Botanic Institute of Barcelona (IBB-CSIC-ICUB), Pg. del Migdia s/n, E08038 Barcelona, Spain.
Ann Bot. 2011 Aug;108(2):263-77. doi: 10.1093/aob/mcr138. Epub 2011 Jun 28.
The geographic distribution of the genus Plectocephalus comprises a single species in Ethiopia, two in North America and possibly four more in South America, in a striking disjunction that is exceptional for genera of the tribe Cardueae. The enormity of this disjunction cast doubts on the precise taxonomic delineation of the genus, which is not unanimously recognized as a natural entity. The aims of this study were to define the generic boundaries of Plectocephalus and to formulate a hypothesis that would explain its natural range.
A combined molecular approach, using nuclear internal transcribed spacers (ITS) and external transcribed spacers (ETS), and plastid trnL-trnL-F, rpl32-trnL(UAG) and ndhF markers, was chosen for phylogenetic reconstruction by maximum parsimony and Bayesian inference.
Phylogenetic analysis shows that Plectocephalus is a natural genus that includes the African species P. varians, together with all the native South American species, currently classified as Centaurea, C. cachinalensis, C. floccosa and C. tweediei. The recognition of Centaurodendron as an independent genus, which we consider appropriate, would make Plectocephalus paraphyletic. Affinities of Plectocephalus should lie with eastern representatives of Centaureinae. Geographic disjunction is explained as a consequence of dispersal via the Bering Land Bridge during the Miocene--Pliocene. The phylogeny of the basal grade of Centaureinae differs from previous phylogenies, and artefacts resulting from differences in mutation rates of annual and perennial taxa are confirmed. Sensitivity of ITS to these differences was the highest observed for all DNA regions used in this study.
The natural status of the genus Plectocephalus is confirmed and several nomenclatural combinations are proposed. New evidence contributes to the debate concerning problems posed by the use of ITS in the phylogenetic reconstruction of groups that differ in terms of their life cycles. Dispersal from Caucasus and Anatolia along the Siberian route and then across the Bering Land Bridge follows a route previously proposed for other taxonomic groups.
属 Plectocephalus 的地理分布包括埃塞俄比亚的一个种、北美洲的两个种，以及南美洲可能还有四个种，这种明显的间断分布在蓟族中是非常特殊的。这种巨大的间断分布使人对该属的精确分类产生了怀疑，因为它并不被一致认为是一个自然实体。本研究的目的是确定 Plectocephalus 的属界，并提出一个假设来解释其自然范围。
选择了一种综合的分子方法，使用核内转录间隔区（ITS）和外转录间隔区（ETS）以及质体 trnL-trnL-F、rpl32-trnL（UAG）和 ndhF 标记，通过最大简约法和贝叶斯推断进行系统发育重建。
系统发育分析表明，Plectocephalus 是一个自然属，包括非洲种 P. varians，以及所有目前被归类为 Centaurea 的南美本地种，即 C. cachinalensis、C. floccosa 和 C. tweediei。我们认为，将 Centaurodendron 作为一个独立的属来识别是合适的，这将使 Plectocephalus 成为并系的。Plectocephalus 的亲缘关系应与 Centaureinae 的东部代表有关。地理间断被解释为中新世-上新世通过白令陆桥扩散的结果。Centaurodendron 的基部等级的系统发育与以前的系统发育不同，并证实了由一年生和多年生分类群的突变率差异产生的假象。在本研究中使用的所有 DNA 区域中，ITS 对这些差异的敏感性是最高的。
确认了属 Plectocephalus 的自然状态，并提出了几个命名组合。新的证据有助于解决在以生命周期不同的类群为对象进行系统发育重建时，ITS 应用所带来的问题的争论。从高加索和安纳托利亚沿西伯利亚路线扩散，然后穿过白令陆桥，遵循了以前为其他分类群提出的路线。
