Habitat Hybridization Between Species Of The Betula Albosinensis Complex And Establishment Of A New Betula Species

Natural hybridization is common in nature,which can lead to speciation and adaptive introgression.Natural hybridization plays a pivotal role in evolution of organisms.Natural hybridization can also drive species to extinction through demographic swamping or genetic swamping,so it is key to study natural hybridization for biodiversity protection.Some studies have shown that habitat disturbance promotes hybridization.Habitat disturbance promotes hybridization by breaking down the isolation between species and creating new habitats for hybrids.Here,we test “hybridization of the habitat” hypothesis by comparing hybridization rate between the disturbed populations and the undisturbed populations.We used the Betula albosinensis complex as a model which included the two hybridizing species: B.albosinensis and the ‘diploid' B.albosinensis.Betula is a genus with some pioneer species,having a wide distribution.Species of this genus are of great ecological and economic values.B.albosinensis and the ‘diploid' B.albosinensis overlapped partially with the latter occupying a higher altitude than the former.The two species can hybridize and some genetic admixture has been detected in co-occurrinig populations.Within some populations,habitat has been disturbed,such as populations LFG,BX,TBS,WLP,DY and QYS whereas in some populations,habitat remains intact,such as populations HHG,MYL,XMX,LHS,TZZ and DTX.In this study,we selected 184 samples from six disturbed populations with 21,41,56,21,14 and 31 samples from LFG,BX,TBS,WLP,DY and QYS,respectively.In addition,we collected 313 samples from six undisturbed populations with 75,60,30,25,76 and 47 samples from HHG,MYL,XMX,LHS,TZZ and DTX,respectively.A total of 497 samples were genotyped at 16 nuclear microsatellite markers.Population structure was analysed using STRUCTURE and a Principal Coordinate analysis.Then we compared the hybridization rate between the disturbed and undisturbed populations based on the results of STRUCTURE.Furthermore,we found 45 the unknown Betula species individuals from WLP,DY and QYS populations.In morphology aspect,the leaf morphology of the unknown Betula species,B.albosinensis,the ‘diploid' B.albosinensis and B.costata were compared.In molecular biology aspect,15 nuclear microsatellite markers were genotyped to infer the genetic structure of the four species.Internal transcribed spacer were sequenced for four samples,which from the Betula unknown species populations.The maximum-likelihood tree was constructed by combining 76 Betula sequences downloaded from GenBank.The main results are as follow:(1)STRUCTURE analysis of the 497 samples showed three clusters: corresponding to B.albosinensis,the ‘diploid' B.albosinensis and a unknown genetic cluster,namely the unknown Betula species.(2)Setting the the q value of 0.9,the proportion of early-generation hybrids accounts for between 29.27 % and 100 % in disturbed populations and between 4 % and 26.67 % in undisturbed habitats.The proportion of early-generation hybrids is much higher in the disturbed populations than that in the undisturbed populations(P = 0.010).(3)The unknown Betula species has very elongated lenticels on the barks of the adult trees compared with that of B.albosinensis.Bark peeled along the elongated lenticels,resulting in strips.In contrast,barks of B.albosinensis peeled in large flakes.Leaf shape of the unknown Betula species is nearly identical with that of B.albosinensis as revealed by PCA analysis.(4)STRUCTURE analysis based on SSRs showed obvious genetic differentiation among the four species.When at the value of K = 2,the unknown Betula species formed a cluster and the ‘diploid' B.albosinensis,B.costata and B.albosinensis formed another;When at the value of K = 3,the three clusters were the unknown Betula species,the ‘diploid' B.albosinensis and the cluster of B.costata and B.albosinensis.The phylogenetic tree based on ITS sequences showed that the four individuals of the unknown Betula species formed a monophyletic clade with moderate support.This clade is nested into a clade with species of section Acuminatae: B.alnoides?B.cylindrostachya?B.hainanensis and B.luminifera.In summary,habitat disturbance promoted hybridization between B.albosinensis and the ‘diploid' B.albosinensis,possibly via creating open habitats.Based on morphology and moluclar data,we tentatively proposed a new Betula species.