Mosquito Transposable Elements and piwi Genes

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Virginia Tech

Vector control is an essential and effective approach for controlling transmission of vector-borne diseases. However, increasing resistance to insecticide and drugs suggests that new strategies to control vector-borne diseases are needed. One possible strategy involves replacing mosquito populations with disease-resistant transgenic mosquitoes. Transposable elements (TEs) are an important component in this new strategy due to their ability to integrate exogenous DNA into chromosomes. They could potentially be useful tools in assisting the spread of disease-resistant genes in mosquito populations.

This research focuses on two related subjects, TEs and their regulation. The first subject is on a Long Terminal Repeat (LTR) retrotransposon in the African malaria mosquito, Anopheles gambiae, namely Belly. The second subject focuses on the characterization of piwi genes in the dengue and yellow fever mosquito, Aedes aegypti.

For the first subject we characterized Belly by identifying the two identical LTRs and one intact open reading frame. We also defined the target site duplications and boundaries of the full-length Belly element. This novel retrotransposon has nine full-length copies in the An. gambiae sequenced genome and their nucleotide similarity suggests that there has been fairly recent retrotransposon. We have shown that Belly is transcribed and translated in An. gambiae. Single LTR circles were recovered from An. gambiae cells, which is consistent with active transposition of Belly.

The second subject focuses on the piwi genes of Ae. aegypti. We found nine potential piwi genes in Ae. aegypti and two in An. gambiae. Phylogenetic analysis suggests that these piwis formed two subgroups and gene duplication within each group occurred after the divergence between the two mosquito species. RT-PCR and transcriptome analysis showed Ago3 as well as all the seven tested piwi genes were expressed either in germline tissues or developing embryos. Differential expression patterns were observed. While most piwis were transcribed in the ovaries, testis, and embryos, two piwis appear to have a zygotic expression. Three piwi genes (piwi 3, piwi 4, and Ago3) were also detected in adult somatic tissues of Ae. aegypti. The expansion of the number of piwi genes in Ae. aegypti compared to An. gambiae and D. melanogaster may be correlated with a larger genome size and greater amount of TEs. The finding of piwi expression in adult somatic tissues is intriguing. It is possible that these piwi genes were expressed in the adult stem cells. It is also possible that they may be involved with anti-viral defense. Both of these hypotheses require further testing.

piwi, Transposable elements, RNAi, Belly