Adaptive gene expression parallelism in the male reproductive tract of two Drosophila species.

While stabilizing selection is likely an important process leading to conserved phylogenetic patterns of gene expression, the role of selection in driving expression divergence among populations and species is much less clear. One approach for identifying adaptation is to document parallel evolution, the independent evolution of similar phenotypes in multiple species in response to similar selective pressures. Latitudinal clines are a classic system for studying adaptation in many species, including Drosophila; multiple species exhibit clines for several phenotypes, such as body and wing size. However, the extent of latitudinal transcriptome variation and the degree to which such variation is shaped by selection remain unclear. Here, we investigate transcriptomes of North American Drosophila melanogaster and Drosophila simulans with a focus on the male reproductive tract. For both species, we sampled accessory glands and testis from lines derived from 2 locations, 1 low latitude (Panama City, Panama), and 1 high latitude (Maine, USA). We observed a striking similarity between species in the directionality and magnitude of latitudinal expression variation in the accessory gland but not in the testis. This suggests that selection has fine-tuned accessory gland transcript abundance in a similar manner in response to latitudinal selection pressures in both species. In addition to gene level parallelism, these species exhibit correlated fluctuations of low vs high latitude expression differences on a larger chromosomal scale. Analysis of whole male transcriptomes from the same population samples suggests that parallel latitudinal selection responses play an important role in expression adaptation for both species.