The impact of inbreeding and parasitism on bumblebees

Abstract

Many bumblebee species are suffering from the effects of habitat fragmentation and population isolation. In some cases, populations have lost genetic diversity due to genetic drift and it is possible they are now at heightened risk of extinction. Inbreeding may be particularly costly to bumblebees because, as Hymenoptera, their complementary sex determination system can lead to the production of sterile or inviable diploid males. However, little is known about the effect that diploid male production has on bumblebee colony fitness. Here, the consequences of brother-sister mating in the bumblebee Bombus terrestris are investigated, and the production of diploid males was found to exert considerable costs at the colony level by reducing productivity and survival. Diploid males may therefore act as indicators of the genetic health of populations, and their detection could be used as an informative tool in hymenopteran conservation. Due to the costs associated with inbreeding, selection may have favoured the evolution of kin recognition systems in bumblebees. Data are presented that suggest that B. terrestris can discriminate between kin and non-kin as gynes were less willing to mate with siblings compared to non-relatives.

Theory predicts that inbreeding may impose further costs on bumblebees through increased levels of parasitism, but empirical data are scarce. The relationship between population genetic diversity and parasite prevalence is assessed using Hebridean island populations of Bombus muscorum and Bombus jonellus. In the more outbred B. jonellus, there was no relationship between parasite prevalence and population heterozygosity. But prevalence of the gut parasite Crithidia bombi and the tracheal mite Locustacarus buchneri were found to be higher in populations of B. muscorum that had lower genetic diversity. In addition to assessing infection status, the activity of the immune system was assessed in each individual bee. However, there was no relationship between population heterozygosity and these immune parameters. This suggests that, in some Hymenopteran species, as populations lose genetic diversity the impact of parasitism will increase, potentially pushing threatened populations closer to extinction. Therefore, preventing population fragmentation by the creation of suitable habitats and by ensuring connectivity between habitat patches are important aspects of hymenopteran conservation.

Finally, this thesis investigates the potential threat of pathogen spread from commercially reared bumblebees used for crop pollination to wild bumblebees. Although no direct evidence for parasite spillover is found, the prevalence of C. bombi was significantly higher in B. terrestris by the end of the season on farms that used commercial bumblebees compared to farms that did not. This high prevalence does suggest that pathogen spillover is a potential threat and it would be preferable to reduce the usage of commercial bumblebees where possible. For example, sowing wild flower mixes could boost natural pollinator populations, which in turn would benefit soft fruit pollination. Overall, this thesis contributes to our knowledge of the consequences of inbreeding in bumblebees and the relationship between genetic diversity and parasite prevalence. It provides a greater understanding of the factors that might be pushing threatened pollinators towards extinction and as a whole provides important information that may inform conservation practitioners, whose aim is to protect the future of our hymenopteran pollinators.