| Apricot is one of the most important trees in Rosaceae,belonging to section Armeniaca(Lam.)Koch,subgenus Prunophora Focke and genus Prunus.Almost all cultivated apricots originated from P.armeniaca.The Kashgar,Hotan and Aksu oasis areas around the Tarim Basin in the southern part of the Xinjiang Uygur Autonomous Region of China are the main apricot-producing areas and contain the greatest abundance of apricot cultivars.The wild apricot forest in Ili,Xinjiang,China,is a relic of a broad-leaved forest from the late Tertiary community that played a decisive role in the domestication and cultivation of apricot worldwide.Based on the study of palynology,cytology,population genomics and phylogeography,we hope to provide new evidence for the origin and evolution of cultivated apricot in Xinjiang.The main findings were as follows:(1)The pollen grains of apricot accessions from different ecological groups was measured and described by light microscopy and scanning electron microscopy.The result showed that pollen grains were isopolar monads,radially symmetric,medium to large in size,prolate or subprolate,and 3-colporate.Most pollen grains exhibited striate exine ornamentation,and a few exhibited striato-reticulate exineornamentation or cerebroid exine ornamentation.The results of the cluster analysis showed that the six ecological groups could be divided into two groups.One group includes the Central Asian,North China and Dzhungar-Ili ecological group accessions.The other group includes the Northeast Asian,European and East China ecological group accessions.The results of the principal component analysis showed that the equatorial diameter,colpus length,colpus width and ridge width were the most important pollen traits based on the pollen metrical variables,and could be used as powerful tools in apricot identification.This study described the morphological characteristics of apricot pollen grains and provided some morphological information for future studies on the phylogenetic relationships of apricots.(2)The results showed that the different ecological group accessions were all diploid(2n=2×=16)by the conventional pressing plate method.The chromosomes of P.armeniaca were small chromosome.No satellite chromosomes were found.All accessions possessed different numbers of median-centromere chromosomes or submedian-centromere chromosomes.According to Stebbins' classification,the karyotype classifications of the selected accessions were 1A or 2A.In the process of evolution,chromosomes progress from symmetrical to asymmetrical,and the degree of karyotype asymmetry in accessions of the North China ecological group was greater than that in accessions of the other groups.Principal component analysis showed that the long-arm/short-arm ratio(MAR)and the karyotype symmetry index(Syi)were the most valuable parameters.Cluster analysis showed that the accessions of the Central Asian and Dzhungar-Ili ecological groups were clustered together.In terms of karyotype characteristics,accessions of the Dzhungar-Ili ecological group and accessions of the Central Asian ecological group were closely related.We analyzed P.armeniaca of different ecological groups from the perspective of cytology,in order to determine its chromosome number and karyotype characteristics and reveal the genetic relationships among P.armeniaca from different ecological groups.With Medicago saliva as the internal standard,Marie's isolation buffer and propidium iodide fluorescent dye were used to evaluate the genomic size of apricot accessions by flow cytometry,and the ploidy was obtained.It was found that there were no polyploids from different ecological groups of P.armeniaca;all accessions were diploids.The genome size of P.armeniaca varied from 0.590 to 0.708 pg,and the average genome size was 0.637 pg.The European ecological group had the largest genome size,,which was the only group significantly different in size from the Central Asian,North China and Dzhungar-Ili ecological group(P=0.05).There was no significant difference in genome size between different populations of wild accessions,and there was no significant correlation with longitude,latitude and altitude.The variation in the genome size of P.armeniaca was only 2.33%,and there was low intraspecies variation among the studied groups;the genome size of P.armeniaca is highly stable.(3)Restriction-site associated DNA sequencing was adopted to sequence in 5 ecological group accessions.The results showed that sequencing generated 417961 high-quality SNPs,91.94%of which were successfully aligned to the reference genome.We used cluster,genetic structure and principal component analyses to examine the genetic diversities and genetic relationships of the 168 accessions.The Dzhungar-Ili ecological group accessions showed the highest genetic diversity in terms of private allele number,observed heterozygosity and nucleotide diversity.We speculate that the Central Asian ecological group accessions were domesticated from the Dzhungar-Ili ecological group accessions.The population structures and gene flow of the North China and European ecological group accessions suggested a genetic background of P.sibirica.We argue that the two groups should be considered hybrid swarms connected to P.sibirica by continuous and extensive gene flow.P.armeniaca originated in Northwest China(Ili Valley),subsequently spread throughout Central Asia,and eventually spread to Europe.In addition,selective sweep signatures in P.armeniaca during domestication from wild to cultivated apricots,combined with differentially expressed genes,underline distinct phenotypic fruit traits,including sugars,aromas,organic acids and carotenoids.The main purposes of this study were to evaluate the genetic diversities,population structures and relationships of apricots in different ecological groups,clarify the phylogenetic relationships between five ecological groups and provide a theoretical basis for further improving and effectively utilizing apricot germplasm resources.(4)Two chloroplast DNA fragments(trnL-trnF and ycf1)and the nuclear ribosomal DNA internal transcribed spacer(ITS)were employed to study genetic variation across 20 populations of Prunus armeniaca L.,to study its phytogeography.The results showed that the level of genetic diversity of P.armeniaca was higher,and the level of genetic differentiation was lower.AMOVA analysis revealed that genetic variation of P.armeniaca mainly among individuals within populations.The value of Nsr was significantly higher than that of GST,indicating that there was a genealogical structure of P.armeniaca.The genetic backgrounds of Prunus sibirica,Prunus mandshurica,and Prunus dasycarpa have the same genotypes(T2,T7-T8 and T12)as P.armeniaca,and they should be associated with P.armeniaca through continuous and extensive gene flow.Haplotypes/genotypes of Central Asian,European and North Chinese populations mixed with large numbers of haplotypes/genotypes of wild apricot populations of Ili River Valley.We suggested that the wild apricot population of the Ili River Valley was the P.armeniaca ancestor of the Central Asian,European and North Chinese populations.The wild apricot populations in Ili River Valley contained ancestral haplotypes/genotypes with the highest genetic diversity,and were located in an area considered a potential glacial refuge for P.armeniaca.Having experienced population expansion at approximately 16.53 kyr,the area provides a suitable climate for the biological population and protects the genetic diversity of P.armeniaca. |
PDF Full Text Request