基于cpDNA片段探讨中-日间断分布双花木属植物的系统发育学

双花木属(Disanthus)植物为东亚特有濒危物种和典型的中-日间断分布成分,基于叶绿体DNA(cp DNA)的3个片段(psb A-trn H、rps16F-R2、psa A-ycf3)联合对双花木属16个居群的164个个体进行系统发育学研究,系统发育的结果支持日本的D.Cercidifolius和中国的D.cercidifolius subsp.Longipes为单系类群,两个群体间无共享单倍型,且结果进一步支持日本群体分为两个地理组,一个来自日本本州岛的中部(居群QF,CY),一个是来自日本本州岛的西南部(居群gz、gd和JH)。PERMUT分析结果表明,双花木属居群间总的遗传多样性Ht(0.725)高于居群内平均遗传多样性场Hs(0.148),居群遗传差异Nst(0.905)大于Gst(0.796),存在极显著的谱系地理学结构。AMOVA进一步表明,双花木属的变异主要来自群体间,即中国和日本的两个群体分化明显,变异占53.08%,组内种群间的变异占24.11%,来自个体间的变异为22.80%。     

七筋姑属种皮形态特征的研究

用扫描电镜对七筋姑属北美全部４个种和东亚七筋姑不同产地的１１个居群进行了种皮微形态特征的观察和研究,结果表明,北美４种中,Ｃｌｉｎｔｏｎｉａ ｕｍｂｅｌｌｕｌａｔａ以其种皮近于平滑、表面分布有大小不等的凹坑而不同于其它各种具条纹状或网穴状的表皮纹饰。北美西部的２个种Ｃ．ｕｎｉｆｌｏｒａ和Ｃ．ａｎｄｒｅｗｓｉａｎａ种皮纹饰相似,关系密切;而北美东部的Ｃ．ｂｏｒｅａｌｉｓ与东亚种Ｃ．ｕｄｅｎｓｉｓ长白     

中国与北美苔藓植物区系关系的探讨

关于中国和北美间苔藓植物区系关系的研究是在20世纪90年代开始的。研究表明中国和北美苔类和藓类的种间关系疏远,这一事实说明两个大陆在第三纪后期分离后,由于地质、地理、气候等因素的差异致使苔藓植物区系成分也产生明显分异。虽然它们在科、属方面差异较小,但两个大陆已各自形成地区特有的苔类和藓类的属。已知的中国特有及东亚特有的苔类属有5个,藓类属有22个;而已知北美地区特有的苔类属有4个、藓类属有5个。北美东部和西部形成于不同地质时期,在苔藓植物方面与中国共同分布的科、属和种存在一定差异。从神农架藓类植物研究表明,从共同分布的种来看,北美东部与中国比北美西部与中国的相似性系数高约6％。从两地区间苔类和藓类植物的雌雄同株或雌雄异株及孢子直径大小而论,这两个因素似乎并不是影响苔藓植物在两大地区间分布的主要原因,但对苔藓植物的分布仍然十分重要。本文作者认为结合中国和北美的地史、地理条件及苔藓植物自身的特点,可以认为白令海峡不是阻碍中国和北美苔藓植物交流的绝对自然屏障;中国和北美苔藓植物区系关系目前的状况,是长期多次交流融合和分异的结果。     

