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.针对鸟纲（Aves）内各主要分类阶元的进化分化过程中的核苷酸替换，本研究开展了相关分析。细胞色素b（cytochrome b, Cytb）基因的转换（transition，即同型碱基A↔G、T↔C替换）与颠换（transversion，即异型碱基A↔T、A↔C、G↔T、G↔C替换）频率，以及二者的数值比值（转换-颠换偏移，transition-transversion shift）均存在广泛变异。 在种级和属级分类阶元中，转换频率显著高于颠换；而在目级阶元的形成过程中，则表现为颠换频率略占优势。这种转换-颠换偏好性的变异，源于鸟类进化谱系中属级及更低分化阶元的转换积累速率加快，而颠换频率则相对稳定。 这意味着，在系统发育的晚期阶段（目级及以上分类群的形成过程），类群形成由自发突变过程驱动；而在早期阶段，进化改变则通过基因调控实现。 不同鸟类目内的同一分类阶元中，核苷酸替换的性质同样存在变异。这种变异表现为：在体型较小的进化年轻类群的科阶元中，颠换频率升高，转换-颠换偏好性的强度降低。 相较于更为古老的大型体型目类群的科，小型、进化演进型且代谢旺盛的鸟类类群，其突变速率升高是颠换频率上升的原因。 鸟类体内的代谢强化不仅是物种多样化与进化演进的驱动因素，同时也会导致突变负荷升高。 针对该现象存在两种可能的解释：其一，点突变可能属于中性事件，其积累未必会提升遗传负荷；其二，代谢的渐进性强化不可避免地成为最具进化演进性的鸟类类群的“掘墓人”。