Comparative analysis of testis transcriptomes in laboratory cohorts of recently diverged allopatric Drosophila nasuta nasuta and Drosophila nasuta albomicans

Expression divergence, primarily defined as the proportion of differentially expressed genes, is one of the key parameters in assessing the evolutionary dynamics of diverging species. Most contributions toward expression differences between species and sexes come from testis tissue, which is the repertoire of rapidly diverging genes that can have a direct impact on speciation. To address the evolutionary dynamics among recently diverged Drosophila species, we analyzed testis transcriptomes of two members of nasuta subgroup of the immigrans species group: Drosophila nasuta nasuta and Drosophila nasuta albomicans. Using RNA-Seq and de novo assembly approach, we found that 96% of the transcriptome exhibited a conserved pattern of gene expression, typical of the recently diverged species'. The 4% of the characterized transcriptomes showing expression divergence had no influence of coding sequence variations. Four of the orthologous testicular genes are under relaxed positive selection, and the remaining 98% (n = 597) exhibit strong purifying selection irrespective of their expression patterns. Overall, our analyses suggest that the recent divergence of this closely related species pair has had a minimal impact on coding sequences and their expression, thereby providing useful insights into early stages of species divergence.