Figure 1.

Young Drosophila genes are strongly biased for expression in the male reproductive system. (A) Example of a young Drosophila gene previously assessed as lethal by RNAi assays (Chen et al. 2010) that exhibits highly restricted expression in the testis, with minor amounts detected in other libraries containing the male reproductive system (e.g., dissected accessory gland, whole male bodies, and unsexed larval/pupal material that contains gonads). (B) We revised the catalog of recently evolved genes in D. melanogaster, doubling their numbers from previous studies. (C) We divided D. melanogaster genes according to evolutionary age and calculated their tissue specificity index (τ) across 93 developmental, tissue, and cell line RNA sequencing (RNA-seq) conditions (195 data sets). As gene age decreases, they tend to become more spatiotemporally restricted. Global median τ (0.61) is indicated for reference; note that the τ distribution is actually highly bimodal (Supplemental Fig. 5). (D) The tissue/stage/cell library of maximal expression (Xmax) was plotted for genes with spatially restricted expression (τ > 0.61) in each evolutionary category. Higher τ of young genes is associated with a prominent trend for expression in the male reproductive system (testis and/or accessory gland). This male restriction is substantially elevated among genes born in the pan-Drosophilid ancestor and increases in cohorts of younger genes born along the lineage toward D. melanogaster.