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.