Population Genetics And Species Delimitation Of The Zygophyllum Xanthoxylon Complex(Zygophyllaceae)

The Zygophyllum xanthoxylon complex,belonging to the family Zygophyllaceae,is one of the most characteristic components in arid regions and contributes greatly to the stability of desert ecosystems.However,due to the influence of deteriorating environment and human activities,their populations were severely fragmented and decreased in the past30 years.Therefore,Some basic studies concerning genetic diversity,genetic structure and population dynamic will facilitate future protection and sustainable utilization of the Z.xanthoxylon complex.In addition,the Z.xanthoxylon complex including Z.xanthoxylon and Z.kaschgaricum is controversial on account of their similar morphological characteristics and indistinct interspecific boundaries.In this study,145 individuals in 32 populations of the Z.xanthoxylon complex were sampled,covering all known distribution areas in China.Genomic SNP data obtained using SLAF-seq technology were used to conduct population genetics and species delimitation studies.Some morphological and micromorphological traits were also observed to provide evidence for the species definition.The population dynamic responding to the past(LGM),present(1950-2000s)and future(2070s)environmental conditions was modeled and predicted in Max Ent with 137 pieces of distribution information in China.The main resutles were as follows:1.Population genetics studies of Z.xanthoxylon complex based on SNP data show that:(1)The results of genetic structure,phylogenetic tree and PCA are highly consistent.All 32 populations are divided into four lineages,GNMQ lineage,EXJ lineage,MXJ lineage and WXJ lineage,characterized by geographic separation and lineage differentiation.But the external morphology is not enough to distinguish these lineages.(2)AMOVA and Fst analysis revealed that although the genetic variation mainly occurred within populations,a large degree of genetic differentiation also occurred among lineages,especially the WXJ lineage in the Tarim Basin was highly differentiated from other lineages.(3)The overall genetic diversity level of Z.xanthoxylon complex was relatively low,which was significantly positively correlated with longitude.The genetic diversity of each lineage from high to low was: GNMQ > EXJ > MXJ > WXJ,that is,decreasing from east to west in China.2.Results of population dynamic modeling with Max Ent displayed highly suitable distribution range of Z.xanthoxylon complex in the LGM(about 22,000 years ago)was very small,and was limited to the Tarim Basin in southern Xinjiang.It is speculated that the Tarim Basin may be its refuge during the LGM.Compared with the LGM,predicted suitable habitats under the current climatic conditions expanded significantly,mainly distributed in the Tianshan Mountains,Helan Mountains,Hexi Corridor,Qaidam Basin and Alxa Plateau.The simulated suitable habitats for Z.xanthoxylon complex in 2070 is not much different from the current distribution range.It can be speculated from the simulation results that during LGM,Tarim Basin was the most suitable area.After that,with the expansion of deserts and the formation of Hexi Corridor,Z.xanthoxylon complex expanded and migrated to the southern slope of Tianshan Mountain,Helan Mountain and Alxa Plateau.3.Morphological evidence of leaves,flowers,fruits and seeds of Z.xanthoxylon and Z.kaschgaricum indicated that these morphological distinctions were greatly affected by water and nutritional conditions,and there was no substantive morphological difference between populations.And molecular evidence for SNPs also displayed they did not form two separate branches but were chimeric with each other on both the phylogenetic tree and the NJ tree.So we propose to merge Z.kaschgaricum into Z.xanthoxylon based on genetic compositions and our morphological analyses.