Southern introgression increases adaptive immune gene variability in
northern range margin populations of Fire-bellied toad
Abstract
Northern range margin populations of the European fire-bellied toad
(Bombina bombina) have rapidly declined during recent decades. Extensive
agricultural land use has fragmented the landscape, leading to habitat
disruption and loss, as well as eutrophication of ponds. In Northern
Germany (Schleswig-Holstein) and Southern Sweden, this decline resulted
in decreased gene flow from surrounding populations, low genetic
diversity, and a putative reduction in adaptive potential, leaving
populations vulnerable to future environmental and climatic changes.
Previous studies using mitochondrial control region and nuclear
transcriptome-wide SNP data detected introgressive hybridization in
multiple northern B. bombina populations after presumed illegal release
of toads from Austria. Here, we determine the impact of this
introgression by comparing the body conditions (as a proxy for fitness)
of introgressed and non-introgressed populations, and the genetic
consequences in two candidate genes for putative local adaptation (the
MHC II gene as part of the adaptive immune system and the stress
response gene HSP70 kDa). We detected regional differences in body
condition. We observed significantly elevated levels of within
individual MHC allele counts in introgressed Swedish populations,
associated with a tendency towards higher body weight, relative to
regional non-introgressed populations. These differences were not
observed among introgressed and non-introgressed German populations.
Genetic diversity in both MHC and HSP was generally lower in northern
than southern populations. Our study sheds light on the potential
benefits of translocations of more distantly related conspecifics as a
means to increase adaptive genetic variability and fitness of struggling
range margin populations without distortion of local adaptation.