Studies On The Systematic Position Of Stracheya Tibetica Benth

Stracheya tibetica Benth. is the only species of the genus Stracheya Benth., whichbelongs to the family Leguminosae, tribe Hedysareae. S. tibetica has a close relationship withthe genus Hedysarum Linn.. Scholars have different opinions on the treatment of S. tibetica, i.e. S. tibetica in a separated monotypic genus, or it should be merged into the genusHedysarum. In order to study the systematic position of S. tibetica, this thesis seeks evidencesfrom the following4aspects: leaflet venations of S. tibetica and related taxa, anatomyfeatures of pod, chromosome numbers of S. tibetica, molecular phylogeny reconstructionbased on ITS sequences. The results are as follows:(1) S. tibetica has a leaflet venation with stout primary vein and major secondary veins,with large and poorly developed areolations, with simple terminal of freely ending veinlet,which has no special morphology compared to the diversified venations of the genusHedysarum. Besides, venation of S. tibetica is similar to that of Hedysarum Sect. Obscura.(2) Pod of S. tibetica has dorsal suture and ventral suture both with one vascular bundleand with extensions outside, which is similar to that of Hedysarum Sect. Obscura. Thepterygoid extension shaped by thickened mesocarp outside the dorsal suture fills in the gapformed by the constriction between the dorsal suture and the ventral suture, so that the pod ofS. tibetica has no obvious joint. Pod with pterygoid extension outside the suture is right theunique feature of Hedysarum Sect. Obscura. Prickles on the pod of S. tibetica and prickles onthe pod of Hedysarum Sect. Fruticosa and Sect. Subacaulia have the same anatomy structure.(3) The chromosome number of S. tibetica is counted to be2n=48, and the basicchromosome number should be x=8, which mean S. tibetica is hexaploid. The basicchromosome number x=8is common in the genus Hedysarum and the tribe Hedysareae butthe hexaploid is seldom reported. The hexaploid of S. tibetica may be the result of adaptationto the cold environment of Tibetan Plateau.(4) In the phylogenetic tree based on ITS sequences, taxa of the genus Hedysarummerged with other genera of the tribe Hedysareae together forming7parallel clades. It isdifficult to tell whether the genus Hedysarum is a monophyletic group. S. tibetica is on one ofthe7parallel clades, so it is difficult to tell which group of Hedysarum is related to S. tibetica. More DNA sequences are needed to study the phylogenetic relationship between S. tibeticaand Hedysarum.In conclusion, evidences from leaflet venation, pod anatomy stucture and chromosomenumbers support the opinion that S. tibetica should be merged into the genus Hedysarum.Furthermore, S. tibetica shows a close relationship with Hedysarum Sect. Obscura. However,the phylogenetic tree based on ITS sequences reveals no clear phylogenetic relationshipbetween S. tibetica and Hedysarum. Further study of molecular phylogenetic relationshipbetween S. tibetica and Hedysarum is needed.