Genome size and GC‐percent were determined by means of a special method of DNA flow cytometry in 154 vertebrate species. For the total dataset, a highly significant positive correlation was found between both parameters. The overall distribution of points is not linear but triangular: a wide range of GC‐percent values is observed at the lower end of genome size range, whereas with an increase in genome size the lower limit for GC‐percent is elevated, gradually approaching the upper limit (about 46%). In teleost fishes, which occupy the lower part of genome size range, the negative relationship between both parameters was observed. Two positive linear relationships were found between mean genome size and GC‐percent of the main vertebrate groups (one includes fishes, amphibians, and mammals, the other consists of reptiles and birds, which show the higher GC‐percent for their genome sizes). Distribution of variance between taxonomic levels indicates that GC‐percent is more evolutionarily conservative than genome size in anamniotes. Anuran amphibians show the greatest part of genome size variability at the lower taxonomic levels as compared to other vertebrates (with no additional variance already above the genus level). The data obtained with different methods are compared. It is shown that the proposed method can provide useful data for studies on genome evolution and biodiversity. Cytometry 31:100–109, 1998. © 1998 Wiley‐Liss, Inc.
A new explanation for the emergence of heavy (GC-rich) isochores is proposed, based on the study of thermostability, bendability, ability to B-Z transition and curvature of the DNA helix. The absolute values of thermostability, bendability and ability to B-Z transition correlated positively with GC content, whereas curvature correlated negatively. The relative values of these parameters were determined as compared to randomized sequences. In genes and intergenic spacers of warm-blooded animals, both the relative bendability and ability to B-Z transition increased with elevation of GC content, whereas the relative thermostability and curvature decreased. The usage of synonymous codons in GC-rich genes was also found to augment bendability and ability to B-Z transition and to reduce thermostability of DNA (as compared to synonymous codons with the same GC content). The analysis of transposable elements (Alu and B2 repeats in the human and mouse) showed that the level of their divergence from the consensus sequence positively correlated with relative bendability and ability to B-Z transition and negatively with relative thermostability. The bendability and ability to B-Z transition are known to relate to open chromatin and active transcription, whereas curvature facilitates chromatin condensation. Because heavy isochores are known to be gene-rich and show a high level of transcription, it is suggested here that isochores arose not as an adaptation to elevated temperature but because of a certain grade of general organization and correspondingly advanced level of genomic organization, reflected in genome structuring, with physical properties of DNA in the gene-rich regions being optimized for active transcription and in the gene-poor regions for chromatin condensation ('transcription/grade' concept).
The resting metabolic rate of passerines is shown to be negatively correlated with genome size when body mass is held constant (r = -0.75, P < 0.01). This finding extends previous conclusion for mammals to this bird order. The result holds when higher taxonomic levels are used instead of the species (for genera, r = -0.76, P < 0.03; for families, r = -0.991, P < 0.01) as well as when the independent contrasts derived from the resolved phylogeny are used instead of the taxa (r = -0.73, P < 0.02), with the evolutionarily older contrasts being more strongly correlated (for the contrasts older than 30 million yr, r = -0.998, P < 0.002). The concept of evolutionary characters consolidation (ECC), previously formulated for mammals, is tested with special reference to the error fraction in the total character variance. In this test, the ECC for the nucleotypic effect cannot be proven for mammals as a whole class, but it holds for the two separate orders tested, rodents and passerine birds. An upper taxonomic limit for the ECC is suggested.
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