Drosophila suzukii in diapause?

Drosophila suzukii is native to Japan, but has recently become a major invasive pest of a wide variety of fruit crops. This vinegar fly uses an enlarged, serrated ovipositor to slice into the skin of fresh fruit and lay its eggs inside, where the larvae feed until maturation. Since its introduction to California in 2008, this fly has been rapidly spreading and is now found in over 35 US states, Canada, South America, and Europe. In Oregon, where Drosophila suzukii populations must face harsher winters, we observed that flies collected in the winter are larger in size and darker in abdominal pigmentation when compared to those collected in the summer.  We have successfully recapitulated the phenotypic plasticity in size and pigmentation using simulated summer and winter conditions in the laboratory, and observed that the winter morph showed increased survival at lower temperatures when compared to the summer morph.  We therefore hypothesized that seasonal phenotypic variation in D. suzukii is associated with altered physiology and metabolism, leading to higher survival for overwintering individuals.  To test this hypothesis and evaluate gene expression differences between the two seasonal morphs, transcriptome libraries were prepared from the summer and winter morphs for RNA sequencing.  Interestingly, mRNA transcripts involved in DNA replication and growth were significantly down-regulated in the winter morph, suggesting that they may be entering a winter diapause, while many genes involved in cell metabolism were up-regulated.  This work provides insight into the mechanisms of increased Drosophila suzukii survival during harsh winter conditions.