Testing an hypothesis of hybrid zone movement for toads in France

Abstract

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.

Strong divergent selection at multiple loci in two closely related species of ragworts adapted to high and low elevations on Mount Etna

“…SFS of the species pair was first analysed under 14 models of isolation with migration and secondary contact ( Figure S2), which were published previously Nevado, Contreras-Ortiz, Hughes, & Filatov, 2018;Tine et al, 2014) or modified from them. SFS of the species pair was first analysed under 14 models of isolation with migration and secondary contact ( Figure S2), which were published previously Nevado, Contreras-Ortiz, Hughes, & Filatov, 2018;Tine et al, 2014) or modified from them.…”

An amphibian species pushed out of Britain by a moving hybrid zone

“…Hybrid zone movement may result in substantial unidirectional introgression of selectively neutral material from the local to the advancing species, leaving a genetic footprint (Scribner & Avise, ; Wielstra et al, ). In France, a weak but statistically significant asymmetry of clines was observed in support of a genetic footprint and southward shift of the B. spinosus – B. bufo hybrid zone (van Riemsdijk, Butlin, et al, ). How long such movements leave genetic traces depends on the effective selection against hybrids, but may also vary depending on other factors related to hybrid zone movement (Currat, Ruedi, Petit, & Excoffier, ).…”

“…Fluorescence‐based genotyping (Semagn, Babu, Hearne, & Olsen, ) was used in the Kompetitive Allele‐Specific PCR (KASP) genotyping system at the SNP (single nucleotide polymorphism) genotyping facility of the Institute of Biology, Leiden University. Primer design, PCR setup and data visualization followed Arntzen et al () and van Riemsdijk, Butlin et al (). SNP's were selected on the basis of species diagnosticity.…”

“…The data for Britain and Norway were analyzed jointly with published data that represent a Bufo spinosus to Bufo bufo hybrid zone in the northwest of France, along with reference populations from Spain, southern France and the Channel island Jersey ( B. spinosus ) and northern France and the Netherlands ( B. bufo ) (Arntzen et al, ; Arntzen, de Vries, Canestrelli, & Martínez‐Solano, ; Arntzen, Wilkinson, Butôt, & Martínez‐Solano, ; van Riemsdijk, Butlin, et al, ). The full panel of SNP's was studied with the exclusion of c10orf2 and klhl that proved not diagnostic, banp that showed a widely displaced cline and egflam that was strongly out of Hardy‐Weinberg equilibrium (van Riemsdijk, Butlin, et al, ). Data for individuals with more than nine SNPs missing were discarded.…”

“…The species involved are the common toad Bufo bufo to the northeast and the spined toad Bufo spinosus to the southwest of their mutual species border (Figure ). Evidence is mounting for this hybrid zone to be moving along the Channel coast (Arntzen et al, ; van Riemsdijk, Butlin, Wielstra, & Arntzen, ). To test this hypothesis this study set out to investigate if—when the Channel had yet to be formed—the Bufo species contact might have extended into Great Britain.…”