Comparative susceptibility of three Western honeybee taxa to the microsporidian parasite Nosema ceranae.

Genetic diversity of a host species is a key factor to counter infection by parasites. Since two separation events and the beginning of beekeeping, the Western honeybee, Apis mellifera, has diverged in many phylogenetically-related taxa that share common traits but also show specific physiological, behavioural and morphological traits. In this study, we tested the hypothesis that A. mellifera taxa living in a same habitat should respond differently to parasites like Nosema ceranae, a microsporidia living in host's midgut. We used the Poulin and Combes' concept of virulence to compare the susceptibility of three A. mellifera taxa to N. ceranae infection. Three criteria were measured 10 days post-infection (dpi): the host mortality, the host sugar consumption and the development success of the parasite (i.e. number of spores produced). Interestingly, we showed that the observed variation in susceptibility to infection by N. ceranae is not linked to honeybee taxa but results from the variability between colonies, and that those differences are probably linked to genetic variations. The use of these three criteria allows us to conclude that the differences in susceptibility are mediated by a genetic variability in honeybee workers from resistance to tolerance. Finally, we discuss the consequences of our findings for beekeeping management.