The compositional transition between the genomes of cold- and warm-blooded vertebrates: codon frequencies in orthologous genes

The genomes of the ancestors of mammals and birds underwent a compositional change in which the gene-richest regions increased their GC levels. Here we investigated this compositional transition by analyzing the levels of G and C in third codon positions, as well as the codon frequencies of orthologous genes from human, chicken and Xenopus. The results may be summed up as follows: (i) GC-poor genes, that did not undergo the compositional transition, showed only minor differences in orthologous sets from Xenopus, human and chicken; this is remarkable in view of the very many nucleotide substitutions that occurred over the long evolutionary times separating these species; (ii) GC-rich genes, that underwent the compositional transition, showed large differences between Xenopus and warm-blooded vertebrates, but not between chicken and human. In other words, the independent changes that occurred in avian and mammalian genes, on the average, were the same. (C) 2000 Elsevier Science B.V. All rights reserved.