to sexual isolation. Yukilevich and Peterson (2019) aggregated their experimentally obtained male courtship and female copulation data with data from prior literature to analyze 66 and 34 species pairs, respectively. They found that males generally tended to be less particular in choosing females for courtship, whereas females were more stringent in copulation preferences and favored copulation with conspecifics. Upon geographical analysis, this sex difference was found to be far more prominent in sympatric species pairs compared to allopatric pairs. Relatedly, females were found to be responsible for only 25% of species-specific sexual isolation in allopatry, while in instances of sympatry, female contribution to sexual isolation skyrocketed to 75%. These results support the conclusion that particularly in sympatric circumstances, females’ mate preference for conspecifics evolves more quickly than males’, which indicates sex-specific evolution of distinct neural and behavioral mechanisms of mating. Some of these sex-specific differences have already been identified in Drosophila melanogaster. For example, male proteins (FruM) derived from sex-specifically spliced transcripts of the gene fruitless are known to regulate normal male courtship and sexual behaviors (Ryner et al. 1996). These FruM proteins are also found to be inhibitors of heterospecific courtship in males (Fan et al. 2013), whereas females do not express these isoforms, highlighting the fact that their mating preferences are at least partly influenced by distinct mechanisms. Furthermore, female mate choice contributed more than male mate choice to species-specific sexual isolation in sympatry, but not allopatry. To explain, the authors suggested that reinforcement may be one explanation for rapid sexual isolation in sympatric females. Because females bear the brunt of reproductive fitness costs, sympatry may hasten evolution of conspecific mating preferences due to the increased probability of heterospecific mating and the lowered fitness associated with carrying hybrid offspring. Consequently, these conclusions lend support to the interference hypothesis, which states that in sympatric species, evolution of female mate preferences in one species reduces the selection pressure for evolution of male mate preferences in the other species. When female preferences that contribute to sexual isolation evolve quickly, there is no need for heterospecific males to be choosy, because regardless, heterospecific females often reject them and force them into conspecific mating. While Yukilevich and Peterson (2019) have identified general trends in sex-specific evolution of mating preferences and contributions to sexual isolation in Drosophila, it is yet to be seen whether these results are an anomaly, or if they are widespread in diverse taxa in nature. Furthermore, the authors found minor differences when comparing the mating preferences between males of allopatric species and females of allopatric species. Therefore, continued study should seek to validate these results in other genera encompassing sympatric and allopatric species, with specific attention to mechanisms of mating preference divergence in each geographical context. Associate Editor: K. Moore Handling Editor: Mohamed A. F. Noor Digests are short (∼500 word), news articles about selected original research included in the journal, written by students or postdocs. These digests are published online and linked to their corresponding original research articles. For instructions on Digests preparation and submission, please visit the following link: https://sites.duke.edu/evodigests/.

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