大血藤科植物的遗传多样性和分子系统学研究

大血藤科(Sargentodoxaceae)是中国特有科，又是单型科，属于比较古老、原始的类群，分布于黄河流域、秦岭及以南各省区，跨越热带、亚热带、温带等广大地区.大血藤属(Sargentodoxa Rehd.et Wils.)原置于木通科中，由于其形态上的特殊性，Stapf(1926)将其从木通科中分出升级为科.作为被子植物中最原始的10个科之一，其起源古老，分布广泛，在长期的演化过程中与复杂的地理环境条件相适应，协同进化.该研究选择nrDNA的ITS区和叶绿体的rdcL作为大血藤科系统发育的分子标记，进行序列分析，并结合大血藤科的植物地理学方面的证据和已有的序列，进行了单叶血藤成立与否、大血藤不同居群遗传多样性和大血藤科植物地理学三方面的探讨.研究结果发现大血藤科内，ITS的长度为634-635之间，其中ITS1为233bp，5.8为163bp，ITS2的长度为238-239bp.5.8S序列则完全相同.单叶血藤与大血藤其它10个居群的核苷酸差异值仅在0.2﹪-3.9﹪之间，远小于种间的核苷酸差异值1.2﹪-10.2﹪，而且在单叶血藤之间还存在着过渡类型即单叶和三小叶在同一植株上的大血藤，因此单叶血藤宜合并到大血藤中去.大血藤的化石首先在北美的Vermont地层发现，时间大约在渐新世(Tiffney，1993).结合大血藤科和木能科的化石资料和分子数据，分析结果表明大血藤与木通科的分歧时间在侏罗纪末期.大血藤科的早期祖先可能在侏罗纪末已经到达冈瓦纳古陆.渐新世后期开始至第四纪初的喜马拉雅造山运动大大改变了东亚的地貌和大气环流，大血藤科植物从欧洲和北美大部分地区消失，在亚洲得以保存.第四纪多次冰川，使北半球植物的分布和发展受到巨大影响.在我国秦岭以南、长江以南的地区、地形比较复杂，气候受冰期影响更小，从而使该地区成为大血藤科植物最重要的保存中心.结合华夏区系现存的大量的多心皮类的类群，大血藤科植物也极可能是在华夏区系起源的，并在侏罗纪末和白垩纪扩散到欧洲和北美地区.

The family Sargentodoxaceae is an endemic and monotypic family to China, belonging to an ancient and primitive group. It is distributed in provinces and regions along the Yellow River, the Qinling Mountains and areas south of them, spanning vast tropical, subtropical and temperate zones. The genus Sargentodoxa Rehd. et Wils. was originally classified in Lardizabalaceae, but due to its unique morphological characteristics, Stapf (1926) separated it from Lardizabalaceae and elevated it to the family level. As one of the 10 most primitive families in angiosperms, Sargentodoxaceae has an ancient origin and wide distribution, and has adapted to and coevolved with complex geographical environmental conditions during long-term evolution.

This study selected the ITS region of nuclear ribosomal DNA (nrDNA) and the chloroplast rbcL as molecular markers for phylogenetic analysis of Sargentodoxaceae, conducted sequence analysis, and combined with phytogeographical evidence of Sargentodoxaceae and available sequences, discussed three aspects: the validity of the monophyllous Sargentodoxa, the genetic diversity of different populations of Sargentodoxa cuneata, and the phytogeography of Sargentodoxaceae.

The results showed that within Sargentodoxaceae, the length of ITS ranges from 634 to 635 bp, with ITS1 being 233 bp, the 5.8S rRNA gene being 163 bp, and ITS2 ranging from 238 to 239 bp. The sequences of the 5.8S rRNA gene are completely identical. The nucleotide difference between the monophyllous Sargentodoxa and the other 10 populations of Sargentodoxa cuneata is only between 0.2% and 3.9%, which is far lower than the interspecific nucleotide difference of 1.2% to 10.2%. Moreover, transitional forms exist among the monophyllous Sargentodoxa, namely Sargentodoxa plants bearing both simple and trifoliolate leaves on the same individual. Therefore, the monophyllous Sargentodoxa should be merged into Sargentodoxa cuneata.

Fossils of Sargentodoxa were first discovered in the strata of Vermont, North America, dating back approximately to the Oligocene epoch (Tiffney, 1993). Combining fossil data and molecular data of Sargentodoxaceae and Lardizabalaceae, the analysis results indicate that the divergence time between Sargentodoxa and Lardizabalaceae occurred at the end of the Jurassic period. The early ancestors of Sargentodoxaceae may have reached Gondwana by the end of the Jurassic.

The Himalayan orogeny from the late Oligocene to the early Quaternary greatly altered the landform and atmospheric circulation of East Asia. As a result, Sargentodoxaceae plants disappeared from most parts of Europe and North America, but were preserved in Asia. Multiple glaciations during the Quaternary period had a profound impact on the distribution and evolution of Northern Hemisphere plants. Regions south of the Qinling Mountains and the Yangtze River in China have relatively complex terrain and their climates were less affected by glaciations, making this area the most important refugium for Sargentodoxaceae plants.

Combined with the abundant extant polycarpous taxa in the Cathaysian flora, Sargentodoxaceae most likely originated in the Cathaysian flora and dispersed to Europe and North America during the late Jurassic and Cretaceous periods.