箭叶淫羊藿同质园栽培居群非腺毛多样性及其分类学启示

淫羊藿是国际上多年来重点研究的药用植物.箭叶淫羊藿(Epimedium sagittatum)是淫羊藿属分布最广,也是形态变异最大的物种,箭叶淫羊藿复合群种分类中存在诸多疑点和争议.为了重建淫羊藿属的系统发育,我们利用数码显微摄影技术对10省16个箭叶淫羊藿同质园栽培居群的叶背非腺毛性状进行了比较研究.结果表明,箭叶淫羊藿非腺毛性状的主要特征包括:细胞数、非腺毛长度、是否存在伸长细胞、顶细胞形态及由此导致的顶细胞直径和壁厚差异.不同居群的非腺毛形态和特征存在巨大差异和丰富的多样性,其中项细胞形状是最重要的特征,并与全长、有无伸长细胞、项细胞比例、顶细胞直径和壁厚等多个性状密切关联.聚类分析将16个居群划分为两个类型,即梭形组和剑形组.变异式样的分析表明个体间和居群间整体呈现连续变异,丰富的多样性可能是多型种的表现.但形态上的分化往往来源于关键性状特征的强烈选择作用引起的居群间适应性分化,同质园栽培个体和居群间的形态差异暗示着适应性相关性状的遗传分化,居群间的形态差异应为地方适应性的结果,这种差异可能是物种形成的雏形.     

基于RAPD标记的5个南京椴居群遗传多样性分析

采用RAPD标记技术,对分布于江苏省(紫金山、牛首山和宝华山)及安徽省(皇藏峪和琅琊山)的5个南京椴(Tilia miqueliana Maxim.)居群的遗传多样性进行了分析,并采用UPGMA聚类方法研究了5个居群的遗传关系.结果表明:使用10条多态性引物从5个居群的总DNA中扩增出169条带,其中多态性条带151条,多态性条带百分率达89.34％.各居群的多态性条带百分率(PPB)、有效等位基因数(Ⅳe)、Nei's基因多样度(h)和Shannon's多样性指数(I)均有明显差异,其中,宝华山居群的各项指标均最高,紫金山和牛首山居群的PPB值最低,琅琊山居群的Ne值最低,紫金山居群的h和I值均最低.5个居群的I、h和Ⅳe值分别为0.2430～ 0.335 1、0.1544 ～0.218 2和1.248 9～1.362 7;在种水平上的I、h和Ne值分别为0.359 4、0.223 6和1.352 9.居群内变异占总变异的75.95％,居群间变异占总变异的24.05％.居群内和居群间的基因多样度分别为0.218 5和0.173 2,物种水平上的基因分化系数为0.207 5,居群间的基因流为1.909 3.各居群间的遗传距离差异较大;其中,皇藏峪与牛首山居群的遗传距离最近,仅为0.026 5;宝华山与紫金山居群的遗传距离最远,为0.134 4.通过聚类分析可将5个南京椴居群分为3支,宝华山和琅琊山居群各自独立为2支,皇藏峪、紫金山和牛首山居群聚为1支,且可进一步分为2个亚支,皇藏峪居群为1个亚支、紫金山和牛首山居群聚为1个亚支.研究结果表明:南京椴居群内的遗传多样性较高,但居群内的变异占主导地位,居群间存在明显的遗传分化.     

粉条儿菜属(肺筋草科)的地理分布及其物种多样性和分化中心

粉条儿菜属(AletrisL.)隶属于肺筋草科,全世界有23种1变种,东亚有18种1变种,北美东南部有5种,为典型的东亚-北美间断分布的属.本文在种(变种)的水平上,研究了粉条儿菜属的地理分布及其分布中心和多样化中心,并对其起源和分化以及现代洲际间断分布格局的成因进行了分析.结果表明,(1)中国共分布有粉条儿菜属植物15种1变种,而广义的横断山地区集中分布有13种1变种,是东亚粉条儿菜属植物分布最为集中的地区,而且包含该属植物各个进化阶段的代表.因此,广义的横断山地区是粉条儿菜属在东亚的分布中心和多样化中心.(2)根据粉条儿菜属及其近缘属的分布格局推测,该属可能在不晚于第三纪早期,起源于古北大陆.东亚和北美的粉条儿菜属植物形态区别明显,应该是隔离分化的结果.(3)该属植物可能曾经广布于北半球,后来地质、气候以及冰川等因素的变化,导致该属在一些地区灭绝,而仅存于东亚和北美东南部.(4)尽管横断山及其周边地区是东亚粉条儿菜属的多样化中心,但该地区很可能并不是粉条儿菜属最早的分化中心,因横断山地区周边的一些特有种可能是在晚近的时期形成的新特有种;另外,东亚粉条儿菜属一些原始的种类主要分布于我国中东部到日本一带.所以,中国中东部到日本一带可能是粉条儿菜属早期的分化中心.     

