Concordant morphological and molecular clines in a contact zone of the Common and Spined toad (Bufo bufo and B. spinosus) in the northwest of France

Abstract

BackgroundHybrid zones are regions where individuals of two species meet and produce hybrid progeny, and are often regarded as natural laboratories to understand the process of species formation. Two microevolutionary processes can take place in hybrid zones, with opposing effects on population differentiation. Hybridization tends to produce genetic homogenization, reducing species differences, whereas the presence of mechanisms of reproductive isolation result in barriers to gene flow, maintaining or increasing differences between taxa.ResultsHere we study a contact zone between two hybridizing toad species, Bufo bufo and B. spinosus, through a combination of molecular (12 polymorphic microsatellites, four nuclear and two mitochondrial SNP markers) and morphological data in a transect in the northwest of France. The results show largely concordant clines across markers, defining a narrow hybrid zone of ca. 30 km wide. Most hybrids in the centre of the contact zone are classified as F2 or backcrossed individuals, with no individuals assigned to the F1 hybrid class.ConclusionsWe discuss the implications of these results for our understanding of the evolutionary history of these species. We anticipate that the toad contact zone here described will become an important asset in the study of hybrid zone dynamics and evolutionary biology because of its easy access and the abundance of the species involved.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-016-0184-7) contains supplementary material, which is available to authorized users.

Testing an hypothesis of hybrid zone movement for toads in France

“…Another 268 individuals from 29 populations, including transect populations (grey circles in Figure a,b) and additional samples from the reference populations were genotyped and included in the data set, to a total of 306 individuals. Distances between transect populations were measured in a straight line, which follows the direction of the transect in Arntzen et al (), using a custom R script. Distances for reference populations were measured with google earth pro v.7.3.0 (Table and Supporting Information Table ).…”

“…This resulted in 32 nuclear DNA SNPs (Supporting Information Table ). Because of limitations due to plate dimensions in the KASP system, the remainder of the samples was analysed with 31 SNPs (by excluding the least well performing marker in the test data set, scaf4 , for which six out of 20 reference individuals had missing data) and we added one previously published diagnostic mtDNA SNP (16S; Arntzen et al, ). We excluded five individuals from the final data set with more than 10 SNPs missing (presumably because of low quality DNA).…”

“…A mean recombination rate between marker pairs of 0.4997 was calculated following formula (6) from Macholán et al (), using the number of chiasmata per bivalent for Bufo bufo (1.95; Wickbom, ), and the number of chromosomes for Bufo bufo ( N = 22). A generation time of 2.5 years for Bufo toads at the latitude of the hybrid zone (mean of three years in females and two years in males; Hemelaar, ), and initial secondary contact at 8,000 years ago following Arntzen et al () were used as input parameters. The width of the hybrid zone was derived from a general sigmoid cline model following the “no‐tails” formula in HZAR (Derryberry et al, ) fitted to the HI .…”

“…While B. bufo survived the last glacial maximum in Italy and the Balkans, B. spinosus survived in southern France and Spain (Arntzen et al, ). Based on four nuclear gene coding markers, twelve microsatellites, and two mitochondrial SNP markers, the width of the hybrid zone (30 km) compared to the dispersal distance (1.3 km/generation) suggested that selection against hybrids restricts gene flow through the hybrid zone (Arntzen et al, ). In the southeast of France, the Bufo hybrid zone was hypothesised to move southwards based on traces of introgression of one nuclear gene coding marker (of four tested), and discordance between nuclear, mtDNA, and morphological clines (Arntzen et al, ).…”

“…In the southeast of France, the Bufo hybrid zone was hypothesised to move southwards based on traces of introgression of one nuclear gene coding marker (of four tested), and discordance between nuclear, mtDNA, and morphological clines (Arntzen et al, ). In the northwest of France, a similar data set including microsatellite data, showed introgression towards the north in two nuclear markers out of four, but was not analysed from the perspective of hybrid zone movement at the time (Arntzen et al, ). This scenario provides the opportunity to test for the hypothesis of hybrid zone movement in the northwest of France.…”