Environmental history impacts on gene expression during diapause development in Megachile rotundata

The alfalfa leafcutting bee, Megachile rotundata, is the world’s most intensely managed solitary pollinator and the primary pollinator of alfalfa in North America. Megachile rotundata emerges from overwintering in late June or early July. Under managed systems, the females construct nests in artificial nest blocks consisting of preformed linear cavities. The females make a series of brood cells within each cavity. Once a cell is completed and provisioned, she will lay a single egg, seal that cell, and then start the construction of the next cell. The larvae will complete development within the cell and either enter diapause as a prepupae or continue development to the adult stage and emerge. The progeny of the second generation adults enter diapause in August or September. These two cohorts (those entering diapause in June/July and those entering diapause in August/September) have very different thermal histories. The prepupae that enter diapause early in the field season are subjected to summer temperatures whereas those that diapause late in the season experience only autumn temperatures before the onset of winter. The objective of this investigation was to determine the degree to which environmental history impacts gene expression during the course of diapause development in M. rotundata. To achieve our objective, early and late diapausing prepupae were collected, and in the late fall, the early and late diapausing groups were further separated into a group that remained outdoors in ambient conditions and a group maintained under cool, constant laboratory conditions. RNAseq was performed on bees sampled in the fall, winter and following spring. Differential expression analysis indicated that differences in thermal history resulted in substantial changes in transcriptional regulation of diapause.