A P element has induced intron formation in Drosophila

We report evidence supporting the hypothesis that some introns could be originated from transposable elements. In the Drosophila montium species subgroup, we recently described a novel example of domestication by the host genome of a P transposable element. The element is a unique truncated P sequence transcribed into a polyadenylated RNA encoding a putative 66-kDa transposition repressor-like protein. Here, we analyze the genomic modifications associated with this transition of a transposable element into a stationary gene that is useful for the host. Study of the transcription modalities of this neogene reveals that the new transcriptional unit harbors a de novo synthesis of a new exon and a new intron upstream of the original P sequence initiation site. The new exon was constructed from the genomic flanking sequence of the P sequence, whereas the first half of the new intron is composed of genomic flanking sequence and the second half is composed of P sequence. This domestication event has involved the capture of a new promoter. An investigation of a large number of species belonging to the melanogaster species group revealed that this P element domestication is restricted to the species of the montium subgroup and that the new exon-intron structure is present in at least three other species. From sequence data, we hypothesize that cryptic acceptor and donor splicing sites present on the P element and flanking sequences have been under selective constraints which have led to the emergence of a new intron.