Washingtonia is a genus of palms currently composed of two species, W. filifera and W. robusta, distributed in the States of Baja California, Baja California Sur, and Sonora in Mexico, and Southern California and Arizona in the United States. The group has been a taxonomic challenge due to a lack of type specimens, incomplete protologues, highly variable vegetative morphology, human dispersal of seeds, limited fieldwork in native populations and poor representation in herbaria. Here, we analyse the population structure and phylogenetic relationships and test whether morphological traits correlate with genetic variation throughout its distributional range. We used genotyping-by-sequencing (GBS) data to identify population structure and delimit species. We further used these data to determine whether morphological traits varied among genetic regions. We analysed 188 individuals from 21 populations of Washingtonia across its distribution range using multivariate and Bayesian methods. Our results showed great consistency in the discovery of four genetic groups: (1) Southern Peninsula, (2) Mid-Peninsula, (3) Northern Peninsula and (4) Sonoran mainland. The geographical limits to these clusters coincide very well with the large natural regions of the Sonoran Desert. Our analyses indicate that Washingtonia populations are highly structured within four major geographical regions. Even when no single morphological trait can be used to determine the genetic identity of Washingtonia palms, leaf greenness, a novel morphological trait, can be useful. Our results provide a robust phylogenetic analysis of Washingtonia settling a taxonomic debate that has lasted over a century.

中文翻译：

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华盛顿（槟榔科）的遗传和形态分化：解开百年棕榈之谜

Washingtonia 是棕榈属，目前由两个物种组成，W. filifera 和 W. robusta，分布在墨西哥的下加利福尼亚州、南下加利福尼亚州和索诺拉州，以及美国的南加州和亚利桑那州。由于缺乏类型标本、不完整的原代词、高度可变的植物形态、人类传播种子、本地人口的实地工作有限以及植物标本馆的代表性不足，该组一直是分类学的挑战。在这里，我们分析了种群结构和系统发育关系，并测试了形态特征是否与整个分布范围内的遗传变异相关。我们使用测序基因分型 (GBS) 数据来识别种群结构和划分物种。我们进一步使用这些数据来确定遗传区域之间的形态特征是否不同。我们使用多元和贝叶斯方法分析了华盛顿州 21 个人口分布范围内的 188 个个体。我们的结果表明，四个基因组的发现具有很大的一致性：（1）南半岛，（2）中半岛，（3）北半岛和（4）索诺兰大陆。这些星团的地理范围与索诺兰沙漠的广阔自然区域非常吻合。我们的分析表明，华盛顿州人口在四个主要地理区域内高度结构化。即使没有单一的形态特征可用于确定华盛顿棕榈树的遗传特性，叶绿度，一种新的形态特征，也是有用的。我们的结果为华盛顿州提供了强有力的系统发育分析，解决了持续了一个多世纪的分类学争论。