Profile of alfalfa leafcutting bee (Megachile rotundata) gene expression and immune response at different temperatures

The significant population losses of the alfalfa leafcutting bees (Megachile rotundata) due to chalkbrood (a disease caused by Ascosphaera aggregate) has led to an interest in bee immunity. Alfalfa leafcutting bees are known to express genes found in the Toll and Imd signaling pathways in response to fungal infection. However, little is known about immune response gene expression patterns when the host is exposed to different environment stresses that might affect their susceptibility to disease. In this study, an oligonucleotide macroarray was designed to test the gene expression patterns of healthy and pathogen-exposed larvae at different temperatures (20, 25, 30, and 35°C). The difference in gene expression levels between healthy and pathogen-exposed larvae were greatest at the highest temperature (30° and 35°C). Prophenyloxidase (PPO); melanization and coagulation; Toll, Imd and JNK pathways; lysosomal enzymes; lipases and other effectors; and iron metabolism are all known to be associated with immune responses in insects, were most highly expressed at 35°C for healthy larvae, but 30°C for pathogen exposed larvae. In addition, genes associated with metabolism and transcription were most highly expressed at 35°C in healthy bee larvae, while genes associated with protein synthesis were most highly expressed at 25°C and 30°C. These genes were greatly down-regulated at 30°C in pathogen-exposed larvae. We clearly found different expression patterns at different temperatures, and between pathogen-exposed and healthy larvae. Further work is needed to determine how temperature, immunity, and pathogen infection processes all interact within the host before we fully understand why higher infection levels occur at lower temperatures.