Background Male genitals have repeatedly evolved left-right asymmetries, and the causes of such evolution remain unclear. The Drosophila nannoptera group contains four species, among which three exhibit left-right asymmetries of distinct genital organs. In the most studied species, Drosophila pachea, males display asymmetric genital lobes and they mate right-sided on top of the female. Copulation position of the other species is unknown. Results To assess whether the evolution of genital asymmetry could be linked to the evolution of one-sided mating, we examined phallus morphology and copulation position in D. pachea and closely related species. The phallus was found to be symmetric in all investigated species except D. pachea , which displays an asymmetric phallus with a right-sided gonopore, and D. acanthoptera, which harbors an asymmetrically bent phallus. In all examined species, males were found to position themselves symmetrically on top of the female, except in D. pachea and D. nannoptera , where males mated right-sided, in distinctive, species-specific positions. In addition, the copulation duration was found to be increased in the nannoptera group species compared to closely related outgroup species. Conclusion Our study shows that gains, and possibly losses, of asymmetry in genital morphology and mating position have evolved repeatedly in the nannoptera group. Current data does not allow us to conclude whether genital asymmetry has evolved in response to changes in mating position, or vice versa. Electronic supplementary material The online version of this article (10.1186/s12862-019-1434-z) contains supplementary material, which is available to authorized users.

16Background: Male genitals have repeatedly evolved left-right asymmetries, and the 17 causes of such evolution remain unclear. The Drosophila nannoptera group contains four 18 species, among which three exhibit left-right asymmetries of distinct genital organs. In the 19 most studied species, Drosophila pachea, males display asymmetric genital lobes and 20 they mate right-sided on top of the female. Copulation position of the other species is 21 unknown. 22Results: To assess whether the evolution of genital asymmetry could be linked to the 23 evolution of one-sided mating, we examined phallus morphology and copulation position in 24 D. pachea and closely related species. The phallus was found to be symmetric in all 25 investigated species except D. pachea, which display an asymmetric phallus with a right-26 sided gonopore, and D. acanthoptera, which harbor an asymmetrically bent phallus. In all 27 examined species, males were found to position themselves symmetrically on top of the 28 female, except in D. pachea and D. nannoptera, where males mated right-sided, in 29 distinctive, species-specific positions. In addition, the copulation duration was found to be 30 increased in nannoptera group species compared to closely related outgroup species. 31 Conclusion: Our study shows that gains, and possibly losses, of asymmetry in genital 32 morphology and mating position have evolved repeatedly in the nannoptera group. Current 33 data does not allow us to conclude whether genital asymmetry has evolved in response to 34 changes in mating position, or vice versa. 35 36 Background 39 Changes in behavior are thought to play important roles in animal evolution [1-3] . 40 How new behaviors evolve and how they are encoded in the genome is little understood. 41 New behaviors can initiate secondary evolutionary shifts in morphology, physiology or 42 ecology ("behavioral drive") [1-9] , for example when they bring an organism into contact 43 with new environmental factors. Behavior can also prevent evolutionary changes because 44 plasticity in behavior might enable individuals to adjust for changed environmental 45 conditions [10-12] . Other investigations suggest that behavior and morphology are both 46 subject to natural selection and that their responses to changes in the environment are 47 D. pachea and D. nannoptera males mate right-sided 126 The position of the male during copulation has not been described for any of the 127 closely related species of D. pachea. In this study, we assessed copulation postures in D. 128 pachea and nine related species: D. acanthoptera and D. nannoptera (sister species of D. 129 pachea), D. machalilla and D. bromeliae (representatives of close outgroup lineages), D. 130 buzzatii and D. mojavensis (members of the repleta species group), as well as 131representatives of other Drosophila species groups (D. tripunctata, D. willistoni and D. 132 melanogaster). Phylogenetic relationships between the ten studied species were 133 estimated with a Bayesian phylogeny ( Supplementary Fig. 6) based on...