Data from: Testing an hypothesis of hybrid zone movement for toads in France

Hybrid zone movement may result in substantial unidirectional
introgression of selectively neutral material from the local to the
advancing species, leaving a genetic footprint. This genetic footprint is
represented by a trail of asymmetric tails and displaced cline centres in
the wake of the moving hybrid zone. A peak of admixture linkage
disequilibrium is predicted to exist ahead of the centre of the moving
hybrid zone. We test these predictions of the movement hypothesis in a
hybrid zone between common (Bufo bufo) and spined toads (B. spinosus),
using 31 nuclear and one mtDNA SNPs along a transect in the northwest of
France. Average effective selection in Bufo hybrids is low and clines vary
in shape and centre. A weak pattern of asymmetric introgression is
inferred from cline discordance of seven nuclear markers. The dominant
direction of gene flow is from B. spinosus to B. bufo and is in support of
southward movement of the hybrid zone. Conversely, a peak of admixture
linkage disequilibrium north of the hybrid zone suggests northward
movement. These contrasting results can be explained by reproductive
isolation of the B. spinosus and B. bufo gene pools at the southern (B.
spinosus) side of the hybrid zone. The joint occurrence of asymmetric
introgression and admixture linkage disequilibrium can also be explained
by the combination of low dispersal and random genetic drift due to low
effective population sizes.