THE COMPOSITIONAL PATTERNS OF THE AVIAN GENOMES AND THEIR EVOLUTIONARY IMPLICATIONS

The compositional distributions of large (main-band) DNA fragments fro m eight birds belonging to eight different orders (including both pale ognathous and neognathous species) are very broad and extremely close to each other. These findings, which are paralleled by the composition al similarity of homologous coding sequences and their codon positions , support the idea that birds are a monophyletic group. The compositio nal distribution of third-codon positions of genes from chicken, the o nly avian species for which a relatively large number of coding sequen ces is known, is very broad and bimodal, the minor GC-richer peak reac hing 100% GC. The very high compositional heterogeneity of avian genom es is accompanied (as in the case of mammalian genomes) by a very high speciation rate compared to cold-blooded vertebrates which are charac terized by genomes that are much less heterogeneous. The higher GC lev els attained by avian compared to mammalian genomes might be correlate d with the higher body temperature (41-43-degrees-C) of birds compared to mammals (37-degrees-C). A comparison of GC levels of coding sequen ces and codon positions from man and chicken revealed very close avera ge GC levels and standard deviations. Homologous coding sequences and codon positions from man and chicken showed a surprisingly high degree of compositional similarity which was, however, higher for GC-poor th an for GC-rich sequences. This indicates that GC-poor isochores of war m-blooded vertebrates reflect the composition of the isochores of the genome of the common reptilian ancestor of mammals and birds, which un derwent only a small compositional change at the transition from cold- to warm-blooded vertebrates. In contrast, the GC-rich isochores of bi rds and mammals are the result of large compositional changes at the s ame evolutionary transition, where were in part different in the two c lasses of warm-blooded vertebrates.