LINE-1 distribution in Afrotheria and Xenarthra: implications for understanding the evolution of LINE-1 in eutherian genomes

Long interspersed nuclear elements (LINEs) comprise about 21% of the human genome (of which L1 is most abundant) and are preferentially accumulated in AT-rich regions, as well as the X and Y chromosomes. Most knowledge of L1 distribution in mammals is restricted to human and mouse. Here we report the first investigation of L1 distribution in the genomes of a wide variety of eutherian mammals, including species in the two basal clades, Afrotheria and Xenarthra. Our results show L1 accumulation on the X of all eutherian mammals, an observation consistent with an ancestral involvement of these elements in the X-inactivation process (the Lyon repeat hypothesis). Surprisingly, conspicuous accumulation of L1 in AT-rich regions of the genome was not observed in any species outside of Euarchontoglires (represented by human, mouse and rabbit). Although several features were common to most species investigated, our comprehensive survey shows that the patterns observed in human and mouse are, in many aspects, far from typical for all mammals. We discuss these findings with reference to models that have previously been proposed to explain the AT distribution bias of L1 in human and mouse, and how this relates to the evolution of these elements in other eutherian genomes.