It takes two transposons to tango: transposable-element-mediated chromosomal rearrangements.

Transposable elements (TEs) promote various chromosomal rearrangements more efficiently, and often more specifically, than other cellular processes(1-3). One explanation of such events is homologous recombination between multiple copies of a TE present in a genome. Although this does occur, strong evidence from a number of TE systems in bacteria, plants and animals suggests that another mechanism - alternative transposition - induces a large proportion of TE-associated chromosomal rearrangements. This paper reviews evidence for alternative transposition from a number of unrelated but structurally similar TEs. The similarities between alternative transposition and V(D)J recombination are also discussed, as is the use of alternative transposition as a genetic tool.

Structure and associated mutational effects of the cysteine proteinase (CP1) gene of Drosophila melanogaster.

The complete structure of the cysteine proteinase (CP1) gene reveals two large 5' introns as well as a small third intron. Deletion studies have shown that null mutations for the locus are female sterile with partial male sterility as well as wing and pigmentation effects. Null alleles can be produced by either deletions to the left or deletions to the right of a P element insertion in the long second intron of the gene. A nearby phenylalanyl tRNA synthetase gene (Pts) was also identified.

The accumulation of P-element-induced recombinants in the germline of male Drosophila melanogaster.

P-element-induced recombination in Drosophila melanogaster occurs premeiotically. Recombinants are therefore expected to accumulate in the stem cells of the germline of P-element-carrying males. We show that both the recombination frequency and the incidence of "clustering" increase with the age of males carrying various P-element derivatives. The combination of end-deleted elements can lead to average recombination frequencies >50% with individual instances of 100% recombination. These elements also lowered the fertility of the carriers. We investigated these features by constructing an analytical and a computer simulation model of the course of events in the germline, incorporating the recently proposed hybrid element insertion (HEI) model of P-element activity. The model is able to predict extreme recombination levels, segregation ratio biases and lowered fertility through cell death in a single analysis.

P-element-induced recombination in Drosophila melanogaster: hybrid element insertion.

It has previously been shown that the combination of two deleted P elements in trans, one containing the left functional end and the second element the right functional end, can lead to high levels of male recombination. This finding strongly suggests that P-element ends from different chromosomes can become associated, followed by "pseudo-excision". We show that two different processes are involved in resolving the pseudo-excision event: (1) the excised P-element ends continue to function as a single unit (Hybrid Element) and insert at a nearby site in the chromosome or into the element itself [Hybrid Element Insertion (HEI)] and (2) free ends that do not contain P elements repair and rejoin [(Hybrid Excision and Repair (HER)]. Both types of resolution can lead to recombination, and this paper concentrates on the HEI class. One type of HEI event predicts the exact reverse complementary duplication of an 8-bp target site, and we have confirmed the existence of such a structure in six independently derived recombinant chromosomes. There is also a high tendency for insertion events to occur within a few bases of the original 8-bp target site, including six apparent cases of insertion into the exact site.