Evaluation Of Genetic Diversity For Germplasm Resources Of Betula Alnoides

Betula alnoides Buch.-Ham.ex D.Don is a valuable timber species with wide ecological adaptability in Southeast Asia and southern China.As a fast-growing hardwoods species with high economic value,its wood is of beautiful texture and excellent processing properties,and its barks are rich of methyl salicylate.Its ecological values are also well recognized in water conservation,maintenance of biodiversity and soil fertility.Up to the end of 2016,there are more than 220 000 hm² of B.alnoides plantations in south China However,heavy differentiation in growth and quality traits with low productivity usually occur in the practice.Elite germplasms are thus urgently needed for this species.The objectives of the present study are to estimate genetic diversity and population genetic structure of B.alnoides,and to screen out superior provenances and families on basis of combined analysis on growth and quality traits in multi-sites.The application of the superior provenances and families will greatly increase the productivity of its plantations in practice and lay a foundation for further studies on genetic improvement and breeding of this species.Here,growth(diameter at breast height,tree height,volume,height to live crown base,crown width)and quality traits(stem form,crown shape,branchiness)for 199 families of 25provenances were assessed at four sites when 10–15 years old to estimate genetic parameters and model height-diameter relationship,then screen out superior provenances and families for plantation forestry of B.alnoides.Using the individual plant of above provenances,families and natural population of Motuo,Xizang,the genetic diversity and population genetic structure of B.alnoides populations were analyzed based on SSR markers.The pedigree structure of the population was assessed and population evolution history was inferred based on chloroplast haplotype analysis.The core collection were selected based on SSR loci.The main results are showed as follows:(1)The genetic variance of growth and quality traits and selection of superior germplasm in multi-sites trials for B.alnoides.Significant site,provenance,family and provenance-site effects were revealed for most growth and quality traits.The mean annual increment of tree height,crown width and branchiness(1.22 m,4.71 m,0.77)of Mengla,and the mean annual increment of diameter at breast height and stem volume(1.50 m?0.0204 m³)of Hua'an were the best in four sites.The performance of provenances were different among sour sites.Provenance Jiangcheng,Yuanyang and Daqingshan provenances in Mengla trial,Baise,Napo and Daqingshan provenances in Pingxiang trial,Daqingshan,Tengchong and Pingguo provenances in Hua'an trial,Daqingshan and Yuanyang provenances in Changning trial were the excellent provenance with better growth performances.In combined analysis,the provenance repeatability(?_p²)and family heritability(?_f²)of all tested traits ranged from 0.026 to 0.636 and 0.148 to 0.578,respectively.The provenance repeatability of height to live crown base,stem form and crown shape are higher than other traits,and the family heritability of growth traits are higher than that of quality traits.The diameter at breast height,tree height,height to live crown base and crown width were strongly correlated with other traits,and were also under relatively higher genetic control and had stronger discriminating ability on genotype differences,these traits should be valued in the selection of multiple traits.Baed on BLUP breeding value of multi-traits,five provenances and 20 families were screened out with selection ratio of 20%and 10%at provenance and family level,respectively.The genetic and realized gains at provenance level ranged from 0.25%to 2.01%and 2.43%to 14.84%,and those at family level ranged from 0.85%to 21.22%and 5.76%to 36.71%,respectively.The superior provenances and families will greatly increase the productivity of its plantations in practice.(2)Simulation of height-diameter relationship and selection of superior provenances.Two base models with the best goodness of fit were screen out from 11 general height-diameter models after model fitting and evaluating,the equations are H^-1=0.021+0.742DBH^-1 and log(H-1.3)=0.204+0.793log(DBH).Sites were introduced as dummy variables into intercept and slope parameters of base models,and provenances were then introduced as dummy variables into base models at each site,final site dummy variables model and provenance dummy variables model at each site were developed.The shape and level of the HD curve were influenced by site(p<0.05),and the tree height variance among sites increased with the increasing diameter at