Intron size and exon evolution in drosophila

We have found a negative correlation between evolutionary rate at the protein level (as measured by [d.sub.N]) and intron size in Drosophila. Although such a relation is expected if introns reduce Hill-Robertson interference within genes, it seems more likely to be explained by the higher abundance of cis-regulatory elements in introns (especially first introns) in genes under strong selective constraints.