Prime importance for newly arrived TEs, which are initially present in
Prime importance for newly arrived TEs, which are initially present in just a few copies in a few individuals, and that have to invade both the genome and the population, but also for TEs that are already established in a species. Hence the reproductive mode is an important factor influencing TE dynamics. TEs have been described as sexually-transmitted parasites [214]. Indeed the model predicts the inability of TEs to invade species in the absence of sex: an element arriving in the genome of an asexual individual would be able to invade this genome, but not to colonize genomes of other lineages during zygote PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/27107493 formation. Moreover, the loss of sexuality of a species already containing TEs may lead to the progressive loss of the TEs, or at least of TE activity, because TE proliferation would cause extinction of the lineage due to detrimental effects [215]. At most, copy-number equilibrium may be attained under certain specific conditions (infinite population and no excision at all). However, in small populations, the TE load leads to extinction, while in larger populations genomes could get rid of the TEs [216]. All these predictions appear to be difficult to demonstrate in nature. Among eukaryotes, the bdelloid rotifers correspond to well-established, ancient, asexual organisms. However, the search for TEs in these species has led to the discovery of several families of Class-I and -II elements [217,218]. The hypothesis suggested is that the presence of TEs results from repeated horizontal transfers [219]. Moreover, TEs appear to be severely confined to specific chromosomal compartments [218]. Ancient asexual haploids are probably best represented by prokaryotes. When compared to eukaryotes, overall they carry a smaller load of mobile elements, which may be explained by enhanced selection due to haploidy and small-sized genomes. However, most prokaryotes nevertheless contain IS elements. In addition to any benefits they may carry (antibiotic resistance, genome plasticity), their persistence could result from a rapid turnover, with frequent horizontal transfers offsetting rapid losses through selection [220]. Asexuality represents the most extreme situation, but nature is full of species with sexual behavior that is somewhere between full asexuality and obligate outcrossing sexuality, notably if we consider their recombination ability. Hence, differences in the ability TEs to invade or to maintain itself in a population are also toHua-Van et al. Biology Direct 2011, 6:19 http://www.biology-direct.com/content/6/1/Page 16 ofbe expected between selfing or out-crossing sexual species [221]. Reduced genetic exchanges (as in selfing populations) leads to greater variation in TE copy number, and thus to stronger natural selection forces [214]. When the effect of selfing was analyzed in different selection PD150606 dose models, contrasting results were observed, with negative correlations between the copy number equilibrium and selfing rate in the transposon insertion model (heterozygous or homozygous) [222], but positive correlations in the ectopic exchange model [206,223]. Under selffertilization (autogamy), genetic exchange is limited and ultimately results in a high level of homozygosity. Langley et al [205] suggested that TEs could accumulate in regions with low levels of recombination. This is observed for the heterochromatic regions (pericentromeric, telomeric). At the population level, effective recombination (including deleterious ectopic recombination.
