Résumé
Narrow hybrid zones are generally subjected to the action of two forces: dispersal, which tends to homogenise the hybridising taxa, and selection against hybrids, which, in contrast, produces steep clines of introgression for diagnostic markers. Although differences between sexes in dispersal abilities or in susceptibility to hybrid counterselection are common in hybrid zones, autosomal genetic differences between males and females have never been reported to our knowledge. Barbus barbus and Barbus meridionalis (Cyprinidae) form a hybrid zone along the Lergue river. By carrying out a genetic analysis of males and females in six samples from two central stations of the hybrid zone using codominant markers (six allozymes and four microsatellite loci), we revealed significant multilocus and monolocus differences between the sexes. This could reflect a genetic difference among sexes within a same cohort, caused either by a survival (or fertility) differential among sexes or by a sex-specific pattern of dispersal. Alternatively, this may be due to genetic differentiation between cohorts, since male and female barbel exhibit different maturation, growth and survival patterns leading to different age distributions among sexes, and particularly among reproducers.