Differential introgression of a female competitive trait in a hybrid zone between sex-role reversed species.

Mating behavior between recently diverged species in secondary contact can impede or promote reproductive isolation. Traditionally, researchers focus on the importance of female mate choice and male-male competition in maintaining or eroding species barriers. Although female-female competition is widespread, little is known about its role in the speciation process. Here, we investigate a case of interspecific female competition and its influence on patterns of phenotypic and genetic introgression between species. We examine a hybrid zone between sex-role reversed, Neotropical shorebird species, the northern jacana (Jacana spinosa) and wattled jacana (J. jacana), in which female-female competition is a major determinant of reproductive success. Previous work found that females of the more aggressive and larger species, J. spinosa, disproportionately mother hybrid offspring, potentially by monopolizing breeding territories in sympatry with J. jacana. We find a cline shift of female body mass relative to the genetic center of the hybrid zone, consistent with asymmetric introgression of this competitive trait. We suggest that divergence in sexual characteristics between sex-role reversed females can influence patterns of gene flow upon secondary contact, similar to males in systems with more typical sex roles.