NEW DROSOPHILA INTRONS ORIGINATE BY DUPLICATION

We have analyzed the phylogenetic distribution of introns in the gene coding for xanthine dehydrogenase in 37 species, including 31 dipteran s sequenced by us. We have discovered three narrowly distributed novel introns, one in the medfly Ceratitis capitata, the second in the will istoni and saltans groups of Drosophila, and the third in two sibling species of the willistoni group. The phylogenetic distribution of thes e introns favors the ''introns-late'' theory of the origin of genes. A nalysis of the nucleotide sequences indicates that all three introns h ave arisen by duplication of a preexisting intron, which is pervasive in Drosophila and other dipterans (and has a homologous position as an intron found in humans and other diverse organisms).