INTRON PHASE CORRELATIONS AND THE EVOLUTION OF THE INTRON-EXON STRUCTURE OF GENES

Two issues in the evolution of the intron/ exon structure of genes are the role of exon shuffling and the origin of introns. Using a large d ata base of eukaryotic intron-containing genes, we have found that the re are correlations between intron phases leading to an excess of symm etric exons and symmetric exon sets. We interpret these excesses as ma nifestations of exon shuffling and make a conservative estimate that a t least 19% of the exons in the data base were involved in exon shuffl ing, suggesting an important role for exon shuffling in evolution. Fur thermore, these excesses of symmetric exons appear also in those regio ns of eukaryotic genes that are homologous to prokaryotic genes: the a ncient conserved regions. This last fact cannot be explained in terms of the insertional theory of introns but rather supports the concept t hat some of the introns were ancient, the exon theory of genes.