Phylogenomic Investigation of CR1 LINE Diversity in Reptiles

摘要

It is unlikely that taxonomically diverse phylogenetic studies will be completed rapidly in the near future for nonmodel organisms on a whole-genome basis. However, one approach to advancing the field of “phylogenomics” is to estimate the structure of poorly known genomes by mining libraries of clones from suites of taxa, rather than from single species. The present analysis adopts this approach by taking advantage of megabase-scale end-sequence scanning of reptilian genomic clones to characterize diversity of CR1-like LINEs, the dominant family of transposable elements (TEs) in the sister group of mammals. As such, it helps close an important gap in the literature on the molecular systematics and evolution of retroelements in nonavian reptiles. Results from aligning more than 14 Mb of sequence from the American alligator (Alligator mississippiensis), painted turtle (Chrysemys picta), Bahamian green anole (Anolis smaragdinus.), Tuatara (Sphenodon punctatus), Emu (Dromaius novaehollandiae), and Zebra Finch (Taeniopygia guttata) against a comprehensive library ∼3000 TE-encoding peptides reflect an increasing abundance of LINE and non-long-terminal-repeat (non-LTR) retrotransposon repeat types with the age of common ancestry among exemplar reptilian clades. The hypothesis that repeat diversity is correlated with basal metabolic rate was tested using comparative methods and a significant nonlinear relationship was indicated. This analysis suggests that the age of divergence between an exemplary clade and its sister group as well as metabolic correlates should be considered in addition to genome size in explaining patterns of retroelement diversity. The first phylogenetic analysis of the largely unexplored chicken repeat 1 (CR1) 3′ reverse transcriptase (RT) conserved domains 8 and 9 in nonavian reptiles reveals a pattern of multiple lineages with variable branch lengths, suggesting presence of both old and young elements and the existence of several distinct well-supported clades not apparent from previous characterization of CR1 subfamily structure in birds and the turtle. This mode of CR1 evolution contrasts with historical patterns of LINE 1 diversification in mammals and hints toward the existence of a rich but still largely unexplored diversity of nonavian retroelements of importance to advancing both comparative vertebrate genomics and amniote systematics.