| Tibetan Giant Cypress,Cupressus gigantea W.C.Cheng et L.K.Fu,is a rare plant species endemic to Tibet,which has been listed in the Chinese National Protection List of Wild Plant (Class I).C.gigantea has long been used as raw materials of Tibetan incense,which is also an good candidate tree species in ecological restoration under drought conditions.In addition,C. gigantea is of important significance for ecological and evolutionary researches,especially for relationship between the uplift of Qinghai-Tibet Plateau and species radiation.Besides a natural park at Linzhi County,the natural distribution of C.gigantea is limited to a 100 km long belt-like riparian area along Yaluzangbu River,which is now at the risk of habitat destroying because of road construction.As a result,studies on conservation biology of C.gigantea are urgently needed.In this study,molecular marker analyses were performed to explore the genetic diversity and genetic structure in C.gigantea,which may provide important insights into making effective conservation strategy.Through GC-MS analyses,the essential oil composition of C.gigantea was compared with that of its conger species C.torulosa D.Don,a common species in southwest China, to evaluate the potential of C.torulosa as raw materials of Tibetan incense.In addition,a few observational experiments on seed germination of C.gigantea were also conducted.The major results are as follows:1) Nine populations(189 individuals) of C.gigantea were sampled throughout its natural distribution and analyzed using AFLP markers.Totally 475 repeatable and clear bands are produced using eight pairs of primers,among which 381 are polymorphic.At species level, the percentage of polymorphic loci(P) is 80.21%,Nei's gene diversity index(He) is 0.2469, and Shannon's diversity index is 0.3762.The levels of population genetic diversity are similar in nine populations(mean He is 0.2031).There is significant genetic differentiation among populations(Gst=0.17).Moreover,significant positive correlation(r=0.78159,p=0.002) is detected between the pairwise genetic distances and geographical distances among populations,which may be partially explained by the isolation by distance model. STRUCTURE analyses indicate no evidence of gene introgression between C.gigantea and C. torulosa.The populations along the Yaluzangbu River are genetically different from those in the natural park at Linzhi county,which should receive more attention in future conservation plan.A sampling strategy based on geographic distance seems robust to collect the germplasm resources of C.gigantea in ex-situ conservation.2) The essential oil composition of C.gigantea was compared with that of C.torulosa through GC-MS analyses.Totally 52 components are detected in essential oil of C.gigantea, accounting for 99.45%of total content.The top five components areβ-Pinene(18.45%), p-menth-1-en-4-ol(17.57%),α-pinene(11.70%) andα-Cadinol(10.26%) andγ-terpinene(5.64%),accounting for 63.62%of total content.In contrast,48 components are detected in essential oil of C.torulosa,accounting for 99.95%of total content.Similarly,the top five components account for 67.60%of total content,four of which are also among the top five in C.gigantea:β-pinene(21.68%),p-menth-1-en-4-ol(15.85%),γ-terpinene(13.3%), andα-pinene(6.32%).In general,the two congeners share 33 common components, especially in the components with high abundance,suggesting that C.torulosa is likely to become a potential material of Tibetan incense in future although further studies are needed.In summary,this study revealed the genetic diversity and genetic structure in C.gigantea, and inter-specific variation in essential oil of C.gigantea and C.torulosa.I also raised some related issues that can be addressed in future studies,including the conservation plan of C. gigantea,the phylogenetic relationship between C.gigantea and C.torulosa,and the industrial techniques of Tibetan incense using C.torulosa as materials. |
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