Department of Biology, Western Washington University, Bellingham, Washington, United States of America.
PLoS One. 2013 Jul 26;8(7):e69814. doi: 10.1371/journal.pone.0069814. Print 2013.
Arctic-alpine plants in the genus Saxifraga L. (Saxifragaceae Juss.) provide an excellent system for investigating the process of diversification in northern regions. Yet, sect. Trachyphyllum (Gaud.) Koch, which is comprised of about 8 to 26 species, has still not been explored by molecular systematists even though taxonomists concur that the section needs to be thoroughly re-examined. Our goals were to use chloroplast trnL-F and nuclear ITS DNA sequence data to circumscribe the section phylogenetically, test models of geographically-based population divergence, and assess the utility of morphological characters in estimating evolutionary relationships. To do so, we sequenced both genetic markers for 19 taxa within the section. The phylogenetic inferences of sect. Trachyphyllum using maximum likelihood and Bayesian analyses showed that the section is polyphyletic, with S. aspera L. and S bryoides L. falling outside the main clade. In addition, the analyses supported several taxonomic re-classifications to prior names. We used two approaches to test biogeographic hypotheses: i) a coalescent approach in Mesquite to test the fit of our reconstructed gene trees to geographically-based models of population divergence and ii) a maximum likelihood inference in Lagrange. These tests uncovered strong support for an origin of the clade in the Southern Rocky Mountains of North America followed by dispersal and divergence episodes across refugia. Finally we adopted a stochastic character mapping approach in SIMMAP to investigate the utility of morphological characters in estimating evolutionary relationships among taxa. We found that few morphological characters were phylogenetically informative and many were misleading. Our molecular analyses provide a foundation for the diversity and evolutionary relationships within sect. Trachyphyllum and hypotheses for better understanding the patterns and processes of divergence in this section, other saxifrages, and plants inhabiting the North Pacific Rim.
高山北极植物属石竹科石竹属（Saxifragaceae Juss.）为研究北方地区物种多样化过程提供了极好的系统。然而，即使分类学家一致认为该属需要彻底重新研究，分子系统学家仍未对包含约 8 至 26 个种的 Trachyphyllum （Gaud.）Koch 组进行研究。我们的目标是使用叶绿体 trnL-F 和核 ITS DNA 序列数据对该属进行系统发育分析，检验基于地理的种群分化模型，并评估形态特征在估计进化关系中的作用。为此，我们对该属内的 19 个种进行了这两个遗传标记的测序。使用最大似然法和贝叶斯分析对 Trachyphyllum 属的系统发育推断表明，该属是多系的，S. aspera L. 和 S. bryoides L. 落在主要分支之外。此外，分析结果支持了对先前名称的若干分类学重新分类。我们使用两种方法检验生物地理假说：i）Mesquite 中的一个合并方法，以检验重建基因树与基于地理的种群分化模型的拟合程度，ii）Lagrange 中的最大似然推断。这些测试强烈支持该分支起源于北美的落矶山脉南部，随后经历了扩散和避难所分化事件。最后，我们在 SIMMAP 中采用了随机特征映射方法来研究形态特征在估计属内种间进化关系中的作用。我们发现，很少有形态特征具有系统发育信息，许多特征具有误导性。我们的分子分析为 Trachyphyllum 属内的多样性和进化关系提供了基础，并为更好地理解该属、其他石竹科植物和北太平洋地区植物的分化模式和过程提供了假设。