breast height.The shape and level of HD relation-ship were both influenced by provenance at Mengla and Pingxiang(p<0.05),and only level of HD relationship was influenced by provenance at Hua'an and Changning(p<0.05).Provenances were clustered based on model parameter at four sites.The 5 provenances with larger DBH than mean and lower level of HD curves were then screen out with the rate of 20%at each site,the selection gains of DBH and tree height ranged from 3.4%to 7.2%and 0.1%to 4.6%respectively.The findings will strongly promote the selection efficiency in term of timber volume and genetic improvement process with HD relationship involved,and applications of the superior provenances will faciliate the large-sized timber cultivation of B.alnoides.(3)The genetic diversity and genetic structure of populations of B.alnoides.Based on 12pairs of SSR primers with high polymorphism amplified product,a total of 185 alleles were revealed in 32 populations.The means of N_a,H_o,H_e and I of 12 loci were 15.42,0.79,0.65 and1.47,respectively.The means of N_a,H_o,H_e and I of 32 populations were 7.78,0.80,0.62 and1.34,respectively.The genetic diversity of Motuo,Xizang(P32)and Xichou,Yunnan(P5)populations were the highest,and that of Nanjian,Yunnan(P28)was the lowest.The generally high level of genetic diversity of populations laid the foundation for the effective selection and utilization of germplasm resources for B.alnoides.AMOVA analysis showed that the genetic variation within populations(96.19%)was much larger than that among the populations.The population differentiation was significant(Phi PT=0.038,p<0.01),and the genetic distance was significantly correlated with geographical distance(latitude and longitude)(p<0.05).Clustering,PCo A and STRUCTURE analysis divided the 32 populations into four major groups.The geographical distribution of the groups was of regional characteristics.(4)The pedigree structure of B.alnoides populations.Based on the fragment difference of chloroplast,significant genetic differentiation among populations were observed(Fst=0.568,p<0.001).Chloroplast haplotype analysis revealed 5 haplotypes.H1 is the central haplotype.H1and H2 exist in 81.25%and 84.38%of the total populations,they are also the dominant haplotypes in Yunnan and Guangxi populations,respectively.The mismatch distribution analysis showed significant expansion event of Yunnan populations.The populations in the western and central Yunnan contained relatively abundant haplotypes,H1 gradually replaced H2 to occupy dominant position in population in the process of extending eastward to Guangxi.(5)Construction of core collection of Betula alnoides.In this study,based on 12 SSR loci of 490 initial collection samples,122 core collection samples were selected by Maximization strategy with the sampling proportion of 24.90%.The reserve percentage of the number of alleles(Na),observed heterozygosity(Ho),expected heterozygosity(He)and Shannon-Wiener diversity index(I)were 94.59%,99.91%,106.59%,119.08%;paired t test showed Na and Ho of core collection were not different from initial collection,while He and I increased significantly(p<0.001);Pco A analysis showed that core collection samples were evenly distributed in initial collection.In a word,these 122 core collection samples with high representation and can be used for subsequent gene mining and genetic improvement of Betula alnoides.Through multi-point experiments,significant interaction of genotype and environment were observed in most of growth and quality traits of Betula alnoides.Based on the combined analysis method of BLUP breeding value and GGE biplot,many excellent provenances and families with wide adaptability to environment were screened out and use for plantation of Betula alnoides.The model fitting revealed significant influence of sites and provenances on tree height-diameter relationship.Consider both height-diameter relationship and DBH growth,some excellent provenances were selected at four sites,which laid the foundation for precise breeding and the cultivation of large diameter timber.Through SSR markers,higher level of genetic diversity and significant genetic differentiation of populations were revealed,and the Motuo population with highest genetic diversity was infered as initial population.The core collection was further constructed to provide the basis for excellent gene mining and genetic improvement.Based on chloroplast fragments,significant genetic differentiation and pedigree structure of populations were revealed,which infered the populations in the western and central Yunnan were the central populations.The results provide the basis for conservation and utilization of the germplasm resources of Betula alnoides.