| Eukaryotic genomes are littered with repetitive DNA sequences called transposable elements (TEs). Though once considered "junk DNA," these elements can greatly impact their host genomes by influencing genome size, providing novel proteins and promoter sequences, as well as disrupting gene function or causing chromosomal rearrangements. However, the impact TEs have on their host's genome depends on their abundance and diversity, which differ greatly among vertebrate genomes. The genome of teleostean fish contain a very diverse community of elements that are represented only by recent inserts found in very low copy number (<100). On the other hand, most mammalian genomes have a very low diversity of TEs dominated by L1 retrotransposons, yet elements in mammals accumulate to reach extraordinary copy numbers (>100,000). This difference accounts, for the most part, for the difference in genome size between these two groups. Until recently, we did not have a good model to study the transition from the small repeat-poor genome of fish to the larger repeat-rich genome of mammals. The first non-avian reptile genome sequence, the lizard Anolis carolinensis (the North American green anole), bridges the large phylogenetic gap between fish and mammals and provides a better understanding of early amniotes genomic evolution. We performed the first comprehensive analysis of TEs in the anole genome. We found that the anole genome contains an extraordinary diversity of active TEs. This genome contains several concurrently active clades of non-LTR retrotransposons (CR1, L1, L2, RTE, and R4) each represented by multiple families. The vast majority of insertions are very young, suggesting that most elements do not reach fixation and when they do, they decay rapidly. In addition the anole genome is inhabited by multiple superfamilies of DNA transposons ( hAT, Helitron, Maverick and Chapaev), some of which were laterally transferred to the anole. We conclude that the genomic landscape of the lizard is strikingly similar to the one of fish and shows little resemblance to mammalian genomes. |
