Effect ofAscosphaera apisinfestation on the activities of four antioxidant enzymes in Asian honey bee larval guts

AbstractAscosphaera apisexclusively infects bee larvae and causes chalkbrood, a lethal fungal disease that results in the sharp reduction in adult bees and colony productivity. However, little is known about the effect ofA. apisinfestation on the activities of antioxidant enzymes in bee larvae. Here,A. apisspores were purified and used to inoculate Asian honey bee (Apis cerana) larvae, followed by detection of the host survival rate and evaluation of the activities of four major antioxidant enzymes. At 6 days post inoculation (dpi) withA. apisspores, white mycelia penetrated the posterior end of the larva, extended to the anterior end, and eventually covered the entire larval body surface, presenting an obvious symptom of chalkbrood disease similar to that occurs inApis melliferalarvae. Additionally, PCR identification showed that the expected fragment was amplified from theA. apis-inoculated larval guts and theA. apisspores, verifying theA. apisinfection ofA. ceranalarvae. The survival rate of larvae inoculated withA. apiswas high at 1–2 dpi, sharply decreased to 4.16% at 4 dpi, and reached 0% at 5 dpi; whereas that of un-inoculated larvae was always high at 1~8 dpi, with an average survival rate of 95.37%, indicating the negative impact ofA. apisinfection on larval survival. Furthermore, in comparison with those in the corresponding un-inoculated groups, the superoxide dismutase (SOD) and catalase (CAT) activities in the 4-day-old larval gut in theA. apis-inoculated groups were reduced (p> 0.05), while those in the 5- and 6-day-old larval guts were significantly decreased (p< 0.05); the glutathione S-transferase (GST) activity in the 4- and 5-day-old larval guts was significantly increased (p< 0.05), while that in the 6-day-old larval gut was reduced (p> 0.05); the polyphenol oxidase (PPO) activity in 4-day-old larval gut was increased (p> 0.05) and that in the 5-day-old larval gut was significantly increased (p< 0.05), whereas that in the 6-day-old larval gut was significantly reduced (p< 0.01). These results together suggested that the activities of SOD and CAT in the larval guts were suppressed during the process ofA. apisinfestation, while the GST activity was induced to activation, and the PPO activity was first enhanced and then inhibited. Our findings not only unravel the response ofA. ceranalarvae toA. apisinfestation from a biochemical perspective, but also offer a valuable insight into the interaction between Asian honey bee larvae andA. apis.