McClintock and the Ac/Ds Transposable Elements of Corn
Yeast TY Elements
Drosophila Transposable Elements
Drosophila P Elements
Bacterial Transposable Elements
Transposable Genetic Elements WWW Links
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
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.
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.
|P-cytotype female x M-cytotype male||M-ctyotype female x P-cytotype male
|No P element movement; non-dysgenic F1 flies||P element movement; hybrid dysgenic F1 flies|
Copyright © 1997. Phillip McClean