Department of Plant and Microbial Biology, North Carolina State University, Raleigh, NC 27695-7612, USA.
Bioinformatics Research Center, North Carolina State University, Raleigh, NC 27695-7566, USA; Department of Statistics, North Carolina State University, Raleigh, NC 27695-8203, USA.
Mol Phylogenet Evol. 2018 Sep;126:1-16. doi: 10.1016/j.ympev.2018.04.001. Epub 2018 Apr 6.
Nyssa sylvatica complex consists of several woody taxa occurring in eastern North America. These taxa were recognized as two or three species including three or four varieties by different authors. Due to high morphological similarities and complexity of morphological variation, classification and delineation of taxa in the group have been difficult and controversial. Here we employ data from RAD-seq to elucidate the genetic structure and phylogenetic relationships within the group. Using the genetic evidence, we evaluate previous classifications and delineate species. We also employ Species Distribution Modeling (SDM) to evaluate impacts of climatic changes on the ranges of the taxa and to gain insights into the relevant refugia in eastern North America. Results from Molecular Variance Analysis (AMOVA), STRUCTURE, phylogenetic analyses using Maximum likelihood, Bayesian Inference, and Splittree methods of RAD-seq data strongly support a two-clade pattern, largely separating samples of N. sylvatica from those of N. biflora-N. ursina mix. Divergence time analysis with BEAST suggests the two clades diverged in the mid Miocene. The ancestor of the present trees of N. sylvatica was suggested to be in the Pliocene and that of N. biflora-N. ursina mix in the end of the Miocene. Results from SDM predicted a smaller range in the southern part of the species present range of each clade during the Last Glacial Maximum (LGM). A northward expansion of the ranges during interglacial period and a northward shift of the ranges in the future under a model of global warming were also predicted. Our results support the recognition of two species in the complex, N. sylvatica and N. biflora, following the phylogenetic species concept. We found no genetic evidence supporting recognitions of intraspecific taxa. However, we propose subsp. ursina and subsp. biflora within N. biflora due to their distinction in habits, distributions, and habitats. Our results further support movements of trees in eastern North America in response to climatic changes. Finally, our study demonstrates that RAD-seq data and a combination of population genomics and SDM are valuable in resolving relationship and biogeographic history of closely related species that are taxonomically difficult.
美国东部的 Nyssa sylvatica 复合种包括几种木本植物，这些植物被不同的作者认为是两种或三种物种，包括三种或四种变种。由于形态高度相似和形态变异的复杂性，该组中分类群的分类和划定一直存在困难和争议。在这里，我们利用 RAD-seq 数据来阐明该组内的遗传结构和系统发育关系。利用遗传证据，我们评估了以前的分类，并划定了物种。我们还利用物种分布模型（SDM）来评估气候变化对分类群范围的影响，并深入了解北美东部的相关避难所。基于 RAD-seq 数据的分子方差分析（AMOVA）、STRUCTURE、最大似然法、贝叶斯推断法和分裂树法的系统发育分析结果强烈支持两分支模式，该模式基本将 N. sylvatica 与 N. biflora-N. ursina mix 的样本分开。BEAST 中的分歧时间分析表明，这两个分支在中新世中期发生分歧。目前的 N. sylvatica 树的祖先被认为是上新世的，而 N. biflora-N. ursina mix 的祖先则是在中新世末期。SDM 的结果预测，在末次冰期最大值（LGM）期间，每个分支现存物种范围的南部范围较小。在间冰期期间，范围向北扩展，在全球变暖模型下，未来范围向北移动。我们的结果支持在该复合种中识别两种物种，即 N. sylvatica 和 N. biflora，这是基于系统发育种概念的。我们没有发现支持识别种内分类群的遗传证据。然而，由于它们在习性、分布和生境方面的区别，我们建议在 N. biflora 中识别亚种 ursina 和亚种 biflora。我们的结果进一步支持了北美东部树木对气候变化的响应。最后，我们的研究表明，RAD-seq 数据以及种群基因组学和 SDM 的结合，对于解决分类困难的密切相关物种的关系和生物地理学历史非常有价值。
