The Maintenance of Reproductive Isolation in a Mosaic Hybrid Zone Between the Fire-Bellied Toads Bombina Bombina and B. Variegata

Abstract

. Mosaic hybrid zones arise when ecologically differentiated taxa hybridize across a network of habitat patches. Frequent interbreeding across a small‐scale patchwork can erode species differences that might have been preserved in a clinal hybrid zone. In particular, the rapid breakdown of neutral divergence sets an upper limit to the time for which differences at marker loci can persist. We present here a case study of a mosaic hybrid zone between the fire‐bellied toads Bombina bombina and B. variegata (Anura: Discoglossidae) near Apahida in Romania. In our 20 × 20 km study area, we detected no evidence of a clinal transition but found a strong association between aquatic habitat and mean allele frequencies at four molecular markers. In particular, pure populations of B. bombina in ponds appear to cause massive introgression into the surrounding B. variegata gene pool found in temporary aquatic sites. Nevertheless, the genetic structure of these hybrid populations was remarkably similar to those of a previously studied transect near Pescenica (Croatia), which had both clinal and mosaic features: estimates of heterozygote deficit and linkage disequilibrium in each country are similar. In Apahida, the observed strong linkage disequilibria should stem from an imperfect habitat preference that guides most (but not all) adults into the habitats to which they are adapted. In the absence of a clinal structure, the inferred migration rate between habitats implies that associations between selected loci and neutral markers should break down rapidly. Although plausible selection strengths can maintain differentiation at those loci adapting the toads to either permanent or temporary breeding sites, the divergence at neutral markers must be transient. The hybrid zone may be approaching a state in which the gene pools are homogenized at all but the selected loci, not dissimilar from an early stage of sympatric divergence.

Genomic Admixture Analysis in European Populus spp. Reveals Unexpected Patterns of Reproductive Isolation and Mating

“…1985; Barton and Gale 1993;Rieseberg et al 1999;Martinsen et al 2001;Wu 2001;Vines et al 2003;Coyne and Orr 2004;Payseur et al 2004;Minder et al 2007;Buerkle and Lexer 2008), there is little experience in the interpretation of genomic patterns of admixture and ancestry in terms of reproductive isolation and speciation (but see Payseur et al 2004;Lexer et al 2007;Nolte et al 2009;Teeter et al 2010). The results of our study allow us to address this topic and to comment on the utility of gene mapping in hybrid zones of wild species in animals and plants.…”

“…A DMIXTURE or hybrid zones between genetically divergent populations are increasingly being explored for their use in studies of adaptation, reproductive isolation, and speciation Martinsen et al 2001;Wu 2001;Vines et al 2003;Payseur et al 2004; reviewed by Coyne and Orr 2004), especially for their potential in identifying recombinants for gene mapping (otherwise known as ''admixture mapping''; Chakraborty and Weiss 1988;Briscoe et al 1994;Rieseberg et al 1999;Reich et al 2005;Slate 2005;Zhu et al 2005;Lexer et al 2007;Nolte et al 2009). In many taxa of animals and plants, recombinants are created by admixture between divergent populations or species in hybrid zones or ecotones (Buerkle and Lexer 2008;Gompert and Buerkle 2009).…”

Ecological differentiation and habitat unsuitability maintaining a ground beetle hybrid zone

“…2007), while exogenous selection operates via organismal adaptation to external environments (Vines et al. 2003; Dodd and Afzal‐Rafii 2004; Nosil et al. 2005).…”

“…2001; Ross and Harrison 2002; Vines et al. 2003; Ruegg 2008). Exogenous selection via environmental factors is often difficult to study in species that have a wide geographic range and/or those that demonstrate cryptic ecological adaptation (Kozak et al.…”

Sperm competition and offspring viability at hybridization in Australian tree frogs, Litoria peronii and L. tyleri

“…Clearly, the reality lies somewhere in between these views with successful introgression of genes occurring at least occasionally during hybridization between species and which generates greater fitness in hybrid offspring compared with purebred offspring. An example of this is (at least sometimes) the case of the fire-bellied toads (Vines et al, 2003, but see Nurnberger et al, 2005. Another set of examples come from hybridization resulting in new, successful species of Lake Malawi cichlids as a result of genetic mosaicism (Salzburger et al, 2002;Seehausen et al, 2003;Seehausen, 2004).…”