Selective Constraints on Intron Evolution in Drosophila

Intron sizes show an asymmetrical distribution in a number of organisms, with a large number of "short" introns clustered around a minimal intron length and a much broader distribution of longer introns. In Drosophila melanogaster, the short intron class is centered around 61 bp. The narrow length distribution suggests that natural selection may play a role in maintaining intron size. A comparison of 15 orthologous introns among species of the D. melanogaster subgroup indicates that, in general, short introns are not under greater DNA sequence or length constraints than long introns. There is a bias toward deletions in all introns (deletion/insertion ratio is 1.66), and the vast majority of indels are of short length (