Transposable element proliferation as a possible side effect of endosymbiont manipulations

Abstract

The mode of reproduction has been predicted to affect the proliferation of transposable elements (TEs). A population that switches from sexual to asexual reproduction could either accumulate TEs because purifying selection becomes less efficient, or a decrease in TE load because the opportunity for horizontal transmission is reduced. A third possibility is that the mechanism that induces asexual reproduction affects TE dynamics as a side effect. We propose two such mechanisms that might explain recently described patterns of TE abundance in sexual and asexual lineages of the parasitoid wasp Leptopilina clavipes. Asexual reproduction in this species is induced by endosymbiotic Wolbachia bacteria. In order to achieve parthenogenesis in its host, Wolbachia might remove methylation or interfere with Argonaute proteins. Both methylation and Argonaute proteins are known to control TE activity in other species. By interfering with either, Wolbachia might therefore secondarily hamper the control of specific TEs.

Keywords: Argonaute; asexual reproduction; methylation; sex; transposable elements.

Figure 1. (A) Cartoon of a normal dividing cell with meotic/mitotic spindle at the top. Complexes of Argonaute proteins (green) and antisense TE fragments capture and destroy TE mRNAs. (B) Dividing cell infected with Wolbachia. Wolbachia (brown) associate with microtubuli (top) and capture Argonaute proteins (green). TE derived mRNAs are left to insert back into the host genome.

Figure 2. Cartoon of chromosomal region in female germline. (A) The sex determination gene tra is methylated as well as a nearby gypsy element. (B) In order to induce female development, Wolbachia produces a demethylase that removes methyl groups. (C) In the absence of methylation, both tra and gyspsy can be transcribed.