Population genomics of Bombus terrestris reveals high but unstructured genetic diversity in a potential glacial refugium

Abstract

Ongoing climate change is expected to cause an increase in temperature and a reduction of precipitation levels in the Mediterranean region, which might cause changes in many species distributions. These effects negatively influence species gene pools, decreasing genetic variability and adaptive potential. Here, we use mitochondrial DNA and RADseq to analyse population genetic structure and genetic diversity of the bumblebee species Bombus terrestris (subspecies Bombus terrestris lusitanicus), in the Iberian Peninsula. Although this subspecies shows a panmictic pattern of population structure across Iberia and beyond, we found differentiation between subspecies B. t. lusitanicus and B. t. africanus, probably caused by the existence of barriers to gene flow between Iberia and North Africa. Furthermore, the results revealed that the Iberian Peninsula harbours a large fraction of B. terrestris intraspecific genetic variation, with the highest number of mitochondrial haplotypes found when compared with any other region in Europe studied so far, suggesting a potential role for the Iberian Peninsula as a glacial refugium. Our findings strengthen the idea that Iberia is a very important source of diversity for the global genetic pool of this species, because rare alleles might play a role in population resilience against human- or climate-mediated changes.