The evolution of two partner LINE/SINE families and a full-length chromodomain-containing Ty3/Gypsy LTR element in the first reptilian genome of Anolis carolinensis
Transposable elements have been characterized in a number of vertebrates, including whole genomes of mammals, birds, and fishes. The Anolis carolinensis draft assembly provides the first opportunity to study retroposons in a reptilian genome. Here, we identified and reconstructed a number of retroposons based on database searches: Five Sauria short interspersed element (SINE) subfamilies, 5S-Sauria SINE chimeras, Anolis Bov-B long interspersed element (LINE), Anolis SINE 2, Anolis LINE 2, Anolis LINE 1, Anolis CR 1, and a chromodomain-containing Ty3/Gypsy LTR element. We focused on two SINE families (Anolis Sauria SINE and Anolis SINE 2) and their partner LINE families (Anolis Bov-B LINE and Anolis LINE 2). We demonstrate that each SINE/LINE pair is distributed similarly and predict that the retrotransposition of evolutionarily younger Sauria SINE members is via younger Bov-B LINE members while a corellation also exists between their respective evolutionarily older SINE/LINE members. The evolutionarily youngest Sauria SINE sequences evolved as part of novel rolling-circle transposons. The evolutionary time frame when Bov-B LINEs and Sauria SINEs were less active in their retrotransposition is characterized by a high retrotransposition burst of Anolis SINE 2 and Anolis LINE 2 elements. We also characterized the first full-length chromoviral LTR element in amniotes (Amn-ichi). This newly identified chromovirus is widespread in the Anolis genome and has been very well preserved, indicating that it is still active. Transposable elements in the Anolis genome account for approximately 20% of the total DNA sequence, whereas the proportion is more than double that in many mammalian genomes in which such elements have important biological functions. Nevertheless, 20% transposable element coverage is sufficient to predict that Anolis retroposons and other mobile elements also may have biologically and evolutionarily relevant functions. The new SINEs and LINEs and other ubiquitous genomic elements characterized in the Anolis genome will prove very useful for studies in comparative genomics, phylogenetics, and functional genetics.
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