Effects of host age on susceptibility to infection and immune gene expression in honey bee queens (Apis mellifera) inoculated with Nosema ceranae

Nosema ceranae is a microsporidium parasite infecting honey bees worldwide. All colony members including workers, drones, and queens can become infected. In this study, we inoculated queens of age 1, 6, and 12 days post-adult emergence, with N. ceranae spores of different doses and allowed them to age an additional 12 days. The results indicated that younger queens were indeed more susceptible to N. ceranae inoculation than older queens. Moreover, this is the first report of the effects of N. ceranae inoculation on immune gene expression in queens of different ages. Our results demonstrated that the expression of genes related to the bee immunity apidaecin, eater, and vitellogenin in the gut and the remaining abdomen was different among queens of different ages when inoculated with N. ceranae. All three ages of queens inoculated by N. ceranae showed upregulation of apidaecin in gut tissue 6 days after inoculation, but only in queens aged 1 day post-emergence were the differences significant. However, transcript levels of eater were increased in all three ages of queens when sampled on day 12, and significant differences were obtained in queens inoculated at 6 and 12 days post-emergence. We clearly show that immune responses to N. ceranae changes as queen age and this knowledge may provide clues for understanding the ability of queens to resist infection by this gut parasite.