基于叶绿体基因证据的民族药滇白珠复合群系统发育关系

滇白珠是我国重要的民族药用植物,广泛分布于长江以南地区,是一个分类困难的复合群。为了探讨其种下分类关系,该文对滇白珠复合群(包括毛滇白珠、秃果白珠和滇白珠3个变种)进行网罗式采样,基本覆盖了该复合群在中国的分布范围,同时包括菲律宾和马来西亚的各1个居群,共计81个居群241个个体,通过联合两个变异位点适中的叶绿体片段rpl33-psa J和trn L-rpl32,构建基于最大似然法(Maximum Likelihood)和贝叶斯法(Bayesian Inference)的系统发育树,以及Neighbor-Net法构建系统发育网络。结果表明:滇白珠复合群内具有明显的遗传差异,这种差异性相比形态,与地域分布相关性更大。系统发育分析显示,滇白珠复合群分为3个支系。其中:一支包括中国台湾和菲律宾南达沃的居群,为变种秃果白珠,符合前人分类结果;一支包括分布于横断山脉区域的居群,由变种毛滇白珠和滇白珠组成;剩余一支包括分布于华东南区域的居群,同样由变种毛滇白珠和滇白珠组成。分子证据支持基于形态分类的秃果白珠作为变种,而不支持毛滇白珠作为变种的处理。这样的遗传分化式样可能是由地理隔离导致,这一结果为...     

基于ISSR标记的福建省钩锥遗传多样性分析

该研究利用ISSR分子标记,对分布于福建省内5个样地(邵武、建阳、建瓯、周宁和屏南)的61个野钩锥(Castanopsis tibetana)单株的遗传多样性进行了分析,并采用聚类分析方法探讨了它们的遗传关系。结果表明:用10条ISSR引物从61个单株的基因组DNA共扩增出158条带,包含145条多态性条带,多态性条带百分率达91.77%,其中引物UBC817、UBC819与UBC842的多态性条带百分率(PPB)为100.0%。各居群的多态性条带百分率(PPB)、有效等位基因数(Ne)、Nei’s基因多样度(H)和Shannon’s多样性指数(I)等各遗传指数差异较大,其中各项遗传指标中最高的为邵武居群,而周宁居群则最低。5个居群的基因分化系数和基因流分别为0.144 0和2.973 0,说明5个居群总遗传变异的14.40%存在于居群间,85.60%存在于居群内。种间总基因多样度分别为0.395 8,种内基因多样度分别为0.338 8,表明钩锥种间遗传多样性较高,且种间变异大于种内变异。各居群间的遗传距离差异较大;其中,邵武与建瓯居群的遗传距离最近,仅为0.081 5;建阳和周宁居群的遗传距离最远,为0.162 9。通过聚类分析可将5个钩锥居群聚为3支,屏南与周宁的居群各自独立聚为2支;来自邵武、建瓯及建阳的居群聚为一支,且可进一步分为两个亚支,建阳居群为1个亚支,邵武和建瓯居群聚为1个亚支。供试的钩锥具有较高的遗传多样性,存在着较为频繁的基因交流。该研究结果较准确地揭示了钩锥种间的遗传多样性。     

基于细胞核rDNA ITS片段的水青冈属的分子系统发育

对山毛榉科水青冈属6种、1亚种、1栽培变种的ITS区片段进行了测序和分析,并对其中2个具有ITS序列多态性的分类群进行了ITS区克隆。水青冈属ITS系统发育树聚成两支,位于基部的是分布于北美的大叶水青冈,另一分支则包括了欧洲和东亚的类群。在欧洲和东亚分支中,又包括两支,其中日本北部的波叶水青冈位于基部,台湾水青冈和欧亚大陆的水青冈形成另外一支。ITS区分析与现行的水青冈属基于形态学性状的属下分类系统有一定差异,而与本属现存物种的地理分布格局较为一致。各类群间TIS区序列差异较小,显示属内现存物种的分化时间不是太长。     

