Jumping genes and transposable elements

Transposable Elements and Horizontal Gene Transfer

Jumping Genes

Hybrid dysgenesis in fruit flies of the genus Drosophila results when males carrying transposable elements (TEs) mate with non-carrier females (reviewed by Blauth et al., 2009). This syndrome results in a multitude of defects, including sterility and gonadal atrophy, and highlights one of the many roles transposable elements play in evolution.

The best characterized TE in Drosophila is the P. element, which has been exploited for decades by molecular biologists as a vehicle for inserting genetic material into laboratory fruit flies during functional studies. P elements have been discovered in invertebrates, vertebrates, fungi, and green algae and represent 1 out of 12 cut and paste, DNA TE superfamilies that have been identified so far (reviewed by Feschotte and Pritham, 2007). The P. element "jumps" around (transposition) in the Drosophila genome in a cut and paste fashion as a DNA element, and is therefore a class II TE (reviewed by Blauth et al., 2009). The P. element contains sequence that produces at least two proteins, including an 87-kDa transposase enzyme that facilitates transposition and a 66-kDa repressor protein that prevents transposition from occurring. Alternative splicing of the third intron determines whether transposase or the repressor is expressed. The expression of these P. element-encoded proteins is developmentally regulated, with transposase expressed only in germline cells and the repressor in all other cells types.

P element insertions have been found within the D. melanogaster cosmopolitan species, but not in other species within the melanogaster subgroup. This suggests P. elements recently invaded this subgroup by horizontal gene transfer, probably from a related species (reviewed by Blauth et al., 2009). When the DNA sequence of the 2900 base pair P. element was compared between D. melanogaster and D. willistoni, only one nucleotide was different. Since P. elements are more widespread in the genomes of D. willistoni than in melanogaster, this finding supports the possibility that the willistoni subgroup is the donor for D. melanogaster. Current estimates suggest the horizontal gene transfer event occurred about 200 years ago and has since been distributed around the world.

TE transposition events have the potential to be catastrophic to their host organism, and therefore must be tightly regulated to prevent not only the extinction of the host but also the parasitic element. Accordingly, the P. element has evolved mechanisms to restrict when and where transposition occurs. One example is the tissue-specific control of third intron alternative splicing that was discussed above. Intronic insertions and deletions also play a role in suppressing P. element transposition (reviewed by Blauth et al., 2009). Possible mechanisms for repressor activity include sequestering transposase in multimeric form, thus rendering the enzyme inactive, or through the production of short polypeptides that interfere with P. element transcription. Other regulatory mechanisms have been suggested, including the presence of a conserved sequence that mediates subcellular sequestration, epigenetic control, and repression by small interfering RNA via the small antisense RNA proteins AUBERGINE and PIWI.

Rationale and Methods

To further investigate the mechanisms by which P. element expression is controlled during D. willistoni and D. melanogaster embryogenesis, the transposase and repressor expression patterns were compared between these two strains using RT-PCR (reverse transcriptase polymerase chain reaction) and DNA sequencing (Blauth et al., 2009). The D. willistoni Wip strain has a P. element inserted into a heterochromatic (transcriptionally inactive) region of the genome, while the D. willistoni 17A2 strain has a P. element inserted into a euchromatic (transcriptionally active) region of the genome. This study with therefore provide a survey of P. element transposase and repressor expression patterns among these different strains.

Results

Surprisingly, only the D. willistoni strains expressed a full length P. element transcript. Although both the repressor and transposase transcripts were expressed, the concentration varied drastically between the two strains. The RT-PCR products from D. melanogaster embryos migrated at unexpected positions in agarose gels and sequencing revealed duplications and low sequence conservation. RT-PCR amplification using sense primers revealed the presence of possible P. element antisense RNA molecules in both D. melanogaster and D. willistoni.

Conclusions

The findings of this study suggest multiple mechanisms may be involved in regulating P. element mobility during fruit fly embryogenesis, including the expression of repressor proteins and anti-sense RNA molecules, and a repressive chromatin state. The detection of both transposase and the repressor transcripts by RT-PCR is probably due to using RNA preparations from whole embryos that contain both germ cells and somatic cells.

Discussion