Asymmetrical introgression in a freshwater fish hybrid zone as revealed bya 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 morp hologically by discriminant function analysis based on samples from the two parental taxa using only three measurements. This morphological hybridizat ion 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 h abitats where the latter cannot (small upstream tributaries), while remaini ng essentially unaffected by the hybridization process. By contrast, B. bar bus is strongly introgressed because its habitat is invaded at least sporad ically by B. meridionalis migrants. There is no behavioral barrier to hybri dization, suggesting that an asymmetrical gene flow is probably partly resp onsible for the asymmetry in introgression. (C) 1999 The Linnean Society of London.