THE SIZE DIFFERENCES AMONG MAMMALIAN INTRONS ARE DUE TO THE ACCUMULATION OF SMALL DELETIONS

In order to investigate the molecular mechanisms that alter intron siz e, we conducted an extensive interspecies comparison of homologous int rons among three mammalian groups: human, artiodactyls, and rodents, T he size differences of introns were statistically significant among al l three groups (longest intron was for human and shortest for rodents) , and appear to be due to the accumulation of small deletions, accordi ng to the separate count of insertion and deletion frequencies, The di stribution of intron size differences also has a shape similar to that for the distribution of insertion/deletion sizes found in pseudogenes . It is suggested that introns are selectively neutral to small-scale changes of the genome size, which inherently contain the bias of favor ing short deletions against short insertions.