Honey bee colonies (Apis mellifera) have been selected for low level of Nosema in Denmark over decades and Nosema is now rarely found in bee colonies from these breeding lines. We compared the immune response of a selected and an unselected honey bee lineage, taking advantage of the haploid males to study its potential impact on the tolerance toward Nosema ceranae, a novel introduced microsporidian pathogen. After artificial infections of the N. ceranae spores, the lineage selected for Nosema tolerance showed a higher N. ceranae spore load, a lower mortality and an up-regulated immune response. The differences in the response of the innate immune system between the selected and unselected lineage were strongest at day six post infection. In particular genes of the Toll pathway were up-regulated in the selected strain, probably is the main immune pathway involved in N. ceranae infection response. After decades of selective breeding for Nosema tolerance in the Danish strain, it appears these bees are tolerant to N. ceranae infections.
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