|其他摘要||Taxus wallichiana var. mairei (Lemée & Léveillé) L. K. Fu & Nan Li is a highly endangered conifer. It is dioecious and wind-pollinated, and the seed is dispersed by birds and mammals. It is the source for taxol, an important anti-tumour substance. However, extant natural populations in China are highly scattered due to exceptional loss of habitat and inappropriate harvest of barks of mature trees in the past decade. The research of the conservation biology of it is urgently needed. In this study, some 494 individuals from 18 populations of T. wallichiana var. mairei were collected across the distribution range, including the East China, Central China, and South China mountain floristic regions.
A total of six haplotypes (H1, H2, H3, H4, H5, H6) were detected in all 494 individuals of 18 populations, in chloroplast DNA trnL-F fragment by PCR-RFLP. H1 and H2 existed in all the populations, but H3 only existed in HSH population, while H4 only existed in LA population, with H5 and H6 coexisting in TW population. The phylogeographic analysis suuggested the H1 and H2 were the ancestor haplotypes, while the others haplotypes were evolved from them. Some 24.49% of the total variation existed among the populations; while 75.51% in the populations suggested by AMOVA. Variation of cpDNA haplotypes suggested a glacial refugium of T. wallichiana var. mairei in the East China region. NCA analysis suggested that the genetic structure of T.wallichiana var mairei resulted from the effect of populaton fragment or the long-distance colonisation. The gene flow was rerestricted.
2. The analysis of genetics diversity base on ISSR
The ISSRs revealed a high genetic diversity with PPB = 98.4%, HE = 0.3456, and I = 0.5134 at the species level in 494 individuals of the 18 populations. Population differentiation was low (GST = 0.4079; θB= 0.4854) as was the gene flow among populations (Nm = 0.7256). 18.29% of the total variation existed in TW population suggested by AMOVA. A UPGMA-dendrogram suggests that the population in Taiwan is the most isolated and a trend of east-west split of the populations. The inbreeding coefficient was low (f = 0.5066). The genetic parameters suggested the effect of the fragment, and the gene flow was disturbed. It seems that Taxus wallichiana var. mairei possessed the much high genetic diversity level than the other studied taxa of Taxus
Both the detection of nDNA and cpDNA suggested the genetic structure of genetic variation of the extant of T. wallichiana var. mairei was the result of fragmentation of a historically continuous distribution, and the fragmentation process was far-flung. The refugium populations still contribute to the genetic structure of the T. wallichiana var. maire, but the primal metapopulation was not due to the expansion from a single refugium. Beside the feature of T. wallichiana var. mairei per se, the current genetic structure of it was co-effected by the fragmentation of the population, the climate change and the geological history of the distribution ariea and the large-scale habitat destruction caused by human activities.
3. Germination study
A preliminary germination test suggested that the endosperm was pale green at the beginning of the seed resurgence. For the species of Taxus, once the cotyledon emerges, it means that the seedling isolates utterly with the endosperms. However, at this stage, the seedlings bear neither functional leaves nor lush root system. Therefore it seems that the seedlings are easy to be burned. This observation is very preliminary and there is no sufficient data in the germination test in the present study. We assume that these characters may have decreased the survival rate of the seedlings in T. wallichiana var. mairei.
4. Conservation stratergy
Based on field survey and the results revealed by ISSR markers and chloroplast haplotype variation, together with other evidence, conservation strategy was proposed for the sustainable use of the endangered plant. It is needed to collect seeds from the wild to enhance the germination rate and the survival rate of the germinated seedlings. The seedlings should be used to ensure the size of the disturbed populations. Conservation of the natural habitat is the key to the survival rate of the seedlings. The survival rate of the seedlings is the base of further populations and is the indemnity for the plant to survive. Artificial pollination between populations can increase gene exchanges. Population JY is the largest population of T. wallichiana var. mairei we collected. The population has a reseanable age structure and higher genetic diversity, and therefore is an excellent candidate for in situ conservation. Although the size of populations LA and HSH are small they possess unique cpDNA haplotypes, and may be valuable for in situ conservation, too.|