Features of evolutionary-genetic differentiation of bird taxa at the level of transitions and transversions of the CytB gene

Nucleotide substitution analysis was performed for nucleotide substitutions during evolutionary divergence across major taxonomic levels within the class Aves. The frequencies of transitions (homotypic A↔G, T↔C substitutions) and transversions (heterotypic A↔T, A↔C, G↔T, G↔C substitutions) of the cytochrome b (Cytb) gene, as well as their numerical ratio (transition-transversion shift) vary widely. A pronounced predominance of transitions over transversions is observed at intraspecific and generic levels, while a slight predominance of transversions characterizes the formation of orders. This variability in transition-transversion bias is driven by an accelerated accumulation of transition at the genus level and lower levels of divergence against the background of relative stability of transversion frequencies within the evolutionary row of the class of birds. This means that at the late stages of phylogenesis (the formation of taxa of the order level and higher), the taxon formation is driven by a spontaneous mutation process, whereas at the early stages, evolutionary changes are realized through gene regulations. The variability in the nature of nucleotide substitutions is also evident across the same taxonomic levels within different avian orders. It manifests itself as an increase of the transversions frequency and a decrease in the magnitude of the transition-transversion bias within families of small-sized evolutionarily young groups. The reason for the increase in transversion frequencies is an increase in mutation rate within small-sized evolutionarily progressive groups of birds with intensive metabolism compared to families of more ancient large-sized order groups. Intensification of metabolism within birds is not only a factor of diversification and evolutionary progress, but also leads to an increase in the mutational load. There are two possible explanations for this phenomenon. First, point mutations may represent neutral events, the accumulation of which does not necessarily increase the genetic load. Second, the progressive intensification of metabolism inevitably becomes the "gravedigger" of the most evolutionarily progressive groups of birds.

本研究针对鸟纲（Aves）内主要分类阶元的进化趋异过程中的核苷酸替换（nucleotide substitution）开展了分析。针对细胞色素b（cytochrome b, Cytb）基因的转换（transition，同型碱基替换：A↔G、T↔C）与颠换（transversion，异型碱基替换：A↔T、A↔C、G↔T、G↔C）的发生频率，以及二者的数值比值（转换颠换偏移）均存在显著差异。在种内（intraspecific）与属级（generic）分类阶元中，转换的发生频率显著高于颠换；而在目级类群的形成过程中，颠换则呈现微弱优势。这种转换颠换偏倚的变异，源于鸟类进化谱系中颠换频率相对稳定的背景下，属级及分化程度更低的分类阶元的转换积累速率显著加快。这意味着，在系统发育（phylogenesis）的晚期阶段（目级及以上类群的形成过程），类群形成由自发突变过程主导；而在进化早期阶段，演化改变则通过基因调控（gene regulation）实现。核苷酸替换性质的变异在不同鸟纲目级类群的相同分类阶元中同样显著。该变异表现为：在小型进化年轻类群的科级分类单元中，颠换频率升高，转换颠换偏倚的幅度降低。相较于更为古老的大型目级类群，这类具备高代谢速率（intensive metabolism）的小型进化进步鸟类类群，其内部突变率升高是颠换频率上升的原因。鸟类体内代谢速率的提升，不仅是类群分化与演化进步的驱动因素，同时也会导致突变负荷（mutational load）的增加。针对这一现象存在两种可行解释：其一，点突变（point mutation）可视为中性事件（neutral event），其积累未必会提升遗传负荷（genetic load）；其二，代谢速率的渐进性提升，终将成为最具进化进步性的鸟类类群的"掘墓人"。