Species Differentiation Of Actaea Purpurea And Its Relatives

Speciation builds a bridge between population evolution and taxa diversification,connecting macroevolution and microevolution.Pollinators,as the medium of natural selection,usually take flower traits as the selection target in ecological speciation,and pollinators play an important role in reproduction of angiosperms.Therefore,ecological speciation mediated by pollinators as the key ecological factors is a special form of ecological speciation.However,there are still many unanswered questions about speciation triggered by pollinators.For example,whether and how do pollinators drive speciation?What are the geographic patterns and genetic divergence processes accompanying pollinator-driven speciation?How does pollinator selection interact with geographic isolation and other selection pressures to drive speciation?In the previous studies of our group,we found that the floral traits of Actaea purpurea(P.K.Hsiao)J.Compton and its related species,especially Actaea japonica Thunb.,had significant divergence,but the variation of vegetative traits was not obvious.Moreover,the main pollinators of the two species are also different.The former was Vespa bicolor Fabricius,and the latter was large flies.Thus,we hypothesized that the divergence of A.purpurea with its closest relatives(A.japonica+Actaea biternata Prantl)was probably triggered by pollinator shifts.Therefore,in this study,I use A.purpurea and its closest relatives as materials to study their degree of genetic divergence,to investigate the factors affecting their genetic divergence,as well as to model their demographic history,so as to elucidate the process of speciation driven by pollinators.In this study,I used cpDNA fragments(trnL-trnF,rpl20-rps12,trnS-trnG)and Genotype-by-Sequencing(GBS)data to first construct a phylogenetic tree to clarify the position of A.purpurea and estimate the divergence time between A.purpurea and its relatives.Then I used neutrality test,Bayesian skyline plot,fastsimcoal and other analyses to trace the historical process of speciation.Finally,I used geographic isolation analysis and Partial Mantel Test to explore the possible causes of genetic divergence between A.purpurea and their relatives.The main research results are as follows.1.By analyzing the phylogenetic relationship between A.purpurea and its relatives,I found that all individuals of A.purpurea are clustered into one monophyletic clade.A.purpurea is sister to A.japonica and A.biternata.Within the clade of A.japonica and A.biternata,A.biternata and A.japonica are sister groups,and the A.japonica in Japan(JP02)is located at the base of the A.japonica clade,indicating that the A.japonica in Japan diverged first.The genetic structure of A.purpurea and its relatives revealed that the genetic divergence of A.purpurea and its relatives was obvious.In the haplotype analysis,a total of 11 haplotypes were obtained.Among them,there are 10 haplotypes in A.purpurea,which is rich in haplotype diversity.A.japonica and A.biternata share one haplotype.The AMO VA also detected that the main genetic variation came from between species.Fixation index based on the GBS showed that the Fst between A.purpurea and A.japonica was 0.64487,and that between A.purpurea and A.biternata was 0.64837,and that between A.japonica and A.biternata was 0.21564.It is proved that there is strong genetic divergence between A.purpurea and its relatives.The genetic diversity analysis,PCA,and Admixture analysis also showed that the genetic diversity of A.purpurea is higher than that of its relatives.The genetic divergence is obvious from A.purpurea and its relatives.A.japonica population in Japan is found to be more similar to A.biternata.2.Based on the analysis of the demographic history of A.purpurea and its relatives,it was found that the divergence time of A.purpurea and its relatives was in the middle of the Pleistocene(cpDNA:1.49 million years ago(Mya)and the GBS:1.70 Mya).The climatic oscillation of Pleistocene is an important driving force for the change of geographical distribution of plants,which may be the reason for the early divergence of A.purpurea and its relatives.Demographic modeling based on GBS data using fastsimcoal indicates that a large-scale population expansion event occurred at the beginning of the separation of A.purpurea and its relatives,followed by a period of lower levels of gene flow from A.japonica(including A.biternata)to A.purpurea,and the gene flow stopped at 37.23 thousand years ago(Kya).Demographic history analysis based on cpDNA fragment suggests that A.purpurea underwent an expansion event at 0.52 Mya and a contraction event at 25 Kya.3.The cause of the existing genetic divergence pattern of A.purpurea and its relatives was explored.After partial mantel test,it was found that the existing genetic divergence of A.purpurea and its relatives was significantly related to geographical isolation and environmental differences(Geographical:r=0.2392,P=0.001;Environment:r=0.4059,P=0.001).The combination of gene flow pattern presence of A.purpurea and its relatives with the strong adaptation of A.purpurea to its pollinators suggest that pollinators are a non-negligible and important factor in their genetic divergence.The parapatric pattern of A.purpurea and its relatives,the gene flow pattern in speciation,and the strong adaptation of A.purpurea to its pollinators suggest that the divergence of A.purpurea and its relatives is a pollinator-driven parapatric speciation.At the early stage of speciation,population expansion and contraction caused by Quaternary climatic oscillations formed geographical barriers,resulting in reduced gene flow between populations.This process could provide an initial arena for subsequent speciation processes.Then,under the selection of pollinators and other abiotic environmental factors,A.purpurea and its relatives further diverged,until they gradually stopped gene exchange and the speciation process was completed.This pattern and characteristics of speciation provide a basis for understanding the process of pollinatordriven speciation and how pollinators,interacting with other factors,drive speciation.