Female discrimination against heterospecific mates does not depend on mating habitat

Environmental differences can cause reproductive isolation to evolve. Distinct habitats can be particularly important for the evolution of genetically based sexual isolation, which occurs when divergent preferences and mating traits reduce mating between species. Yet, we know little about environmental effects on the potentially plastic expression, and thus the current maintenance, of sexual isolation. This is especially intriguing in the context of reverse speciation, where previously isolated taxa begin hybridizing and merge. Environmental change could weaken reproductive isolation underlain by plastic traits even before any genetic change occurs. Here, we examine how differences in mating habitats affect the expression of both female discrimination between species and male traits that underlie sexual isolation. We used 2 species pairs of threespine stickleback fish (Gasterosteus spp.): an intact species pair and a formerly distinct but now hybridizing species pair, where habitat change presumably triggered reverse speciation. The expression of female discrimination was fairly insensitive to habitat, despite the central importance of habitat differences to the initial evolution of sexual isolation. Only the ecotype being subsumed by hybridization showed habitat sensitivity, suggesting this plasticity may have contributed to reverse speciation either as a cause or consequence of gene flow. Also, we found plasticity in male courtship across habitats that could further erode sexual isolation. Thus, environmental differences may play different roles in the genetic evolution versus plastic maintenance of sexual isolation, with implications for the forward versus reverse processes of speciation.