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.

Data collection was carried out following the Methods section in Brock et al. No severe genetic bottleneck in a rapidly range-expanding bumblebee pollinator.

Bottleneck analysis

The bottleneck analyses were carried out using worker microsatellite genotyping data from Crowther et al. (2019) and Dreier et al. (2014). Analysis code and full details of how to perform the bottleneck analyses can be found here.

References

Crowther, L.P., Wright, D.J., Richardson, D.S., Carvell, C. and Bourke, A.F., 2019. Data from: "Spatial ecology of a range-expanding bumble bee pollinator". https://figshare.com/articles/Data_from_Spatial_ecology_of_a_range-expanding_bumble_bee_pollinator_/7284803.

Dreier, S., Redhead, J.W., Warren, I.A., Bourke, A.F., Heard, M.S., Jordan, W.C., Sumner, S., Wang, J. and Carvell, C., 2014. Microsatellite genotype data for five species of bumblebee across an agricultural landscape in Buckinghamshire, UK. NERC Environmental Information Data Centre. (Dataset). https://doi.org/10.5285/6a408415-0575-49c6-af69-b568e343266d.

Diploid male analysis 

The diploid male analysis was carried out using male microsatellite genotyping data found in this Dryad directory. Analysis code and full details of how to perform the diploid male analysis analyses can be found here.

This Dryad directory contains two data files (.csv format) and two read me files (.docx format):

File: Raw_male_typing_matrix.csv

Description: Raw genotypes across 14 microsatellite markers from initial genotyping and regenotyping reactions for all sampled males used in this study. Full details can be found in the respective Raw_male_typing_matrix_README.docx file.

File: Consensus_male_genotypes.csv

Description: Consensus genotypes across 14 microsatellite markers for all sampled males used in this study. Full details can be found in the respective Consensus_male_genotypes_README.docx file.