Parallel transcriptome adaptation during recent evolution of the Drosophila testis.

Understanding how natural selection shapes gene expression during species expansion remains a central question in evolutionary biology. Parallel evolution, defined as the independent emergence of similar phenotypes in different species or populations in response to similar selective pressures, provides a powerful framework for inferring the role of selection. Drosophila melanogaster has been a focal model system for investigating genomic and phenotypic variation driven by selection, whereas its sister species, D. simulans, has been largely understudied. D. melanogaster originated in sub-Saharan Africa and experienced a major out-of-ancestral range expansion approximately 13,000 years ago, dispersing into Europe, and then colonizing the Americas and Australia only very recently. D. simulans is thought to have originated in Madagascar, although the exact timing of its global expansion remains unclear; it, too, is thought to have very recently colonized the Americas and Australia. Here, we present the first comparative continent-scale transcriptomic analysis of the male reproductive tract, testis and accessory gland, in these 2 species, including ancestral range populations for both, to investigate the role of migration and selection in shaping gene expression variation on a global scale.