Nest architecture and male aggression drive sexual isolation between three-spined stickleback ecotypes.

To understand how sexual isolation contributes to the evolution of reproductive barriers, we examined the pre- and post-zygotic reproductive barriers between ecotypes of the three-spined stickleback (Gasterosteus aculeatus) from North Uist, where the species has undergone an adaptive radiation. Using an experimental design that separated the effects of ecotype, population of origin, and body size, we identified a pre-zygotic barrier attributed solely to ecotype; pairings within ecotypes were more likely to result in mating than those between ecotypes, regardless of population origin or relative body size of males and females. This effect was associated with reproductive incompatibilities linked to the failure of armoured females to enter the unusually small nests of armourless males, along with high levels of aggression from armoured males directed at armourless females. In vitro crosses did not reveal post-zygotic reproductive barriers associated with ecotype or population of origin. These findings indicate that sexual isolation, linked primarily to male traits and involving the coupling of barrier effects, is a significant obstacle to gene flow among the ecotypes, supporting an emerging view that pre-zygotic sexual isolation tends to evolve rapidly and at an early stage during ecological divergence.