Alien gene introgression promotes rapid adaptation: an unconscious experiment on Fire-bellied toad (Bombina bombina) at the northern edge of its distribution range

TI 349/13-1

This proposal focusses on genomic sources of adaptive variation for rapid change (thematic group 3), by evaluating particular hypotheses, i.e., (1) small isolated populations at the margin of a species distribution might be constrained by a lack of standing genetic variation and (2) introgression of allochthonous alleles may enhance the adaptive potential of such populations. In case of introgression, heterozygosity and allelic diversity increase instantly and sexual reproduction creates mosaic genotypes combining different sets of authochthonous and allochthonous alleles, providing the raw material for rapid adaptation. Our study organism is the Fire-bellied toad Bombina bombina, which occurs in Europe in two distinct evolutionary lineages, i.e., a southern lineage distributed in the Balkan states and Austria, and a northern lineage reaching from Ukraine through Poland to the edges of the northern distribution range in Northern Germany and Southern Sweden. Initial genetic analyses at 20 unlinked genetic loci unraveled an introgression of southern lineage (putatively Austrian) genotypes into three local northern lineage populations, presumably due to independent unofficial (and illegal) release by amateur herpetologists. Interestingly, these introgressed populations (1) are fixed for introgressed alleles at several loci and (2) have a positive population trend, such that they outperform local authochthonous populations. This scenario provides a replicated unconscious experiment, mimicking long-distance dispersal and subsequent introgression from a southern lineage (potentially adapted to warmer summer conditions) to a northern population. We will perform genomic scans (RAD-seq) and expression profiling (RNA-seq) for both the authochthonous and the introgressed Bombina populations to discern, on a genomic scale, introgressed alleles/expression patterns from authochthonous ones. In genome-wide association studies (GWAS), we will relate these patterns to fitness related traits (body size, mass, condition). These data will be fed into theoretical models developed by our collaborator (Prof. Stephan) to discern the relative importance of selective sweeps vs. polygenic selection in rapid adaptation. In-depth analysis of candidate genes (including MHC and heat shock protein genes) will unravel the functional classes, in which introgressed (allochthonous) alleles will be favored by selection, and in which ones authochthonous alleles are maintained. This will provide us with a causal understanding of the reason, why introgressed populations of the Fire-bellied toad at the northern edge of the distribution range outperform the non-introgressed ones. Ultimately, the project focusses on the relevance of introgression as a mechanism to increase the amount of standing genetic variation and thus to contribute adaptive genetic material for rapid adaptation.

Publications

• De Cahsan B, Westbury MV, Drews H, Tiedemann R (2019) The complete mitochondrial genome of a European fire-bellied toad (Bombina bombina) from Germany. Mitochondrial DNA Part B: Resources 4, 498-500. DOI: 10.1080/23802359.2018.1547143