Parasite infection of specific host genotypes relates to changes in prevalence in two natural populations of bumblebees.

The antagonistic relationship between parasites and their hosts is strongly influenced by genotype-by-genotype interactions. Defense against parasitism is commonly studied in the context of immune system-based mechanisms and, thus, the focus in the search for candidate genes in host-parasite interactions is often on immune genes. In this study, we investigated the association between prevalence of parasite infection and host mitochondrial DNA (mtDNA) haplotypes in two natural populations of bumblebees (Bombus terrestris). The two most common haplotypes of the host populations, termed A and B, differ by a single nonsynonymous nucleotide substitution within the coding region of cytochrome oxidase I, an important player in metabolic pathways. We screened infection by Nosema bombi, a common endoparasite of bumblebees, and the corresponding host mtDNA-haplotype frequencies in over 1400 bumblebees between 2000 and 2010. The island population of Gotland showed lower mtDNA diversity compared to the mainland population in Switzerland. Over time, we observed large fluctuations in infection prevalence, as well as variation in host haplotype frequencies in both populations. Our long-term observation revealed that N. bombi infection of specific host genotypes is transient: We found that with increasing infection prevalence, proportionally more individuals with haplotype B, but fewer individuals with haplotype A were infected. This suggests that the presence of N. bombi in specific host genotypes relates to infection prevalence. This may be a result of parasite competition, or differential resilience of host types to ward off infections. The findings highlight the important role of host mtDNA haplotypes in the interaction with parasites.