Repeated evolution of asymmetric genitalia and right-sided mating behavior in theDrosophila nannopteraspecies group
Background
Male genitals have repeatedly evolved left-right asymmetries, and the causes of such evolution remain unclear. TheDrosophila nannopteragroup 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 inD. pacheaand closely related species. The phallus was found to be symmetric in all investigated species exceptD. pachea, which display an asymmetric phallus with a right-sided gonopore, andD. acanthoptera, which harbor an asymmetrically bent phallus. In all examined species, males were found to position themselves symmetrically on top of the female, except inD. pacheaandD. nannoptera, where males mated right-sided, in distinctive, species-specific positions. In addition, the copulation duration was found to be increased innannopteragroup 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 thenannopteragroup. Current data does not allow us to conclude whether genital asymmetry has evolved in response to changes in mating position, or vice versa.
