Big congratulation to Alba Marino for Best poster prizes at ‘The mobile genome: genetic and physiological impacts of transposable elements’ !
Abstract:
Genome size is a widely variable trait ranging seven orders of magnitude across the tree of life. While transposable elements (TEs) are known to be pivotal determinants of genome size, the evolutionary mechanisms underlying their proliferation or reduction in different lineages – and ultimately genome inflation or downsizing – are not well understood. TEs may disrupt genes, interfere with expression levels, induce chromosomal rearrangements, and increase the mutational risk: hence, they are considered as mostly neutral or deleterious to their host. In 2007, Michael Lynch postulated that the removal of selfish elements is as effective as the strength of selection, which is stronger in species with larger effective population sizes (Ne). Hence, larger genomes are expected in species with small Ne due to the random fixation of detrimental TEs (Mutational Hazard Hypothesis, MHH). Some empirical studies have found that genome size and TE content are associated with Ne variation, while others have not, leaving the question unresolved. Here, we study the impact of genetic drift on genome size variation and TE accumulation on short evolutionary times using two independent datasets with contrasted patterns of genome size or Ne, composed of 33 Papilionidae and 76 Passeriformes species. Indeed, comparisons across closely related species should reduce the variability of selective effects potentially intervening at broader phylogenetic scales. In Papilionidae, in spite of their 6-fold genome size variation, we fail to find an association between genetic drift indices (dN/dS, heterozygosity) and genome size, overall or recently accumulated TE content. Interestingly, larger genomes present more frequent lateral transfers of TEs. While more homogeneous in genome size, Passeriformes exhibit multiple transitions from mainland to insular lifestyle, thus providing contrasted Ne. No link between relaxed selection and genome size is observed in birds either. However, species undergoing stronger drift accumulate more TEs, and notably display higher loads of full-length, young elements.Our findings are consistent with a nearly-neutral accumulation of TEs in the short term in birds, as predicted by the MHH. However, this does not seem a general trend shared across taxa, nor reflected on longer-term differences in genome size.