Three-partner conversion induced by the P-element transposase in Drosophila melanogaster

Abstract

Conversion of one P-derived transposon into another has already been shown to occur with a measurable frequency. However, the mechanism responsible for such replacements has remained controversial. We previously proposed a mechanism involving three partners. We assumed that after excision of the P-element inserted at the target site, the double-strand break was repaired using, first, the homologous P sequences on the sister chromatid, and second, a remote template, the donor P-derived transposon. However, two other mechanisms have been proposed. The first involves two partners only, the broken end and the remote template, while the second involves transposition of the donor into the target P-element, followed by a double recombination event. Here we describe the conversion of a defective P-element using as a remote template an enhancer-trap element that is itself unable to transpose because it lacks 21 bp at its 5' end. This result makes it possible to exclude the possibility that this conversion event occurred after transposition. The new allele was molecularly and genetically characterized. The occurrence of a polymorphism at position 33 of the P-element sequence and of an imperfect copy of the template on the 3' side of the converted transposon confirmed that the sister chromatid was absolutely necessary as a partner for repair. Our results show that targeting of a marked P-element is possible, even when this element is unable to transpose. This provides a means of improving recovery of conversion events by eliminating unwanted transpositions catalyzed by the P transposase.

The evolutionary life history of P transposons: from horizontal invaders to domesticated neogenes

“…At least four distinctive exit strategies to avoid silencing-induced sequence erosion are known: (1) the resurrection of silenced transposons by means of interelement selection and ectopic recombination between distinct subfamily members (Jordan and McDonald 1998), (2) the reactivation of silenced genomic parasites by stochastic loss of host-encoded suppressor alleles (Pelisson et al 1994;Prud'homme et al 1995), (3) the escape via horizontal transfer into the naive genome of an unprotected host species (Flavell 1999), and (4) molecular domestication see below). Recombination between different elements has been observed recently, first in retrotransposons (Jordan and McDonald 1998;Saxton and Martin 1998), retroviruses such as HIV (Gao et al 1998;Laukkanen et al 2000), and also in DNA transposons (Miller et al 1999;Peronnet et al 2000). Sequence analyses of genomic parasites derived from yeast, Drosophila, mouse, and humans have shown that new active copies were generated by ectopic recombination between different variants present in the genome.…”

“…Hybrid elements of DNA transposons were detected recently. In D. melanogaster the resurrection of an active canonical P element through recombination of defective copies has been described by Peronnet et al (2000). In the obscura group, sequence comparisons indicate a hybridogenic origin of the T-type P elements (Miller et al 1999).…”