The impact of diapause and stress on transposable element silencing in two agricultural bees

Diapause is a complex physiological phenomenon that allows insects to weather stressful environmental conditions. The regulation of diapause is accordingly complex, including signaling pathways that involve both small RNA and mRNA and affect the cell cycle, stress resistance, and developmental timing. Transposable elements, mobile genetic elements that replicate within the genome, are also thought to be stress responsive and regulated by the small RNA pathway. Therefore, we asked what the relationship was between environmental stress, diapause status, and transposable element expression in two species of agriculturally important bees, Megachile rotundata and Osmia lignaria. We characterized the TE content of the genomes of both species, then evaluated the expression of TEs during temperature stress, general environmental stress, and diapause stage. We found that the genomic TE content of the two species was very different, and M. rotundata has higher copy number TEs compared to O. lignaria. We also found that both diapause stage and temperature stress had large effects on TE expression. The fold change of TEs tended to be larger in those expressed during diapause, however there was only a small majority that were upregulated during diapause. This suggests that stress and diapause do not break down to a simple up-regulation or down-regulation of TEs, but rather that the TE, the genomic position of its insertions, and the exact heterochromatin formation of the organism at any given environmental state or life stage may affect overall expression of TEs.