Transposable elements are evolutionary cousin of viruses. They make copies and insert themselves into another locus in the genome.
As such, organisms have developed defence mechanism to counteract them. The Piwi-interacting RNA (piRNA) pathway is a highly conserved such defence system in metazoan.
piRNAs are 24 - 32nt long small RNA and silence transposon mRNA with PIWI-clade Argonaute proteins. The piRNA pathway is predominantly active in the germline where majority of transposons are active. However, piRNA pathway is also active in the ovarian somatic cells of Drosophila species because of one type of retrotransposons called gypsy.
The prototypical gypsy retrotransposon only carries gag and pol genes while one class of gypsy retrotransposons in Drosophila carries an additional ORF called envelope.
The envelope gene resembles those from other insect DNA viruses and it allows the transposon to transmit across the cell membrane.
The somatic piRNA pathway is distinct from its germline counterpart, especially in the mechanism of piRNA biogenesis, and has only been studied in Drosophila.
However, by an in silico survey, we found that envelope-gypsy retrotransposons are widespread in Dipteran insect species outside Drosophila. We speculate that gypsy retrotransposons have been horizontally transferred across different genera in >100 million years of Diptera evolution.
But, the piRNA pathway cannot be horizontally transferred unlike transposons. Is the somatic piRNA different between species that acquired envelope-gypsy at different times in evolution? If so, are there any common features?
We characterised the genomic distribution of gypsy retrotransposons and the somatic piRNA pathway of a variety of Dipteran species to answer this question.
Species we studied include Sheep blowflies, Queensland fruit flies, hover flies, black soldier flies, and Maralia mosquitoes.