Molecular phylogeny, recent radiation and evolution of gross morphology of the rhubarb genus Rheum (Polygonaceae) inferred from chloroplast DNA trnL-F sequences
Molecular phylogeny, recent radiation and evolution of gross morphology of the rhubarb genus Rheum (Polygonaceae) inferred from chloroplast DNA trnL-F sequences
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DOI：
10.1093/aob/mci201
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发表时间：
2005-09-01
期刊：
影响因子：
4.2
通讯作者：
Liu, JQ
中科院分区：
文献类型：
--
作者：
Wang, AL;Yang, MH;Liu, JQ
背景与目的大黄属植物约有60余种，分布于青藏高原的山地和荒漠区及其邻近地区，是一个高度多样化的属。该属是青藏高原温带属广泛多样性的一个很好的例子，其中驱动多样性的力量仍然未知。迄今为止，大黄属下的分类主要是基于形态特征。然而，它可能已经受到趋同进化的栖息地压力下，和一些部分的系统位置是不清楚的，特别是组Globulosa，其中有球状花序，和组Nobilia，其中有半透明的苞片。最近的孢粉学研究发现，大黄属的外壁模式和目前的分类之间存在着实质性的矛盾。本研究的两个具体目标是（1）评估一些具有独特形态的模糊部分的可能关系，以及（2）检查在整个属中可能发生的低遗传分化的辐射物种形成以及大黄广泛的多样化时间与过去地理事件之间的相关性。方法对大黄属8个组中7个组的26种29个个体的叶绿体DNA trnL-F区序列进行测定和比较。根据获得的序列进一步构建了系统发育关系。关键结果尽管形态高度多样化，但该DNA片段的遗传变异相对较低。分子系统发育与宏观形态、花粉外壁模式和传统分类极不一致，但将掌叶组的所有样品、穗状组的3个种和大黄组的少数种鉴定为相应的单系群。单型组Globulosa显示了一个暂定的分支内，包括5个种的组大黄。所有分析均揭示了R. nobile和R. alexandum，只有两个种的组Nobilia由拥有大苞片的限制。基于trnL-F序列差异的谱系的粗校准暗示了大黄在大约100年内的广泛多样性。结论：青藏高原自晚第三纪以来的大规模隆升，加上第四纪气候的振荡，可能促进了大黄属小种群和孤立种群的快速物种形成，并使大黄属独特或罕见的形态特征得以固定。这种快速的辐射，结合渐渗杂交和网状进化，可能导致了形态上不同物种之间的cpDNA单倍型的转移，并可能解释了形态分类和分子系统学之间的不一致。
Background and Aims Rheum, a highly diversified genus with about 60 species, is mainly confined to the mountainous and desert regions of the Qinghai-Tibetan plateau and adjacent areas. This genus represents a good example of the extensive diversification of the temperate genera in the Qinghai-Tibetan plateau, in which the forces to drive diversification remain unknown. To date, the infrageneric classification of Rheum has been mainly based on morphological characters. However, it may have been subject to convergent evolution under habitat pressure, and the systematic position of some sections are unclear, especially Sect. Globulosa, which has globular inflorescences, and Sect. Nobilia, which has semi-translucent bracts. Recent palynological research has found substantial contradictions between exine patterns and the current classification of Rheum. Two specific objectives of this research were (1) to evaluate possible relationships of some ambiguous sections with a unique morphology, and (2) to examine possible occurrence of the radiative speciation with low genetic divergence across the total genus and the correlation between the extensive diversification time of Rheum and past geographical events, especially the recent large-scale uplifts of the Qinghai-Tibetan Plateau.Methods The chloroplast DNA trnL-F region of 29 individuals representing 26 species of Rheum, belonging to seven out of eight sections, was sequenced and compared. The phylogenetic relationships were further constructed based on the sequences obtained.Key Results Despite the highly diversified morphology, the genetic variation in this DNA fragment is relatively low. The molecular phylogeny is highly inconsistent with gross morphology, pollen exine patterns and traditional classifications, except for identifying all samples of Sect. Palmata, three species of Sect. Spiciformia and a few species of Sect. Rheum as corresponding monophyletic groups. The monotypic Sect. Globulosa showed a tentative position within the clade comprising five species of Sect. Rheum. All of the analyses revealed the paraphyly of R. nobile and R. alexandrae, the only two species of Sect. Nobilia circumscribed by the possession of large bracts. The crude calibration of lineages based on trnL-F sequence differentiation implied an extensive diversification of Rheum within approx. 7 million years.Conclusions Based on these results, it is suggested that the rich geological and ecological diversity caused by the recent large-scale uplifts of the Qinghai-Tibetan Plateau since the late Tertiary, coupled with the oscillating climate of the Quaternary stage, might have promoted rapid speciation in small and isolated populations, as well as allowing the fixation of unique or rare morphological characters in Rheum. Such a rapid radiation, combined with introgressive hybridization and reticulate evolution, may have caused the transfer of cpDNA haplotypes between morphologically dissimilar species, and might account for the inconsistency between morphological classification and molecular phylogeny reported here.