Introgression of Austrian fire-bellied toads (Bombina bombina) into northern German populations confirmed by complete mitochondrial genomes and transcriptome-wide Single Nucleotide Polymorphisms (SNPs)

Abstract

The fire-bellied toad (Bombina bombina) is a small amphibian species inhabiting the lowlands of Central and Eastern Europe. Continual population fragmentation has caused a decrease in genetic diversity and gene flow in margin populations in Northern Germany. Previous research using mitochondrial control region data uncovered a translocation of allochthonous toads from Austria into a single northern German population, Hogsdorf. Microsatellite and MHC data also confirmed this introgression, but were limited in revealing the true extent of this translocation for all investigated populations. Here, we utilize complete mitochondrial genomes and nuclear data, in the form of transcriptomes, to unravel the full extent of admixture from this translocation. The increased number of loci enabled us to uncover signs of introgression in four out of the five northern populations investigated. Moreover, as hybrids between B. bombina and its sister species, B. variegata, have been reported in Austria, we also investigated whether B. variegata alleles were translocated into the northern gene pool. We found evidence for hybridization between Austrian B. bombina and B. variegata, as well as traces of B. variegata alleles in one of the five German populations included in this study. These findings present the first reported case of introgressed relict B. variegata alleles from the Southern lineage being detected in Northern Germany.