2 Results and Discussion Statistical Features Identied from Comparison of Homologous Introns

In order to develop an understanding of the evolutionary history of introns, we compared intron sequences from homologous genes of di erent vertebrate species. We observed the following statistical features of introns. From interspecies comparisons, the length difference of an intron from each gene exhibits considerable variations, which is in consistent with the previous observation on the wide distribution of insertion and deletion sizes. The distribution pro les also indicate that 68 % of introns are longer in human than in rodentia, although such a signi cant shift in the intron length is not observed in mouse/rat and human/artiodactyla comparisons. From the analysis of diversities in base compositions and sequence data between intron and coding regions, a similar behavior was observed between intron and the third codon position, although intron sequences appeared to be more constrained. Those descriptive analysis will help us to understand the functional constraints and the recent history of introns.