P element domestication: a stationary truncated P element may encode a 66-kDa repressor-like protein in the Drosophila montium species subgroup.

Functional P transposable elements can be separated into two distinct classes: mobile elements, which present the canonical structure, with transposase and repressor functions, and immobile P sequences truncated in 5' and 3' by loss of the terminal inverted repeats and exon 3, which retain only the repressor function. This second class was first described in some species of the Drosophila obscura group. Here, we describe a new truncated immobile P sequence cloned from one species of the Drosophila montium subgroup (D. tsacasi) that produces a polyadenylated RNA with a coding capacity for a 66-kDa "repressor-like" protein. The results from a number of different comparisons between P-homologous sequences concerning both coding and noncoding regions strongly suggest that the obscura and montium immobile P sequences as well as the T-type P subfamily derive from the same ancestral mobile P element family. Study of the flanking regions of these immobile P sequences shows that the two immobilizations were produced by two independent events. Our results provide evidence that the molecular domestication of a transposable element family may recur in a species lineage.