Characterization of a hybrid zone between two annual killifish genus Austrolebias from the Biosphere Reserve and Ramsar Sites in South America

This study attempts to characterize species boundaries in the hybrid zone between two taxa of the South American killifish genus Austrolebias (Cyprinodontiformes: Rivulidae), A. charrua and A. reicherti, to test mechanisms underlying secondary contact and introgression. The hybrid zone spans in a narrow area of approximately 106 km2 in the Patos-Merin System. Present analyses were based on seven microsatellite loci, and one mitochondrial marker and morphology. Discriminant analysis of principal components (DAPC) and morphology detected five hybrid populations. Structure and Newhybrids analyses suggested the existence of a bimodal hybrid zone, in which hybrids predominantly consist of individuals genetically similar to one or other parental genotype, with few intermediates. The existence of bidirectional backcrossing was evident through AMOVA analyses using both nuclear and mitochondrial markers, haplotype network, and long-term asymmetric gene flow estimates. Several mechanisms may underlie the patterns of mitochondrial and nuclear gene flow such as differential fitness of intraspecific and interspecific cyto-nuclear combinations and selection against female function in interspecific backcrosses. Finally, present results highlight the role of natural hybridization and introgression to increase morphological and genetic variability contrasting with inbreeding and genetic drift which occur in each temporal pond where these endangered killifish species live.