Similar selective factors affect both between-gene and between-exon divergence in Drosophila

As a consequence of alternative splicing, a gene's exons will have different frequencies of inclusion into mature mRNA and different patterns of expression. These differences affect their patterns of evolutionary divergence. Using the recently reannotated genome of Drosophila melanogaster and the genome sequences of four closely related species of the melanogaster subgroup, we investigated the effect of alternative splicing, inclusion level (defined as the number of transcripts an exon is found in), and expression pattern on exon evolution across divergence times ranging from 1 to 12.5 Ma. Genes undergoing alternative splicing have a broader pattern of expression associated with a lower divergence rate in comparison with genes with a single annotated protein isoform. Within genes undergoing alternative splicing, we report a significant effect of inclusion level on exon evolution, as alternatively spliced exons are less conserved than constitutively spliced exons. More generally, there are significant negative correlations between inclusion level and exon evolutionary rates that can be associated with relaxation of selection. A significant effect of expression pattern on evolution rates is also observed. Overall, we found that similar selective factors such as the expression level and the pattern of expression are affecting both gene and exon evolution.