用核糖体DNA的ITS序列探讨裂叶沙参的系统位置——兼论ITS片段在沙参属系统学研究中的价值

A REEVALUATION OF THE STATUS OF A . LOBOPHYLLA BASED ON ITS SEQUENCE, WITH REFERENCE TO THE UTILITY OF ITS SEQUENCE IN ADENOPHORA

本文采用PCR直接测序方法,对来自沙参属全部2组7亚组的10个种和作为外类群的风铃草属2个种的核糖体DNA ITS片段进行了序列分析。在重点探讨裂叶沙参分类地位的同时,分析了ITS片段序列在沙参属系统发育重建中的价值。结果表明,在沙参属中,ITS片段在长度、GC含量和位点变异量上均比较一致;长度539bp～541bp,GC含量57％～60％,信息位点只占总位点的3.9％。采用PAUP软件进行的系统发育分析表明,裂叶沙参A.lobophylla与大花盘亚组的A.himalayana组成一支,而不是象以往形态学和杂交试验所推断的与泡沙参A.potaninii或A.stenanthina近缘。可见,ITS序列进一步支持将裂叶沙参移出泡沙参复合体甚至移出有齿亚组的推论,但同时也表明将其作为A.stenanthina的近缘种是不适宜的。尽管本研究所测定的类群已涵盖了整个沙参属,但种间序列的两两比较表明,沙参属种间在ITS片段上的分化很小(0.0～3.9％),相比之下,沙参属类群与风铃草属类群间的分化却很高(17.8％～19.2％)。这大概和沙参属起源较晚、遗传分化较小有关。本文还就ITS片段在沙参属和桔梗科系统学研究中的价值进行了讨论。

Adenophora Fisch. (Campanulaceae)is a medium-sized and diverse genus, which occurs mainly in eastern Asia, particularly in China. Although several taxonomic systems have been proposed, there is still much debate on the delimitation and systematic arrangement of the species within the genus, due mainly to high variation in morphology, habitats, phe-notypic plasticity, and potential hybridization. A. lobophylla Hong is a rare perennial endemic to several counties of Sichuan, China. Since this species was recognized, much work has been done on its taxonomic status, includ .ng morphology, crossing experiment and al-lozymes. However, evidence available is not concordance so far, and its relationship with other species within the genus remains largely uncertain. In the present paper, the internal transcribed spacer(ITS) region of 18s^26s nuclear ribosomal DNA was sequenced in 10 representatives of Adenophora and two outgroup species of Campanula to assess its utility for phylogenetic reconstruction in Adenophora, in addition to reevaluation of the taxonomic status of A. lobophylla. The results indicate that there exist high sequence alignability and length conservation among ITS sequences, with informative sites being 3.9% and length ranging from 539 to 541 bp and G + C content varying from 57% to 60% . Phylogenetic analyses using Fitch parsimony show that A. lobophylla is closely related to A. hi-malayana instead of A . potaninii or A . stenanthina as revealed by previous morphological work and crossing experiment. The ITS phylogeny is generally congruent with other evidence in that A . lobophylla should be removed from the A . potaninii complex or even sub-sect. Microdiscus. Its appropriate placement, however, should be further explored. In pair-wise comparisons at unambiguously aligned sites, sequence divergence was relatively low a-mong Adenophora species with the values ranging from 0.0 to 3.9 % in spite of the fact that the species surveyed covered all of the sections and subsections of the genus. In contrast, high sequence divergence is found among Adenophora and Campanula species with the values from 17.8% to 19.2% . The low divergence among Adenophora species is justified considering its recent origin, and also suggests that ITS sequence would provide a promising source of nuclear phylogenetic markers to reconstruct the phylogeny within Campanulaceae on the tribe and family levels.