Mobile genetic elements and sexual reproduction

Abstract

Transposable elements (TE) are prominent components of most eukaryotic genomes. In addition to their possible participation in the origin of sexual reproduction in eukaryotes, they may be also involved in its maintenance as important contributors to the deleterious mutation load. Comparative analyses of transposon content in the genomes of sexually reproducing and anciently asexual species may help to understand the contribution of different TE classes to the deleterious load. The apparent absence of deleterious retrotransposons from the genomes of ancient asexuals is in agreement with the hypothesis that they may play a special role in the maintenance of sexual reproduction and in early extinction for which most species are destined upon the abandonment of sex.

Deleterious transposable elements and the extinction of asexuals

“…Some other parasitic protists may lack DTEs but without more information on the degree of genome sequence completeness, the deleterious nature of the elements that they contain, the possibility of facultative or cryptic sex, and the degree of inbreeding, no clear generalization emerges. (89) Moreover, simple comparisons of DTE content between related outbreeding and inbreeding species may have limited relevance to whether their proliferation might proceed unchecked in asexuals, in part because mutational selection, owing to dominance effects, may be stronger but selection due to ectopic exchange weaker in inbreeders than in outbreeders-and also because meiotic silencing systems may be active in both. In contrast to the finding of enhanced TE concentration in non-recombining portions of the genomes of sexual species, no clear conclusion regarding the effect of asexuality or inbreeding on TE content emerges from the limited available comparisons between relatively recent asexual populations and their sexual progenitor(s) or between closely related outbreeding and inbreeding species.…”

Distant horizontal gene transfer is rare for multiple families of prokaryotic insertion sequences

“…The incidence of such horizontal transfer determines the evolutionary fate of disease agents (Anderson and May 1991), and the same may hold for families of transposable elements. However, we know little about this incidence, especially among distantly related species, even though a rich literature exists on the evolution of transposable elements (Sawyer and Hartl 1986;Ajioka and Hartl 1989;Charlesworth and Langley 1989;Vonsternberg et al 1992;Wilke and Adams 1992;Blot 1994;Maside et al 2000;Bartolome et al 2002;Vieira et al 2002;Edwards and Brookfield 2003;Fingerman et al 2003;Petrov et al 2003;Witherspoon and Robertson 2003;Pasyukova et al 2004;Vieira and Biemont 2004;Arkhipova 2005;Garfinkel 2005;Maside et al 2005;Sanchez-Gracia et al 2005;Wagner 2006;Touchon and Rocha 2007) An unrelated reason to study the evolutionary dynamics of transposable elements is that it may shed light on the evolution of prokaryotes themselves. Pervasive horizontal gene transfer is the major challenge in reconstructing prokaryotic phylogenies from gene trees (Doolittle 1999;Lake et al 1999;Snel et al 1999;Gogarten et al 2002;Lawrence and Ochman 2002;Brown 2003;Daubin et al 2003;Philippe and Douady 2003;Daubin and Ochman 2004;Delsuc et al 2005;Kurland 2005;Lerat et al 2005;Ochman et al 2005).…”

Transposable elements as genomic diseases

“…For instance, we now know that the simple question why hosts do not rid themselves of transposable elements does not have a simple answer. First, transposable elements may on occasion cause beneficial mutations [3][4][5][6][7][8] , which may drive their fixation in a population, although their effects are, on average, deleterious [9][10][11][12][13][14][15] . Second, even in largely clonally reproducing asexual organisms, transposable elements can be shuttled from genome to genome through horizontal gene transfer 16 .…”

“…For many human diseases, the restriction of infection, for example through vaccination, can lead to the extinction of the disease. Much the same may hold for some transposable elements and organisms where horizontal transfer or sexual reproduction probably ceased a long time ago 12,[31][32][33][34][35] . Ancient intracellular prokaryotic symbionts provide a case in point.…”

“…Related examples include the bdelloid rotifers, eukaryotes that probably have reproduced asexually for many million years. In these species, the incidence of deleterious retrotransposons, which rely on spreading through sexual recombination, is reduced 12,35 .…”