木薯己糖激酶MeHXK5的催化功能鉴定

该研究利用ISSR分子标记,对分布于福建省内5个样地( 邵武、建阳、建瓯、周宁和屏南)的61个野钩锥(Castanopsis tibetana)单株的遗传多样性进行了分析,并采用聚类分析方法探讨了它们的遗传关系。结果表明： 用10条ISSR引物从61个单株的基因组DNA共扩增出158条带,包含145条多态性条带,多态性条带百分率达91.77%,其中引物 UBC817、UBC819与UBC842的多态性条带百分率（PPB）为100.0%。各居群的多态性条带百分率(PPB)、有效等位基因数(Ne)、Nei’s基因多样度(H)和Shannon’s多样性指数（I）等各遗传指数差异较大,其中各项遗传指标中最高的为邵武居群,而周宁居群则最低。5个居群的基因分化系数和基因流分别为0.144 0和2.973 0,说明5个居群总遗传变异的14.40%存在于居群间,85.60%存在于居群内。种间总基因多样度分别为0.395 8,种内基因多样度分别为0.338 8,表明钩锥种间遗传多样性较高,且种间变异大于种内变异。各居群间的遗传距离差异较大; 其中,邵武与建瓯居群的遗传距离最近,仅为0.081 5; 建阳和周宁居群的遗传距离最远,为0.162 9。通过聚类分析可将5个钩锥居群聚为3支,屏南与周宁的居群各自独立聚为2支;来自邵武、建瓯及建阳的居群聚为一支,且可进一步分为两个亚支,建阳居群为1个亚支,邵武和建瓯居群聚为1个亚支。供试的钩锥具有较高的遗传多样性,存在着较为频繁的基因交流。该研究结果较准确地揭示了钩锥种间的遗传多样性。     

Evolution of biogeographic disjunction between eastern Asia and eastern North America in Phryma (Phrymaceae)

This study examines molecular and morphological differentiation in Phryma L., which has only one species with a well-known classic intercontinental disjunct distribution between eastern Asia (EA) and eastern North America (ENA). Phylogenetic analysis of nuclear ribosomal ITS and chloroplast rps16 and trnL-F sequences revealed two highly distinct clades corresponding to EA and ENA. The divergence time between the intercontinental populations was estimated to be 3.68 ± 2.25 to 5.23 ± 1.37 million years ago (mya) based on combined chloroplast data using Bayesian and penalized likelihood methods. Phylogeographic and dispersal-vicariance (DIVA) analysis suggest a North American origin of Phryma and its migration into EA via the Bering land bridge. Multivariate analysis based on 23 quantitative morphological characters detected no geographic groups at the intercontinental level. The intercontinental populations of Phryma thus show distinct molecular divergence with little morphological differentiation. The discordance of the molecular and morphological patterns may be explained by morphological stasis due to ecological similarity in both continents. The divergence of Phryma from its close relatives in the Phrymaceae was estimated to be at least 32.32 ± 4.46 to 49.35 ± 3.18 mya.     

Phylogenetics and biogeography of eastern Asian-North American disjunct genus Pachysandra （Buxaceae） inferred from nucleotide sequences

Pachysandra is an eastern Asian-North American disjtunct genus with three species, two in eastern Asia （Pachysandra axillaris and Pachysandra terminalis） and one in eastern North America （Pachysandra procurnbens）. Although morphological and cytological studies suggest a close affinity of Pprocumbens with P axillaris, molecular data from nuclear and chloroplast DNA regions have provided conflicting signals. In this study, we tested previous phylogenetic hypotheses using sequences of nuclear ribosomal DNA internal transcribed spacers and chloroplast ndhF gene from multiple individuals of each of the three species. We also estimated the time of divergence between eastem Asia and eastern North America. Our results support the morphological and cytological conclusion that P procumbens is more closely related to P axillaris than to P terminalis. The estimated time of divergence of P axillaris and P procumbens was 14.6±5.5 mya, consistent with estimates from many other eastern Asian-North American disjunct genera. The migration of Pachysandra populations from eastern Asia to North America might have occurred by way of the North Atlantic land bridge.     

