Variation and heritability in immune gene expression by diseased honeybees

Social insects are frequent targets for pathogens and have consequently evolved diverse ways to minimize disease impacts, one of which is the innate immune response. Here, a 4-generation mating scheme was carried out to assess heritability and variation in a honeybee (Apis mellifera) immune trait, the production of the key antimicrobial peptide abaecin. Larval offspring from controlled crosses (n = 576 offspring from 36 singly inseminated queens) as well as offspring of field colonies (896 individuals in 53 colonies) were challenged individually with a widespread bee pathogen, the gram-positive bacterium Paenibacillus larvae. After bacterial challenge, transcript levels for the gene encoding abaecin were quantified and then compared using known pedigrees and colony environments. Considerable variation among highly related siblings (r = 0.75) indicates that subtle allelic differences in immune pathway genes can have large effects on transcriptional profiles. Abaecin levels were moderately heritable (h(2) = approximately 0.3-0.4), reflecting high amounts of standing genetic variation, and suggesting that this and other immune traits are amenable to selective programs aimed at improving honeybee health. The results help efforts to determine the relative effectiveness of social versus individual defenses by social insects toward their pathogens.