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 dipterans sequenced by us. We have discovered three narrowly distributed novel introns, one in the medfly Ceratitis capitata, the second in the willistoni and saltans groups of Drosophila, and the third in two sibling species of the willistoni group. The phylogenetic distribution of these introns favors the "introns-late" theory of the origin of genes. Analysis of the nucleotide sequences indicates that all three introns have 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).