Movements of adult fish in a hybrid zone revealed by microsatellite genetic analysis and capture-recapture data

• 1 Barbus barbus and Barbus meridionalis (Cyprinidae) form a hybrid zone in the Lergue river (southern France). We attempted to characterise the pattern of adult dispersal with the aim of understanding the evolutionary interactions maintaining this hybrid zone. In a previous capture/recapture study, movements between localities at a distance of 2.5 km or more appeared extremely rare. We therefore decided to investigate adult movement using a different method, based on genetic markers.
• 2 As this hybrid zone has produced allele frequency clines, massive population movements would produce temporal variations in genetic composition at a given locality. In order to determine the relationship between gene frequency and position, we surveyed four diagnostic or semidiagnostic microsatellite loci over the hybrid zone and estimated the cline produced by introgression. Then, we focused on a single locality at the centre of the hybrid zone and established its introgression index over 11 periods during two years.
• 3 The introgression index varied between periods producing significant Fst. A synthetic hybrid index, based on principal component analysis of the logit frequencies, was used for regression analyses and appeared significantly correlated with the river flow. This may be explained by displacement of adult fishes in response to flow increase. Using the information from the cline, we estimate that the most important gene frequency changes among dates, if created exclusively by population movement, correspond to a distance of 1500 m.
• 4 Additionally, we performed recaptures on a finer geographic scale than previously, around the central locality. No recaptured fishes were observed at 1875 m or more downstream, and at 875 m or more upstream, from the central locality. A high proportion of recaptured fishes (20% and 12%) was found in the nearest points (312 m downstream and 437 m upstream, respectively).
• 5 Thus, we established that (i) individual movements appear limited to a few hundred metres and (ii) individuals in a location tend to move in the same direction at the same time, probably in response to the same environmental factor.