Study On Natural Hybridization And Gene Introgression Of Two Sympatric Species Of Hemiculter

Natural hybridization is a common phenomenon in flora and fauna,especially when two closely related species are sympatric and reproductive isolation is incomplete.and introgressive hybridization may occur if the hybrid offspring are fertile can continue to backcross with the parents.Introgression hybridization promotes speciation and adaptive introgression,but can also lead to species extinction and reduce species diversity.Besides,introgressive hybridization affects the accurate construction of phylogenetic relationships,thereby affecting the accurate identification of species.Therefore,in taxonomy terms,it was important to explore the impact of introgressive hybridization on species boundary maintenance and on current taxonomic standards.Hemiculter tchangi and H.leucisculus are small fish of the Hemiculter with a similar life history and morphological characteristics.Studies have shown that H.tchangi and H.leucisculus are sympatry in the upper reaches of Yangtze river,providing opportunities for natural hybridization and gene introgressive.However,whether there is a natural interbreed between H.tchangi and H.leucisculus has been not yet reported.Therefore,this study took sympatric H.tchangi and H.leucisculus as the research objects,combined mitochondrial cytb,three nuclear markers(rho,rag2 and myh6)and morphological traits to study the population genetic structure,phylogenetic relationship,population history and gene introgression between species.compared the phenotypic differences of hybrid offspring and purebred species,and discussed the mechanism and its evolutionary significance.The main findings and conclusions are as follows:The population genetic diversity of H.tchangi and H.leucisculus was high,and the genetic diversity from upstream to downstream showed an increasing trend.Haplotype network and phylogenetic tree by mitochondrial markers showed that H.tchangi and H.leucisculus are two highly differentiated species,the most recent common ancestor is approximately 2.97 Mya.Neutral tests and Bayesian skyline results suggested that historical population expansion may have occurred in H.leucisculus,while H.tchangi population may have experienced recent sudden population shrinkage,leading to significant differences in species abundance.The phylogenetic relationship showed that the morphology of 42 individuals did not match the mitochondrial type to which they belonged,indicating that there was extensive mitochondrial haplotype sharing between H.tchangi and H.leucisculus.Fst between sympatric species slightly smaller than the allopatric population and the existing gene exchange recently,indicating that bidirectional introgression of mitochondria has occurred between H.tchangi and H.leucisculus.STRUCTURE analysis found 61 genome mixed and 22 mt-nuclear discordance individuals,most of which were identified by New Hybrids as F2 generation or backcross offspring with H.leucisculus,proving that H.tchangi and H.leucisculus had bidirectional introgression of interspecific nuclear genes.Genetic data indicated that the intensity of introgressive hybridization varied by geographical location,and the number of hybrids tended to increase gradually from upstream to downstream,and hybridization may be caused by differences in species abundance.H.tchangi and H.leucisculus were well differentiated morphologically and could be accurately distinguished by morphology,while the hybrid offspring did not show the intermediate morphology of parent species and were more similar to the morphological characteristics of H.leucisculus.Morphological discrimination analysis was difficult to distinguish between hybrid and parent species,and due to the occurrence of introgressive hybridization,there is a mismatch between morphological discrimination and genetic assignment results between H.tchangi and H.leucisculus.In addition,the age structure and fitness of hybrids were not significantly different from parent species.In summary,this study revealed the ongoing introgressive hybridization between H.tchangi and H.leucisculus,but in the face of continuous gene flow,H.tchangi and H.leucisculus still have a high degree of genetic and morphological differentiation,and the species boundary is maintained.Due to the occurrence of introgressive hybridization,it was difficult to distinguish between hybrid and parental species in morphological discriminant analysis,and hybrid individuals are more similar to the morphological characteristics of H.leucisculus,and there is a mismatch between morphological discrimination and genetic assignment results between H.tchangi and H.leucisculus.This study provided case evidence for introgressive hybridization between Hemiculter,which on the other hand will help to understand the role of interspecific gene flow in species boundary maintenance,and provide insights for better understanding of the interaction of related sympatric species and the development of timely strategies for future species conservation.