| Toona ciliata Roem., a deciduous or semi-deciduous tree, is a valuable timber species in China and listed as a level Ⅱ national key protected wild plant. Commonly known as Chinese mahogany, the tree grows rapidly in the early stage. Given the huge developmental potential of this species, it has become the focus of a development and utilisation program targeting the planting of fast-growing timber species in southern China. This study focuses on natural provenances and individual trees. For the first time the germplasm resources of T. ciliata were collected throughout China and from the east coast of Australia. Research using SRAP and SSR molecular markers and on its phenotypic traits, afforestation and seedling stage characteristics revealed geographic variation and its trend, and clarified the degree of genetic diversity and genetic differentiation between T. ciliata provenances and individuals. The results provides an important theoretical basis for germplasm resources conservation, breeding strategy and sustainable development genetic improvement work of T. ciliata. The main results of this study are described below.1. Fourteen phenotypic traits of 30 provenances in China were analysed. These traits differed significantly among individuals and provenances, and 52.46% of the total variation resided among provenances. Most fruit and seed traits were strongly genetically controlled. Longitude variation explained the main trend in the majority of phenotypic traits. With the exception of petiole length, the sizes of leaves, fruits and seeds decreased from east to west. Leaf blade size was significantly positively correlated with size seed and fruit size. According to principal component and cluster analyses, the 30 provenances were divided into two groups:one comprising central and east China provenances, and the other comprising the southwest and south China provenances. The characteristics of different traits was obviously.2. Wood density ranged from 0.2804-0.5346 g/cm3. The mean provenance wood density was 0.4222 g/cm3. The fibre length of T. ciliata was 443.10-646.58μm; i.e. fibres were short. Wood density and fibre morphology differed significantly among provenances; these properties in the southwest and south China provenances were superior to those of the central China and east China provenances, and their processing and utilisation value was greater.3. Traits differed significantly among provenances and families at the seedling and young growth stages. The heritability of seedling height, ground diameter and crown breadth were>0.85 at the seeding stage, while family heritability of growth traits were>0.5 across afforestations. The interaction between provenance and environment was also highly significant. The heritability and standard errors of tree height and DBH in Zengcheng(Guangong), Taolin (Hunan) and Dongmen (Guangxi) were 0.8110±0.0787,0.7620±0.0796,0.8660±0.0552 and 0.7330±0.118,0.8320±0.054,0.8760±0.0329, respectively. The genetic gain of provenance selection was large.4. The distributions of height and ground diameter were fitted to an "S-shaped" curve, and differed significantly among provenances at the seedling stage. During the fast-growth period, the duration of height and base diameter growth in the eastern provenance was longer than that in the western provenance, while the extent of growth was smaller.5. Sequence-related amplified polymorphism (SRAP) markers and simple sequence repeats (SSRs) were used to investigate genetic diversity among 40 T. ciliata provenances in China and Australia A total of 505 polymorphic bands were amplified using 24 primer pairs. Based on SRAP, the average polymorphism information content (PIC) value was 0.41, and the average Nei’s gene diversity index (H) was 0.3770. Shannon’s information index (I) within provenances ranged from 0.1575 to 0.4675, and the average I value among provenances was 0.5569. For 12 SSR loci, there were a total of 147 alleles, and the PIC was 0.723. No significant linkage disequilibrium was detected between SSR loci, suggesting independent segregation among provenances. T. ciliata exhibited few alleles and medium genetic diversity, as evidenced by the fact that the number of alleles (Na) and effective alleles (Ne) were 2.2500-5.8333 and 1.9362-3.3765, respectively. An ANOVA indicated total variation within provenances of 75.31%. In the current study, the Nm value of T. ciliata was 0.605, revealing that gene flow among provenances occurred and that the extent of differentiation was low. The habitat fragmentation and the difference in flowering period between east and west China resulted in geographic isolation and thus a low frequency of gene exchange. This increased the geographic variation among provenances.In STRUCTURE analysis, the 40 provenances were divided into two groups. Group Ⅰ included provenances from Yunnan, Sichuan, Guangxi, Guizhou, Yunfu and Australia, while group Ⅱ comprised those from Hubei, Hunan, Zhejiang, Jiangxi, Fujian, and Anhui. Based on the SRAP data, the unweighted pair group method of arithmetic averages (UPGMA) cluster showed that the 40 provenances could be classified into four types. Cluster Ⅰ consisted of the 14 provenances from central China and east China, while LC formed Cluster Ⅱ separately. Cluster Ⅲ provenances were mainly from south China and southwest China, but the two provenances from Guangdong province were not classified together. Group IV was composed of provenances from Guangdong (YF) and 10 Australian provenances. This clustering is consistent with the principal coordinate analysis biplot and with the STRUCTURE analysis.The correlation between the similarity coefficients matrix of SRAP and SSR was r=0.6883; therefore, SRAP and SSR were identical. The regional characteristics of genetic structure were more unequivocal and the genetic relationships among the 40 provenances were well defined based on the integrated SRAP and SSR marker data. Isolation-by-distance (IBD) patterns were revealed in the 30 Chinese provenances by Mantel test. |
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