Adaptive genome defense mechanisms in the ovarian somatic cell niche silences ancient infectious retroelements

Abstract

Transposable elements (TEs) are kept silent in the germline of most animals by the PIWI interacting RNA (piRNA) pathway. The piRNA pathway is a form of adaptive immunity and has been proposed to be in an 'evolutionary arms race' with the TE elements that they silence. The best example of this, are Ty3/gypsy retrotransposons in Drosophila melanogaster, that acquired env proteins, called errantiviruses. These errantiviruses are expressed in the ovarian somatic cell niche and utilize their membrane fusion proteins to enter the adjacent germline cells, thus avoiding the highly conserved germline piRNA pathway. As an adaptation to this, Drosophila melanogaster have duplicated piRNA biogenesis factors, and have started to express piRNAs targeting these elements in the somatic cell niche as well. This thesis will establish three main points. Firstly, rapid divergence of piRNA biogenesis, and loss of piRNA biogenesis factors, does not impact efficient transposon silencing in Drosophila. Secondly, errantiviruses and env containing retrotransposons are ubiquitous, and actively transposing in almost every metazoan branch. Finally, somatic transposon defence mechanisms are highly conserved to silence errantiviruses, and they have likely existed in the same cell niche over long periods of time. Show more