Natural Sciences, Biological Sciences, Genetics and Genomics, Naturvetenskap, Biologi, Genetik och genomik, Evolutionary Biology, Evolutionsbiologi

Hybrid zones present a unique framework to study the genetic basis of reproductive isolation and speciation. The European fire-bellied toad (Bombina bombina) and yellow-bellied toad (B. variegata) hybridize in a zone across several thousand kilometres in Central Europe. The Bombina system has been examined for more than a century and its scientific legacy comprises the development of the ultilocus cline theory and the aintenance of hybrid zones theory. To set the scene for future genomic research we review the scientific literature on the Bombina hybrid zone, with emphasis on studies that provided insights into genetic processes. Our review emphasises that the two species are highly genetically diverged and reduced hybrid fitness prevents unrestricted gene flow between the two parental species (endogenous selection). However, environmental variables influence the location of hybrid populations (exogenous selection). Depending on region, the transects are described as clinal, tension, mosaic, and residual hybrid zones. The era of genomics has opened a plethora of yet unutilized analytical possibilities in Bombina research, including high resolution population genomics, understanding the genetic basis of reproductive isolation, phenotype-genotype associations, and elucidating the roles of endo- and exogenous selection in maintaining the hybrid zone. Because the hybrid zone occurs across many different environmental settings and genetic backgrounds, it should be possible to distinguish common genetic barriers to gene flow from those arising due to local adaptation. We argue that the Bombina zone offers an excellent opportunity to gain deep genomic insights into the selection processes that maintain the species boundary.