Moore Michael J, Jansen Robert K
Section of Integrative Biology and Institute of Cellular and Molecular Biology, University of Texas at Austin, 1 University Station A6700, Austin, TX 78712, USA.
Mol Phylogenet Evol. 2006 Jun;39(3):668-87. doi: 10.1016/j.ympev.2006.01.020. Epub 2006 Feb 21.
Although the deserts of North America are of very recent origin, their characteristic arid-adapted endemic plant lineages have been suggested to be much older. Earlier researchers have hypothesized that the ancestors of many of these modern desert lineages first adapted to aridity in highly localized arid or semi-arid sites as early as the late Cretaceous or early Tertiary, and that these lineages subsequently spread and diversified as global climate became increasingly arid during the Cenozoic. No study has explicitly examined these hypotheses for any North American arid-adapted plant group. The current paper tests these hypotheses using the genus Tiquilia (Boraginaceae), a diverse North American desert plant group. A strongly supported phylogeny of the genus is estimated using combined sequence data from three chloroplast markers (matK, ndhF, and rps16) and two nuclear markers (ITS and waxy). Ages of divergence events within the genus are estimated using penalized likelihood and a molecular clock approach on the ndhF tree for Tiquilia and representative outgroups, including most of the major lineages of Boraginales. The dating analysis suggests that the stem lineage of Tiquilia split from its nearest extant relative in the Paleocene or Eocene ( approximately 59-48 Ma). This was followed by a relatively long period before the first divergence in the crown group near the Eocene/Oligocene boundary ( approximately 33-29 Ma), shortly after the greatest Cenozoic episode of rapid aridification. Divergence of seven major lineages of Tiquilia is dated to the early-to-mid Miocene ( approximately 23-13 Ma). Several major lineages show a marked increase in diversification concomitant with the onset of more widespread semi-arid and then arid conditions beginning in the late Miocene ( approximately 7 Ma). This sequence of divergence events in Tiquilia agrees well with earlier researchers' ideas concerning North American desert flora assembly.
尽管北美沙漠形成的时间非常近，但有观点认为其适应干旱的特有植物谱系要古老得多。早期研究人员推测，这些现代沙漠谱系中的许多祖先早在白垩纪晚期或第三纪早期就首次在高度局部化的干旱或半干旱地区适应了干旱环境，并且随着新生代全球气候变得越来越干旱，这些谱系随后扩散并多样化。尚无研究针对任何北美适应干旱的植物类群明确检验这些假说。本文使用北美沙漠植物类群——紫草科的蒂基利亚属（Tiquilia）来检验这些假说。利用来自三个叶绿体标记（matK、ndhF和rps16）和两个核标记（ITS和蜡质基因）的组合序列数据，估计了该属得到有力支持的系统发育。利用惩罚似然法和分子钟方法，在蒂基利亚属以及包括紫草目大多数主要谱系在内的代表性外类群的ndhF树上，估计了该属内分化事件的时间。年代测定分析表明，蒂基利亚属的主干谱系在古新世或始新世（约5900万 - 4800万年前）与其最近的现存亲属分化。随后是一段相对较长的时期，直到始新世/渐新世边界附近（约3300万 - 2900万年前）冠群首次分化，这一时期正值新生代最大规模的快速干旱事件之后不久。蒂基利亚属七个主要谱系的分化时间定在中新世早期至中期（约2300万 - 1300万年前）。几个主要谱系显示出多样化的显著增加，这与始于中新世晚期（约700万年前）更广泛的半干旱然后干旱条件的开始相伴。蒂基利亚属的这一分化事件序列与早期研究人员关于北美沙漠植物群组装的观点非常吻合。