Timing the eastern Asian-eastern North American floristic disjunction: molecular clock corroborates paleontological estimates

Sequence data of the chloroplast gene rbcL were used to estimate the time of the well-known eastern Asian-eastern North American floristic disjunction. Sequence divergence of rbcL was examined for 22 species of 11 genera (Campsis, Caulophyllum, Cornus, Decumaria, Liriodendron, Menispermum, Mitchella, Pachysandra, Penthorum, Podophyllum, and Phryma) representing a diverse array of flowering plants occurring disjunctly in eastern Asia and eastern North America. Divergence times of putative disjunct species pairs were estimated from synonymous substitutions, using rbcL molecular clocks calibrated for Cornus. Relative rate tests were performed to assess rate constancy of rbcL evolution among lineages. Corrections of estimates of divergence times for each species pair were made based on rate differences of rbcL between Cornus and other species pairs. Results of these analyses indicate that the time of divergence of species pairs examined ranges from 12.56 +/- 4.30 million years to recent (<0.31 million years), with most within the last 10 million years (in the late Miocene and Pliocene). These results suggest that the isolation of most morphologically similar disjunct species in eastern Asia and eastern North America occurred during the global climatic cooling period that took place throughout the late Tertiary and Quaternary. This estimate is closely correlated with paleontological evidence and in agreement with the hypothesis that considers the eastern Asian-eastern North American floristic disjunction to be the result of the range restriction of a once more or less continuously distributed mixed mesophytic forest of the Northern Hemisphere that occurred during the late Tertiary and Quaternary. This implies that in most taxa the disjunction may have resulted from vicariance events. However, long-distance dispersal may explain the disjunct distribution of taxa with low divergence, such as Menispermum.     

Morphological and molecular comparisons ofCampsis grandiflora andC. radicans (Bignoniaceae), an eastern Asian and eastern North American vicariad species pair

Morphological and molecular comparisons were made forCampsis grandiflora (Thunb.)K. Schumann (Bignoniaceae) from eastern Asia andC. radicans (L.)Seemann from eastern North America. Chloroplast DNA (cpDNA) variation was surveyed with 20 restriction endonucleases. The cpDNA divergence between the two vicariad species was 2.44%, which is the highest reported among North Temperate disjunct taxa and one of the highest reported for infrageneric taxa. Detailed morphological comparisons also suggest a high level of divergence. Cluster analyses based on 22 morphological characters and 39 OTUs revealed two distinct groups corresponding with the two species. The average taxonomic distance between the two species was 1.806. Analyses of variance (ANOVA) revealed that 12 of the 18 quantitative characters differed significantly ( 0.01) betweenC. grandiflora andC. radicans. Divergence time based on cpDNA data was estimated as 24.4 million years. The Bering land bridge hypothesis was favored over the North Atlantic land bridge hypothesis based on the estimated divergence time and the geological history of the North Temperate region. The high levels of morphological and cpDNA divergence are not consistent with morphological stasis, which has been proposed as a common mode of evolution for North Temperate disjunct taxa.     

