Recurrent Exon Shuffling Between Distant P-Element Families

Two independent stationary P-related neogenes had been previously described in the Drosophila obscura species group and in the Drosophila montium species subgroup. In Drosophila melanogaster, P-transposable elements can encode an 87 kDa transposase and a 66 kDa repressor, but the P-neogenes have only conserved the capacity to encode a 66 kDa repressor-like protein specified by the first three exons. We have previously analyzed the genomic modifications associated with the transition of a P-element into the montium P-neogene, the coding capacity of which has been conserved for around 20 Myr ( Nouaud, D., and D. Anxolabéhère. 1997. Mol. Biol. Evol. 14:1132-1144). Here we show that the P-neogene of some species of the montium subgroup presents a new structure involving the capture of an additional exon from a very distant P-element subfamily. This additional exon is inserted either upstream or downstream of the first exon of the P-neogene. As a result of alternative splicing, these modified neogenes can produce, in addition to the repressor-like protein, a new protein which differs only by the NH2-terminal region. We hypothesize that this protein diversity within an organism results in a functional diversification due to the selective advantage associated with the domestication of the P-neogene in these species. Moreover, the autonomous P-element which provides the additional exons is still present in the genome. Its nucleotide sequence is more than 45% distant from the previously defined P-type element (M-type, O-type, T-type) and defines a new P-type element subfamily referred to as the K-type.