No severe genetic bottleneck in a rapidly range-expanding bumblebee pollinator

Genetic bottlenecking can limit the success of populations colonising new ranges. However, successful colonisations can occur despite bottlenecking, a phenomenon known as the genetic paradox of invasion. Eusocial Hymenoptera such as bumblebees (Bombus spp.) should be particularly vulnerable to genetic bottlenecking, since homozygosity at the sex-determining locus leads to costly diploid male production. The Tree Bumblebee (B. hypnorum) has rapidly colonised the UK since 2001 and has been highlighted as exemplifying the genetic paradox of invasion. Using microsatellite genotyping, combined with the first genetic estimates of diploid male production in UK B. hypnorum, we tested two alternative genetic hypotheses (‘bottleneck’ and ‘gene flow’ hypotheses) for B. hypnorum’s colonisation of the UK. We found that the UK population has not undergone a recent severe genetic bottleneck and exhibits levels of genetic diversity falling between those of widespread and range-restricted Bombus species. Diploid males occurred in 15.4% of reared colonies, leading to an estimate of 21.5 alleles at the sex-determining locus. Overall, the findings show that this population is not bottlenecked, instead suggesting that it is experiencing continued gene flow from the continental European source population with only moderate loss of genetic diversity, and does not exemplify the genetic paradox of invasion. Methods Data collection was carried out following the Methods section in Brock et al. No severe genetic bottleneck in a rapidly range-expanding bumblebee pollinator.