Gene Introgression And Genetic Structure Of Sympatric Distribution Plants In Ligularia And Cremanthodium

As a common phenomenon in nature,natural hybridization is widely believed to play an important role in the origin of species diversity and the formation and evolution of species.The hybrid zone,where natural hybridization takes place,has long been considered as the species evolution and natural laboratory of evolution and the reaction of biological evolution process window,to study the mechanism of reproductive isolation between species,investigating the origin of species diversity,evolution of adaptation,and evolution mechanism provides a good platform.As one of the hot spots of ecological diversity in the world,natural hybridization is one of the potential driving forces to promote the rich species diversity in Hengduan Mountain region(HMR).Complex geography and climate change in history,and now the frequent human activities in this area,may result in the region with the domain of species distribution and reproductive isolation mechanism is not completely established.Natural hybridization events,many natural hybrid species in this area,mainly in the form of the hybrid zone.Ligularia and Cremanthodium,which are widely distributed in Asteraceae in HMR,are not monophyletic genus,but are in the Ligularia-Cremanthodium-Parasenecio complex(LCP complex).Natural hybridization events have been recorded between Ligularia and Cremanthodium,and between Ligularia species in the sympatric distribution of Ganzi County in Sichuan and Bigutianchi in Yunnan,which located in the HMR.In this study,the sympatric distribution plants of Ligularia and Cremanthodium in these two regions were taken as research objects to analyze the genetic structure and gene introgression within and between species,so as to reveal the maintenance and selection of interspecific boundaries among sympatric species,as well as the genetic relationship between Ligularia and Cremanthodium,The main research results and conclusions are as follows:1.Study on the hybrid zone between L.nelumbifolia and C.stenoglossumThe hybrid zone where natural hybridization has been proved by molecular methods between L.nelumbifolia and C.stenoglossum.A total of 80 individuals of L.nelumbifolia,C.stenoglossum and their hybrid population of two morphological types were sequenced by ddRAD,and 2,540 SNPs were obtained for study.Combined with the STRUCTURE analysis results,principal coordinate analysis(PCoA)and maximum likelihood(ML)tree analysis results show that natural hybridization occurred between L.nelumbifolia,C.stenoglossum,and all of the hybrid population showed the genetic components mixed by parental species L.nelumbifolia and C.stenoglossum.NEWHYBRIDS analysis showed that both L.nelumbifolia and C.stenoglossum were determined to be pure parents,while all the hybrid offspring were identified as F1s.In combination with the wild investigation,the seeds of hybrid progeny were not collected,suggesting that there was a strong reproductive isolation between parent L.nelumbifolia and C.stenoglossum in this hybrid zone,and post-zygotic isolation may be the main mechanism of reproductive isolation,so as to maintain the genetic integrity between species.At the same time,the natural hybridization of L.nelumbifolia and C.stenoglossum provided a new perspective into the re-circumscription and re-delimitation of the phylogenetic relationship between Ligularia and Cremanthodium.2.Study on gene introgression and gene structure of many species sympatric distributed in BigutianchiThe Bigutianchi hybrid zone was a mixture of natural hybridization between L.duciformis and L.cyathiceps,and between L.duciformis and L.yunnanensis.In this hybrid zone,L.cymbulifera,L.subspicata,L.tsangchanensis,L.pleurocaulis and C.helianthus were also distributed,a total of 100 individuals of these five populations were sequenced by ddRAD,and 3,027 SNPs were obtained for analysis.STRUCTURE analysis results show that,L.cymbulifera and L.pleurocauli have no gene flow with other populations,and C.helianthus,L.tsangchanensis and L.pleurocaulis may share some genetic components from a common ancestor,they may be origin from a common ancestor,and combined with the TreeMix results,C.helianthus and L.tsangchanensis,L.cymbulifera,and L.subspicata,may have gene flow in history.Genetic components from other populations such as L.tsangchanensis and C.helianthus were detected in some individuals of L.subspicata.One individual of L.tsangchanensi showed a genetic component of the other four populations of L.cymbulifera,L.subspicata,L pleurocaulis and C.helianthus.The results of principal coordinate analysis and maximum likelihood tree show that the five populations cluster into a group.Then,among all the individuals mentioned before,ten individuals of L.subspicata and L.tsangchanensis were randomly selected.These 20 individuals were used the same methods as the L.duciformis,L.cyathiceps,L.yunnanensis and their hybrid descendants Type A,B,C,D,which have been performed ddRAD sequencing before.A total of 88 individuals were combined to perform analysis using ddRAD sequencing data.In addition,2,854 SNPs were obtained for later analysis.STRUCTURE analysis showed that L.subspicata and L.cyathiceps had similar genetic components,and there were three individuals in L.subspicata may be the hybrid offspring between L.subspicata and L.cyathiceps.Most of the hybrid individuals from Type A,B,C,D showed genetic components from both parental species.Principal coordinate analysis showed that L.duciformis,L.cyathiceps,L.yunnanensis,L.subspicata and L.tsangchanensis clustered into a group respectively,and the hybrid offspring of L.duciformis and L.yunnanensis,L.duciformis and L.cyathiceps clustered into parents or between parents.NEWHBRIDS analysis of L.subspicata and L.cyathiceps showed that the three individuals suspected to be the hybrids of L.subspicata and L.cyathiceps could not be identified as progeny or parents.Based on the results of the two parts,it can be inferred that the Bigutianchi area is dominated by the natural hybridization of L.duciformis,L.cyathiceps and L.yunnanensis,but in this area there are some individuals of L.subspicata and L.tsangchanensis that are intact in morphology but with gene introgression.The remaining sympatric species maintain their genetic integrity and interspecific boundaries with other populations.The difference in habitat between the naturally hybridized species and the non-hybridized species,as well as the characteristics of the species themselves,may be the main reasons for the maintenance of the hybrid zone.