Climate change is predicted to affect host-parasite interactions. For example, parasite prevalence is expected to increase with increasing temperature. Global population declines of bumblebees, important pollinators, have been partly attributed to infection by the microsporidian parasite, Nosema bombi, but the role of climate change remains obscure.
In this study, we investigated the association between changing temperatures, prevalence of N. bombi infection and host mitochondrial DNA genotypes (‘haplotypes’) in a natural population of bumblebees (Bombus terrestris) in Switzerland. The two most common haplotypes of this population, ‘A’ and ‘B’, differ by a single nonsynonymous nucleotide substitution within cytochrome oxidase I. Previous studies have demonstrated that individuals with haplotype B show significantly stronger activity of phenoloxidase, an enzyme that plays an important, temperature-dependent role in the immune function of bumblebees. With this study we wanted to address the following question: Is N. bombi infection of specific host genotypes associated with changing temperatures?
In order to address this, we screened infection by N. bombi queens and the corresponding host mtDNA-haplotype frequencies in 876 bumblebees between 2000 and 2010. We also recorded daily maximum temperatures in spring during those eleven years, and tested for correlations between temperature and infection prevalence of specific host genotypes.
Results/Conclusions
Our long-term observation revealed that N. bombi infection of specific host haplotypes is correlated with prevalence and temperature: We found that with increasing infection prevalence, proportionally more queens with haplotype B (r = 0.590, p = 0.030), but fewer queens with haplotype A were infected (r = ‒ 0.694, p = 0.011). Furthermore, infection of specific host haplotypes is correlated with temperature at different time points. With increasing temperatures more queens with haplotype B were infected (r = 0.608, p = 0.02) and fewer with haplotype A (r = ‒ 0.607, p = 0.026).
These findings suggest that N. bombi infection of specific host genotypes could become affected by climate change. This may be due to differential resilience of host-types to ward off infections. It is plausible that during warmer years, queens who possess haplotype A elicit a comparatively stronger immune response (perhaps due to differential phenoloxidase activity) than those with haplotype B. Consequently, haplotype-A queens may be more resilient to infection during warmer years. These results suggest a relationship between climate change and parasite infection of specific host genotypes, but they need to be investigated further, especially given the considerable variation in haplotype frequencies over space and time.