The conservation genetics of the bumblebees Bombus muscorum and Bombus jonellus in a model island system

Abstract

This thesis compares the population genetics of two bumblebee species (B. muscorum and B. jonellus) in a model island system (Hebrides, UK). Both species show significant levels of population structuring (B. muscorum, θ = 0.13 and B. jonellus, θ = 0.034) and isolation by distance. Pairwise comparisons between populations suggest that B. jonellus disperse > 50 km relatively frequently. By contrast, B. muscorum populations are spatially structured on a smaller scale, and are estimated to disperse > 8 km only infrequently. Populations of both species show signs of recent population bottlenecks under some microsatellite mutation models, and under all models bottlenecks were more frequent for B. muscorum. Diploid males were found at low frequency in B. muscorum but were not detected for B. jonellus. However, we find triphoid individuals in both species, indirectly confirming that diploid males were present in the previous generation. This is the first time that triphoids have been detected in any wild bee populations. For both species triploid frequencies were negatively correlated with population size, and those restricted to less than 13km² of suitable habitat were particularly at risk. Estimated total triploid frequencies peaked at 20% with respect to normal diploid workers, and were higher in B. muscorum than in B. jonellas, perhaps due to the greater dispersal range of the latter species. These results indicate that closely-related species exhibit cryptic but fundamental differences in aspects of their ecology which influence their susceptibility to habitat fragmentation. Observed differences may in part explain differential declines of mainland populations of bumblebees and will greatly inform future conservation strategies.

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