Abstract
AbstractHoneybees require a functioning immune system to defend against microbial pathogens. The American foulbrood pathogen,Paenibacillus larvae, is lethal to honeybees and one of the main causes of colony collapse. This study investigated the immune responses ofApis melliferaandApis ceranahoneybees against the bacterial pathogenP. larvae. Both species of honeybee larvae exhibited significant mortalities even at 102∼ 103cfu/mL ofP. larvaeby diet-feeding, althoughA. melliferaappeared to be more tolerant to the bacterial pathogen thanA. cerana. Upon bacterial infection, the two honeybee species expressed both cellular and humoral immune responses. Hemocytes of both species exhibited characteristic spreading behaviors by cytoskeletal extension along with F-actin growth, and formed nodules uponP. larvaeinfection. Larvae of both species also expressed an antimicrobial peptide called apolipophorin III (ApoLpIII) in response to bacterial infection. However, these immune responses were significantly suppressed by a specific inhibitor to phospholipase A2(PLA2). Each honeybee genome encodes four PLA2genes (PLA2A∼PLA2D), representing four orthologous combinations between the two species. In response toP. larvaeinfection, both species significantly up-regulated PLA2enzyme activities and the expression of all four PLA2genes. To determine the roles of the four PLA2s in the immune responses, RNA interference (RNAi) was performed by injecting gene-specific double stranded RNAs (dsRNAs). All four RNAi treatments significantly suppressed the immune responses, and specific inhibition of the two secretory PLA2s (PLA2AandPLA2B) potently suppressed nodule formation andApoLpIIIexpression. These results demonstrate the cellular and humoral immune responses ofA. melliferaandA. ceranaagainstP. larvae. This study suggests that eicosanoids play a crucial role in mediating common immune responses in two closely related honeybees.
Publisher
Cold Spring Harbor Laboratory