Two transects reveal remarkable variation in gene flow on opposite ends of a European toad hybrid zone

Speciation entails a reduction in gene flow between lineages. The rates at which genomic regions become isolated varies across space and time. Barrier markers are linked to putative genes involved in (processes of) reproductive isolation, and, when observed over two transects, indicate species-wide processes. In contrast, transect-specific putative barrier markers suggest local processes. We studied two widely separated transects along the 900 km hybrid zone between Bufo bufo and B. spinosus, in northern and southern France, for ~1200 RADseq markers. We used genomic and geographic cline analyses to identify barrier markers based on their restricted introgression, and found that some markers are transect-specific, while others are shared between transects. Twenty-six barrier markers were shared across both transects, of which some are clustered in the same chromosomal region, suggesting that their associated genes are involved in reduced gene flow across the entire hybrid zone. Transect-specific barrier markers were twice as numerous in the southern than in the northern transect, suggesting that the overall barrier effect is weaker in northern France. We hypothesize that this is consistent with a longer period of secondary contact in southern France. The smaller number of introgressed genes in the northern transect shows considerably more gene flow towards the southern (B. spinosus) than the northern species (B. bufo). We hypothesize that hybrid zone movement in northern France and hybrid zone stability in southern France explain this pattern. The Bufo hybrid zone provides an excellent opportunity to separate a general barrier effect from localized gene flow-reducing conditions.