| The mechanism of adaptive evolution of organisms has always been a hot topic of research in evolutionary biology,and the accurate identification of species is an important prerequisite for the study of adaptive evolution.The genus Acrossocheilus belongs to the Cyprinidae family of fishes,and the species of this genus are widely distributed in several regions of China and have important economic value.However,there are still many problems in the systematic classification of the species of this genus.It is difficult to classify species of this genus by morphological characters alone because of the small morphological differences between closely related species of this genus,the lack of distinctive features between species,and the differences between male and female,juvenile and adult traits of the same species,which hinder the identification and differentiation of different species.Therefore,it is necessary to systematically classify species of the genus Acrossocheilus with the help of molecular techniques.In addition,species of the genus Acrossocheilus are distributed in different temperature zones,such as tropical,subtropical and temperate zones,and as a variable temperature animal,its body temperature and metabolic rate are closely related to abiotic factors such as environmental temperature,and mitochondria play a crucial role in the balance of energy and heat production in organisms.Thus,it is hypothesized that the adaptive divergence of different species of the genus Acrossocheilus is most likely to leave genetic evidence in the mitochondrial genome,and it is also hypothesized that the environmental factors leading to the divergence of species in this genus may be temperature-related.In this study,different molecular classification methods(GMYC,bPTP,ABGD,BPP)were used to systematically classify the species of this genus at the molecular level based on the mitochondrial protein-coding genes of 14 species in the genus Acrossocheilus to clarify the number and phylogenetic relationships of the species of this genus;Then,different selection pressure analysis methods(CodeML,MEME,FEL,SLAC,FUBAR,TreeSAAP)were used to examine whether the mitochondrial protein-coding genes of this genus were affected by natural selection at both gene and codon levels,and to identify the types of selection to which they were subjected;Finally,a generalized linear model was applied to explore the correlation between Ka/Ks ratios and various environmental factors,thus verifying the correlation between adaptive evolution of species of the genus Acrossocheilus and various environmental factors.The main findings were as follows:The results of the species taxonomic analysis based on four molecular taxonomic methods showed that all individuals of the genus Acrossocheilus in this study were defined into 12 Molecular Operational Taxonomic Units(MOTUs).In addition,we found that neither the genus Acrossocheilus nor the genus Onychostoma formed an independent monophyletic structure in the phylogenetic tree,and the evolutionary relationship between them and the classification of each species still needs to be further verified.Natural selection tests showed that although purifying selection was widespread in the mitochondrial protein-coding genes of species of the genus Acrossocheilus,some codons and amino acid loci were still subject to positive selection,suggesting that the mitochondrial protein-coding genes of species of the genus Acrossocheilus may have been involved in their adaptive evolution under different environmental conditions.In addition,the results of the generalized linear model correlation analysis between the mean Ka/Ks of each gene and environmental factors in this experiment showed that there was a significant correlation between the mean Ka/Ks of the ATP6 gene and the mean diurnal temperature difference(maximum temperature-minimum temperature),and the mean Ka/Ks of the COX2 gene was correlated with several precipitation-related environmental variables.The above results suggest that the adaptive evolution of species of the genus Acrossocheilus in different geographical environments may be influenced by factors such as temperature and precipitation of the environment.This experiment explores the evolutionary relationships of species of the genus Acrossocheilus at the molecular level and provides a new basis for the classification of species of this genus.At the same time,the study of the adaptive evolutionary mechanisms of Acrossocheilus species can help to reveal the role of abiotic factors in the differentiation of the species,which in turn can provide a basic study on how the species of this genus can cope with some future human-induced environmental changes. |
