Phylogeographic Structure Of Toona Ciliata（meliaceae）in China Inferred From CpDna Markers And Ecological Niche Model Analyses

Toona ciliata is a deciduous/semi-deciduous tree species of the family Meliaceae and genus Toona.It is distributed in the subtropical and tropical areas in China and is a precious timber species,known as the "Chinese mahogany".Owing to its low nature regeneration and over exploitation,the species is currently facing a continuous recession and listed as an endangered tree species,one of the nationally protected wild plants at level Ⅱ.Based on the previously preliminary reports assayed with nuclear SSR and SRAP markers,this study employed chloroplast DNA markers to further investigate the geographical variation of T.ciliata.Two markers successfully screened were the non-coding trn L-trn L fragment at the large single copy（LSC）region and the non-coding psb A-trn H fragment between LSC and inverted repeats（IR）.Four hundred and forty-seven samples in total were collected from twenty-nine populations across the range-wide distribution of the species in China.The amplified sequences of trn L-trn L and psb A-trn H fragments had a finally edited length of 520 and 505 bp,27 and 72 polymorphic sites,respectively.The haplotype diversity was 0.898 for trn L-trn L fragment and 0.977 for psb A-trn H.Because of the characteristics of uniparental inheritance,a low mutation rate and haploid of cpDNA,the high polymorphic haplotypes observed in this study could indirectly reflect a long-term evolutionary process of speciation and the accumulation of multiple mutants.Most distinct haplotypes were distributed in the southwestern regions.Signifiant phylogeographic structure occurred in the whole distribution of the species,with Nst being signifiantly greater than G_st（0.3733 vs.0.1496 for trn L-trn L and 0.7163 vs.0.2566 for psb A-trn H）.Analysis of molecular variance（AMOVA）showed that the inter-population variation Φ_st accounted for 53.71% of the total genetic variation.Significant but weak effects of isolation by distance（IBD）existed across the range-wide populations.Tajima’s D and Fu’s F values indicated that populations at several geographical locations slightly expanded to the central-north but shrank in the southern regions.The historical migration route of forming the present distribution could occur from the southwest to northeast regions.The ecological niche model analysis supported the general conclusion derived from the analyses with cpDNA markers.A map for the area distribution of the species adaptation predicts that Yunnan Province is a highly suitable area for T.ciliata growth in the contemporary and future times.The southwestern regions in Guizhou,Guangxi and Sichuan Provinces are moderately suitable for growth,and the rest areas in the species distribution in China are less suitable for growth.Populations in Guangxi and Guangdong Provinces tend to expand in future distribution,but the fitness of T.ciliata in Zhejiang,Anhui and Hunan Provinces tends to decline.Based on the comprehensive information from both cpDNA markers and niche model analyses,it is suggested that genetic resource management of T.ciliata should focus on the regions with high genetic diversity in the southwest areas.The distribution of rare haplotypes occrrring in the northeast regions is also worthy of attention.Conservation with plantation in ex situ should concentrate on the effects of environmental factors,with a priority given to the temperature,followed by the moisture factor.