Differential introgression across newt hybrid zones: Evidence from replicated transects

Abstract

Genomic heterogeneity of divergence between hybridizing species may reflect heterogeneity of introgression, but also processes unrelated to hybridization. Heterogeneous introgression and its repeatability can be directly tested in natural hybrid zones by examining multiple transects. Here, we studied hybrid zones between the European newts Lissotriton montandoni and two lineages of Lissotriton vulgaris, with replicate transects within each zone. Over 1,000 nuclear genes located on a linkage map and mitochondrial DNA were investigated using geographical and genomic clines. Overall, the five transects were all similar, showing hallmarks of strong reproductive isolation: bimodal distribution of genotypes in central populations and narrow allele frequency clines. However, the extent of introgression differed between the zones, possibly as a consequence of their different ages, as suggested by the analysis of heterozygosity runs in diagnostic markers. In three transects genomic signatures of small‐scale (~2 km) zone movements were detected. We found limited overlap of cline outliers between transects, and only weak evidence of stronger differentiation of introgression between zones than between transects within zones. Introgression was heterogeneous across linkage groups, with patterns of heterogeneity similar between transects and zones. Predefined candidates for increased or reduced introgression exhibited only a subtle tendency in the expected direction, suggesting that interspecific differentiation is not a reliable indicator for the strength of introgression. These hierarchically sampled hybrid zones of apparently different ages show how introgression unfolds with time and offer an excellent opportunity to dissect the dynamics of hybridization and architecture of reproductive isolation at advanced stages of speciation.

Massive introgression of major histocompatibility complex (MHC) genes in newt hybrid zones

“…There could simply be too little hybridization in the OUT zone to allow extensive MHC introgression, even if favoured by selection. The presence of admixed individuals, though rare even in syntopy, indicates nonzero recent or ongoing hybridization, although genomes of the two species are highly congealed (Zieliński et al, ). Introgression from the L. vulgaris lineage inhabiting the OUT zone has been strong enough to cause a regional mtDNA replacement in L. montandoni (Zieliński et al, ).…”

“…Here, we compared MHC clines to the average genome-wide (SNP) clines using the following approach. For each individual the proportion of the genome derived from each species estimated with Admixture (Alexander, Novembre, & Lange, 2009) was taken from Zieliński et al (2019). The proportion of MHC alleles derived from each species was estimated using Structure (Falush, Stephens, & Pritchard, 2007) with MHC alleles encoded as binary dominant data.…”

“…Data on genome‐wide admixture, available for 96% of individuals genotyped in MHC, based on 2,849 SNPs (minor allele frequency, MAF ≥ 0.05) in ca. 1,100 protein‐coding genes are from Zieliński et al ().…”

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

“…Hybrid zone studies have a prominent place in evolutionary research because they provide opportunities to study the mechanisms that keep species apart in the face of gene flow (Abbott et al, ; Ravinet et al, ). They can be seen as natural arenas where combinations of mutations from different species are functionally tested (Zieliński et al, ). Most recognized hybrid zones have come into existence upon the secondary contact of groups of organisms that are morphologically and genetically differentiated.…”

Pervasive introgression of MHC genes in newt hybrid zones

“…The two lineages within L. vulgaris diverged more than 1 Mya (Pabijan et al, 2017; Zieliński et al, 2016), and each of them has experienced a distinct history of hybridization with L. montandoni (Zieliński et al, 2016). In particular, the IN zone appears older (Zieliński et al, 2019). Within each zone we sampled two transects separated by a distance of 100-200 km, which is two orders of magnitude larger than the per generation dispersal distances of these newts.…”