Asymmetrical introgression in a freshwater fish hybrid zone as revealed by a morphological index of hybridization

Two cyprinid species, Barbus barbus and Barbus meridionalis , form a hybrid zone in the River Lergue (southern France). Hybrids were distinguished morphologically by discriminant function analysis based on samples from the two parental taxa using only three measurements. This morphological hybridization index showed a strong asymmetry in introgression between the two taxa. B. meridionalis can live in the habitats typical of B. barbus and also in habitats where the latter cannot (small upstream tributaries), while remaining essentially unaffected by the hybridization process. By contrast, B. barbus is strongly introgressed because its habitat is invaded at least sporadically by B. meridionalis migrants. There is no behavioral barrier to hybridization, suggesting that an asymmetrical gene flow is probably partly responsible for the asymmetry in introgression.