Spreading introgression in the wake of a moving contact zone

An increasing number of studies describe moving hybrid zones. This raises the issue of their actual frequency and emphasizes the need for methods that enable the detection of zone movements without historical records. Asymmetric introgression, usually considered as a signature of geographical shift, might be misleading when applied to mitochondrial or potentially non-neutral markers. We investigated mitochondrial and genomic introgression, using 30 AFLP derived markers, in a well-documented moving avian contact zone between two warblers. We found no instances of cross-species transmission of mitochondrial DNA but we detected nuclear introgression. Introgression levels were higher in the expanding species. Highest introgression was observed in populations that recently became allopatric than in current sympatric populations, which suggests that alien genetic material mainly spread at the time just before the receding species became extinct. We propose that either local recruitment or positive selection on some loci contribute to this pattern. Furthermore, we propose that, when the rarefaction of sexual partners drive the hybridization process, movement could be revealed by introgression peaking on the rear edge of the moving zone, or in its close vicinity.