Xiang Q Y, Soltis D E, Soltis P S
Department of Botany, Washington State University, Pullman, Washington, 99164-4238,
Mol Phylogenet Evol. 1998 Oct;10(2):178-90. doi: 10.1006/mpev.1998.0524.
One of the most remarkable examples of intercontinental disjunction of the North Temperate Flora involves eastern Asia and eastern and western North America. Although there has been considerable interest in this phytogeographic pattern for over 150 years (e.g., Gray, 1859; Li, 1952; Graham, 1972; Boufford and Spongberg, 1983; Wu, 1983; Tiffney, 1985a, 1985b), relationships among taxa displaying the disjunction remain obscure. Understanding phylogenetic relationships is, however, a prerequisite for historical biogeographic analyses of this distributional pattern. To understand better the relationships of taxa displaying this intercontinental disjunction, phylogenetic analyses were conducted using a variety of DNA data sets for species of four genera (Cornus, Boykinia, Tiarella, and Trautvetteria) that occur in eastern Asia, eastern North America, and western North America. An area cladogram was constructed for each of the four genera, all of which show a similar pattern of relationship: the eastern Asian species are sister to all North American species. An identical phylogenetic pattern is also found in three other taxa exhibiting this disjunction (Aralia sect. Aralia, Calycanthus, and Adiantum pedatum). The congruent phylogenetic pattern found in these seven diverse genera raises the possibility of a common origin of the eastern Asia, eastern and western North America disjunction. The data are in agreement with the long-standing hypothesis that this well-known floristic disjunction represents the fragmentation of a once continuous Mixed Mesophytic forest community and suggest that the disjunction may have involved only two major vicariance events: an initial split between Eurasia and North America, followed by the isolation of floras between eastern and western North America. However, congruence between phylogenies and geographic distributions does not necessarily indicate an identical phytogeographic history. Taxa exhibiting the same phylogenetic pattern may have originated at different geological times. Analysis of divergence times using the molecular clock indicates that species of Cornus, Boykinia, and Calycanthus may have diverged at different geological times, suggesting that the floristic disjunction involving eastern Asia and North America may not be simple; it may have involved multiple historical events at very different geological times in different genera.
北温带植物区系洲际间断分布最显著的例子之一涉及东亚以及北美东部和西部。尽管150多年来人们一直对这种植物地理格局颇感兴趣（例如，格雷，1859年；李，1952年；格雷厄姆，1972年；布福德和斯庞伯格，1983年；吴，1983年；蒂夫尼，1985a，1985b），但呈现间断分布的类群之间的关系仍不明确。然而，了解系统发育关系是对这种分布格局进行历史生物地理学分析的前提。为了更好地理解呈现这种洲际间断分布的类群之间的关系，我们利用多种DNA数据集，对分布于东亚、北美东部和北美西部的四个属（山茱萸属、博伊金草属、黄水枝属和特拉特韦特草属）的物种进行了系统发育分析。为这四个属分别构建了区域分支图，所有这些属都显示出相似的关系模式：东亚物种是所有北美物种的姐妹群。在另外三个呈现这种间断分布的类群（五加属五加组、夏蜡梅属和掌叶铁线蕨）中也发现了相同的系统发育模式。在这七个不同的属中发现的一致的系统发育模式增加了东亚、北美东部和西部间断分布有共同起源的可能性。这些数据与长期以来的假说相符，即这种著名的植物区系间断分布代表了曾经连续的中生混交林群落的破碎化，并表明这种间断分布可能只涉及两次主要的地理隔离事件：欧亚大陆和北美之间的最初分裂，随后是北美东部和西部植物区系的隔离。然而，系统发育与地理分布之间的一致性并不一定表明具有相同的植物地理历史。呈现相同系统发育模式的类群可能在不同的地质时期起源。利用分子钟对分歧时间进行分析表明，山茱萸属、博伊金草属和夏蜡梅属的物种可能在不同的地质时期发生了分歧，这表明涉及东亚和北美的植物区系间断分布可能并不简单；它可能在不同的属中涉及了不同地质时期的多个历史事件。
