Lizards and LINEs: Phylogeography and genome evolution of the green anole (Anolis carolinensis).

摘要

The sequencing of the green anole lizard (Anolis carolinensis ) genome has already provided insights into how vertebrate genomes have evolved since the phylogenetic split between reptiles and mammals ∼300 million years ago. For instance, the diversity and abundance transposable elements (TEs) in the Anolis genome, particularly the non-LTR retrotransposons (nLTR-LTs), shows more similarity to fish than to mammals, which suggests that mammals have significantly diverged from the amniote ancestor in terms of genome structure. The fate of TEs in a genome relies on the relative strengths of purifying selection against deleterious elements and genetic drift in host populations. Surprisingly, the geographic distribution and demographic history of populations within A. carolinensis has largely escaped scrutiny. We studied the patterns of mitochondrial and nuclear DNA sequences and found that there are five evolutionary lineages of green anoles, which diverged ∼2 million years ago. Climatic shifts during the early Pleistocene may have driven their early diversification, particularly on ancient island refugia in what is now Florida, where a remarkable phylogeographic diversity of green anoles is found. More recently, the dispersal of green anoles onto the continental mainland led to a dramatic westward range expansion across the Gulf Coastal Plain. These insights into the evolutionary history of A. carolinensis allowed us to infer the population dynamics of nLTR-RTs in the Anolis genome. While nLTR-RTs are rare in Anolis, we find that they reach fixation in populations quite readily. We also find that full-length (FL) nLTR-RT insertions may be subjected to purifying selection as they are found at lower population frequencies than truncated (TR) insertions. This suggests deleterious effects of ectopic recombination or the process of retrotransposition are limiting the copy number of FL elements in Anolis. Finally, we find that FL elements are much more likely to be fixed in populations of small effective size, where purifying selection may not be acting as efficiently due to strong genetic drift. While FL elements are subjected to purifying selection, the fixation of TR elements suggests that another mechanism, such as DNA loss, may account for the relative paucity of nLTR-RTs in the Anolis genome.