DE-NOVO INSERTION OF AN INTRON INTO THE MAMMALIAN SEX-DETERMINING GENE, SRY

Two theories have been proposed to explain the evolution of introns wi thin eukaryotic genes, The introns early theory, or ''exon theory of g enes,'' proposes that introns are ancient and that recombination withi n introns provided new exon structure, and thus new genes, The introns late theory, or ''insertional theory of introns,'' proposes that anci ent genes existed as uninterrupted exons and that introns have been in troduced during the course of evolution, There is still controversy as to how intron-exon structure evolved and whether the majority of intr ons are ancient or novel. Although there is extensive evidence in supp ort of the introns early theory, phylogenetic comparisons of several g enes indicate recent gain and loss of introns within these genes. Howe ver, no example has been shown of a protein coding gene, intronless in its ancestral form, which has acquired an intron in a derived form. T he mammalian sex determining gene, SRY, is intronless in all mammals s tudied to date, as is the gene from which it recently evolved. However , we report here comparisons of genomic and cDNA sequences that now pr ovide evidence of a de novo insertion of an intron into the SRY gene o f dasyurid marsupials, This recently (approximately 45 million years a go) inserted sequence is not homologous with known transposable elemen ts. Our data demonstrate that introns may be inserted as spliced units within a developmentally crucial gene without disrupting its function .