Studies On The Origin Of P.Ussuriensis And P.pyrifolia Cultivars Based On SSAP And SSR Markers

The genus Pyrus was thought to be of Tertiary or earlier origin in the mountainous regions of western and southwestern China.Based on the geographic distribution,Pyrus species are divided into two native groups:Occidental pears and Oriental pears.The most commonly cultivated Asian pears are P.ussuriensis Maxim.,P.sinkiangensis T.T.Yu,P.pyrifolia Nakai which including Chinese white pear,Chinese sand pear and Japanese pear.Chinese white pear and sand pear have been considered to be the same germplasm,which displayed the same genetic composion.Xinjiang pear has been proved to be of hybrid origin between Oriental pears and Occidental pears.On the origin of Japanese pear,different scholars had different views.P.ussuriensis cultivars are commonly cultivated in northeastern China and some parts of northern China,but its origin was still unknown.In a previous study without accessions of wild P.ussuriensis samples,cultivated P.ussuriensis was found to be composed of two major genepools,one was from P.pyrifolia,while the other was suspected to be from wild P.ussuriensis.Besides,the gene introgression might happen from P.pashia to Chinese sand(white)pear.In this study,we used SSR and SSAP markers associated with retrotransposons to elucidate the origin of P.ussuriensis cultivars and the role of P.pashia played in the Chinese sand(white)pear origin.The main results were as follows:A total of 92 Pyrus accessions were analyzed by using 12 SSAP markers.A total of 1143 scorable bands were produced.Of these,871 bands were polymorphic(i.e.,76.20%),which indicated the good polymorphism of the markers.The results of dendrogram and STRUCTURE diaplayed Pyrus was divided into two large divisions:Occidental pears and Oriental pears.Additionally,when the Dice similarity coefficient was 0.56,all P.pyrifolia accessions fell into one group,and most wild and cultivated P.ussuriensis accessions were clustered into the other group,which were subdivided into two subgroups at a Dice coefficient of 0.61.Three P.ussuriensis cultivars(’Jinxishanli’,’Qingmian’,and ’Ruanerli’)were scattered in P.pyrifolia group.In STRUCTURE plot when K=3,Ussurian pear cultivars mainly consisted of two major genepools,of which half was derived from P.pyrifolia and the other from wild P.ussuriensis).In this study,we explored the genetic relationships of P.pyrifolia and wild and cultivated P.ussuriensis using 16 SSRs.Mean number of polymorphic alleles(na)and effective alleles(ne)were 13.63 and 5.29,respectively.Observed heterozygosity(Ho),expected heterozygosity(He),Shannon’s information index(I)and PIC were 0.534,0.72,1.812 and 0.752,respectively.NJ tree showed that all the accessios divided into two groups.In the first group,all wild P.ussuriensis and a majority of cultivated P.ussuriensis independently clustered into two subgroups.And other P.ussuriensis cultivars were mixed with Chinese white pears and sand pears to form different subgroups in the other group.In STRUCTURE plot,wild P.ussuriensis showed a relatively pure genepool,while P.pyrifolia dominated by the other genepool when K=2.P.ussuriensis cultivars consisted of these two genepools.We selected 92 Pyrus accessions to study the role of P.pashia in the origin of P.pyrifolia cultivars.The dendrogram indicated that all the P.pashia samples clustered together independently.The result of STRUCTURE revealed that P.pashia had the relatively pure genetic background which mainly dominated by one genepool.Additionally,P.pyrifolia mainly consisted of the other genepool,and the genepool of P.pashia included.This indicated P.pashia involved the origin of P.pyrifolia to some extent.Based on the genetic distance of different geographic population(P1-P9),the introgression level decreased gradually with the distance from the southwest where P.pashia distributed naturally.In summay,we used SSAP and SSR markers to elucidate the hybridization origin of cultivated P.ussuriensis and analyze the relationships of P.pashia and P.pyrifolia.The results provided the theoretical basis of pear cultivar breeding and improvement.However,in the present study,the accessions were collected from limited extents.In the future study,much more pear collections of wild P.ussuriensis and P.pashia covering different distribution areas should be included.