McClintock and the Ac/Ds Transposable Elements of Corn

Yeast TY Elements

Drosophila Transposable Elements

Drosophila P Elements

Bacterial Transposable Elements

Study Questions

Transposable Genetic Elements WWW Links

Genetic Topics

Drosophila P Elements

One class of Drosophila transposable elements that has generated a lot of interest is the P element class. These are small transposons with terminal 31-bp inverted repeats, and the element generates 8-bp direct repeats of target DNA sequences upon insertion. The complete element is 2907 bp and is autonomous because it encodes a functional transposase. Incomplete members P elements have lost the transposition ability because the transposase has been mutated. But if a complete (autonomous) element exists in the same cell as an incomplete (non- autonomous) element, then the incomplete element can transpose because of the presence of the transposase in the cell. This relationship among complete and incomplete P elements is similar to that of the Ac/DS family in corn.

Some natural populations contain a few P elements, but others contain up to 50 copies. Interestingly, strains captured before 1950 do not contain any P elements. These are called "empty" strains. These strains may represent the primitive condition and P elements may have invaded these natural populations. Close relatives of the lab species D. melanaogaster have maintained the empty condition, but more distantly-related strains acquired the element. It has been suggested that the element entered the species by piggy-backing on viruses that naturally infect Drosophila.

P element movement within the fly is a maternally inherited trait. Movement is suppressed in flies with the P cytotype, but is permissible in the M cytotype. Furthermore, P-cytotype flies contain P elements, whereas M-cytotype elements do not. When movement does occur it happens in the germ line, that is those cells which produce the gametes. This movement of P elements creates many different changes in phenotype, but few are fatal. The progeny are described as being dysgenic which refers to biological deficiencies that are a result of the P element movement. Movement is suppressed in somatic cells where the effects would be more damaging. This condition of germ line abnormalities which generates mutations, chromsosomal breakage and sterility is called P-M hybrid dysgenesis and is mediated by P element movement.

P-cytotype female x M-cytotype maleM-ctyotype female x P-cytotype male
No P element movement; non-dysgenic F1 fliesP element movement; hybrid dysgenic F1 flies
It is currently thought that P elements encode a transposase as well as a repressor of the transposase activity. The expression of the repressor protein is also thought to be a maternally-expressed trait. P-cytotype flies contain large numbers of P elements. Thus, the females would contain high levels of the repressor protein. Therefore, in a cross between a P-cytotype female and a M- cytotype male the high level of repressor expressed by the female would prevent P element movement and hybrid dysgenesis. Conversely, a M-cytotype female does not express the repressor and when they are mated with a P-cytotype female, P element movement can occur and the progeny would be dysgenic.

Copyright © 1997. Phillip McClean