石山苣苔属(苦苣苔科)花形态演化及分类学意义

Floral evolutionary trend of Petrocodon(Gesneriaceae)and its significance in classification

关键词:  分类, 花部特征, 苦苣苔科, 同塑性演化, 喀斯特植物, 石山苣苔属

Key words:  classification, floral characters, Gesneriaceae, homoplasious evolution, karst flora, Petrocodon

摘要:

石山苣苔属(苦苣苔科)约30种,主要分布于我国南部的石灰岩地区。目前该属已知物种数虽少但花形态极其多样,是该科中分类最为困难的类群之一。基于分子证据,其它8个属中花形态迥异的一些物种被并入石山苣苔属。然而,该属花形态的演化趋势缺乏系统性的研究,传统分类对属的界定与分子系统学研究结果相矛盾的原因,以及是否有形态特征支持新界定的石山苣苔属还不清楚。该研究中,总共编码了19种石山苣苔属植物和9种报春苣苔属植物的35个形态特征,其中包括26个花部形态特征,在分子系统树上追踪了它们的演化路径。结果表明:无论属内还是属间,多数花部形态特征,尤其以往属的分类界定特征,在演化过程中变化频繁且发生了高度同塑性演化,这是导致传统形态分类不自然的关键因素。此外,在观察研究的所有特征中,花丝和柱头的差异可能在石山苣苔属植物共同祖先中经历了演变,或可用于区分石山苣苔属与其姐妹报春苣苔属的大多数种类。因此,在苦苣苔科植物的分类学研究中应当慎用这些花部性状作为分类依据,而且应对形态特征进行广泛地观察研究,在密集的取样和分辨率更高、更可靠的系统树上追踪它们的演化规律。更为重要的是,需要进一步研究导致复杂形态性状演化的内在分子调控机理和外在的自然选择动力,最终更加深入地理解石山苣苔属等典型喀斯特植物的演化过程和机理。

Abstract:

Petrocodon Hance is a genus(ca. 30 species)in the angiosperm family Gesneriaceae, which are mainly distributed in the limestone region of South China. The genus is special for its remarkable floral diversity, and is also one of the most taxonomy-difficult groups in the family. Although the genus was redefined to accommodate species with distinctive floral morphologies from eight other genera based on molecular phylogenetic studies, the evolutionary trends of the floral characters have not been studied comprehensively yet; the major causes of the incongruence between traditional taxonomy and molecular phylogenetic studies remain largely unexplored; and the morphological characters that can support the redefined Petrocodon keep unknown, either. In the present study, we coded 35 morphological characters(including 26 floral characters)of 19 Petrocodon species and 9 outgroup taxa of Primulina, traced the evolutionary trends and addressed the taxonomic implications in Petrocodon based on phylogenetic tree. We found that most of the floral characters, especially those used for delimiting traditional genera, evolved frequently and highly homoplasiously within and across the genus, which were the key factors that misled the traditional classifications. Further, we proposed that among all the characters we examined, the shape of filaments and stigmas may be used to distinguish most taxa of Petrocodon from its closest relative Primulina. The present study indicates that caution must be taken when using these floral traits in future classification of Gesneriaceae. Moreover, extensive examinations of the morphological characters and their evolutionary trends need to be carried out and traced upon denser sampling and more robust phylogenetic tree. More importantly, efforts should also be taken to investigate the underlying molecular regulatory networks and driving forces underpinning the complicated floral evolution of Petrocodon. All these will finally improve the understandings of the evolutionary processes and mechanism of this group of typical karst flora and its allies.