Biased Gene Conversion and GC-Content Evolution in the Coding Sequences of Reptiles and Vertebrates

choirunnisa febriani hasim;

Mammalian and aviangenomes are characterized by asubstantial spatial heterogeneity of GC - content, which is of ten interpretedas reflecting the effect of local GC - biased gene conversion(gBGC), ameiotic repair bias that favors Gand Cover A and T alleles in high recombining genomic regions. Surprisingly, the first fully sequenced nonavian sauropsid (i.e., reptile), the green anole Anolis carolinensis, revealed a highly homogeneous genomic GC-content landscape, suggesting the possibility that gBGC might not be at work in this lineage. Here, we analyze GC-content evolution at third-codon positions (GC3) in 44 vertebrates species, including eight newly sequenced transcriptomes, with a specific focus on nonavian sauropsids. We report that reptiles, including the green anole,haveagenome-wide distribution of GC3 similar to that of mammals and birds, and we infer a strong GC3 - heterogeneity tobe already present in the tetrapodancestor. We further show that thedynamic of coding sequence GC-content is largely governed by karyotypic features in vertebrates, notably in the green anole, in agreement with the gBGC hypothesis. The discrepancy between third-codon positions and noncoding DNA regarding GC-content dynamics in the green anole could not be explained by the activity of transposable elements or selection on codon usage. This analysis highlights the unique value of third-codon positions as an insertion/deletion-free marker of nucleotide substitution biases that ultimately affect the evolution of proteins.