Differentiation of bird taxa at the level of transition-transversion bias in the СYTB gene sequencies

The study of DNA marker variability provides robust insights into historical relationships, evolutionary rates, and the magnitude of genetic differentiation among taxa. This is particularly relevant for the youngest and most diverse group of vertebrates, traditionally classified as the class Aves. One of the core principles of molecular evolution theory is the transition–transversion bias, i.e., the paradoxical predominance of homotypic nucleotide substitutions (transitions) over heterotypic substitutions (transversions) during spontaneous mutational processes and speciation, a pattern that becomes progressively attenuated with increasing genetic divergence. This phenomenon exhibits group-specific features at higher taxonomic levels, allowing the identification of evolutionary characteristics unique to each lineage. The present study investigates patterns of evolutionary transition–transversion bias in birds using the mitochondrial CYTB gene as a model. The results demonstrate that the magnitude of the bias varies both along the evolutionary hierarchy depending on taxonomic level and among avian families. A predominance of transitions over transversions is observed at the population, species, genus, and family levels, whereas a slight predominance of transversions is detected at the order level. Temporal variation in transition– transversion ratios is driven by saltatory accumulation of transitions at the population, species, and genus levels against a background of relatively stable transversion frequencies. Th is pattern may be explained by genetic saturation effects or the peculiarities of the mutation process at different levels of divergence. Within Aves, nucleotide substitution patterns are characterized by an increase in transversion frequency and a decrease in transition frequency, accompanied by a reduction in the overall transition–transversion bias in small-bodied species from evolutionarily young families, compared with more ancient, large-bodied lineages with lower metabolic rates. The increase in transversion frequency in small-bodied, evolutionarily progressive bird groups is associated with elevated metabolic rates, leading to increased cellular oxygen availability and, consequently, higher levels of oxidative DNA damage. Metabolic intensification in birds thus represents not only a driver of enhanced genetic diversity and evolutionary advancement, but also evidently results in an increased mutational load, which may ultimately contribute to the evolutionary extinction of historically young, small-bodied taxonomic groups.