Phylogenetics and biogeography of eastern Asian–North American disjunct genus Pachysandra (Buxaceae) inferred from nucleotide sequences

Abstract Pachysandra is an eastern Asian–North American disjunct genus with three species, two in eastern Asia (Pachysandra axillaris and Pachysandra terminalis) and one in eastern North America (Pachysandra procumbens). Although morphological and cytological studies suggest a close affinity of P. procumbens with P. axillaris, molecular data from nuclear and chloroplast DNA regions have provided conflicting signals. In this study, we tested previous phylogenetic hypotheses using sequences of nuclear ribosomal DNA internal transcribed spacers and chloroplast ndhF gene from multiple individuals of each of the three species. We also estimated the time of divergence between eastern Asia and eastern North America. Our results support the morphological and cytological conclusion that P. procumbens is more closely related to P. axillaris than to P. terminalis. The estimated time of divergence of P. axillaris and P. procumbens was 14.6±5.5 mya, consistent with estimates from many other eastern Asian–North American disjunct genera. The migration of Pachysandra populations from eastern Asia to North America might have occurred by way of the North Atlantic land bridge.     

MOLECULAR DIVERGENCE BETWEEN ASIAN AND NORTH AMERICAN SPECIES OF LIRIODENDRON (MAGNOLIACEAE) WITH IMPLICATIONS FOR INTERPRETATION OF FOSSIL FLORAS

Botanists have long been aware of the floristic similarities between eastern Asia and eastern North America. Most who have considered this classic disjunction pattern have suggested that it arose through range disruption of a flora that was once more widely distributed in the Northern Hemisphere. There is less agreement on the timing of this process, with suggestions ranging from the Paleocene to the Neogene. In this study, molecular markers from two different plant genomes were used to assess the degree of genetic divergence between the two interfertile, morphologically similar species of the genus Liriodendron, i.e., L. tulipifera and L. chinense. Resulting molecular divergence estimates were translated into approximate dates of separation, independent of evidence from the fossil record. Allozyme data (Nei's genetic identity = 0.434) suggested a divergence time of 10–16 million years before present, whereas sequence divergence in the plastid genomes (1.24%) led to an estimate of approximately 11–14 million years before present. A review of the paleobotanical literature indicated that the fossil floras that included, or might have included Liriodendron could not have survived in Beringia after the late Miocene and the onset of southward-migrating Arctic air masses on the North American continent. This interpretation suggests a minimum time of separation of approximately 13 million years before present. Thus, both molecular data sets and the paleobotanical evidence concur in suggesting a divergence time of 10–16 million years before present. Interspecific compatibility and relative morphological stasis must have, therefore, persisted from at least the late Miocene. We emphasize the need for similar studies in other genera, especially those that have both a reasonable Tertiary fossil history and extant species in mesic temperate refugia in Asia, Europe, and western as well as eastern North America.     

Evolution of the Eastern Asian and Eastern North American disjunct genus Symplocarpus (Araceae): insights from chloroplast DNA restriction site data

Chloroplast DNA (cpDNA) variation was surveyed with 20 restriction endonucleases for the eastern Asian and eastern North American disjunct genus Symplocarpus (Araceae). The cpDNA phylogeny reveals a sister group relationship between S. foetidus from eastern North America and S. renifolius from eastern Asia. The cpDNA divergence between the two intercontinental sister species is 0.61%, which suggests an estimated divergence time of 6.1 million years ago during the late Miocene. The Bering land bridge hypothesis is compatible with the estimated time of divergence for the migration of Symplocarpus between eastern Asia and North America. Furthermore, a single origin of the exothermic spadices in Symplocarpus is suggested by the phylogeny. The cpDNA data also provide independent support for the recognition of three species within the genus.     

Integrated Fossil and Molecular Data Reveal the Biogeographic Diversification of the Eastern Asian-Eastern North American Disjunct Hickory Genus (Carya Nutt.)

The hickory genus (Carya) contains ca. 17 species distributed in subtropical and tropical regions of eastern Asia and subtropical to temperate regions of eastern North America. Previously, the phylogenetic relationships between eastern Asian and eastern North American species of Carya were not fully confirmed even with an extensive sampling, biogeographic and diversification patterns had thus never been investigated in a phylogenetic context. We sampled 17 species of Carya and 15 species representing all other genera of the Juglandaceae as outgroups, with eight nuclear and plastid loci to reconstruct the phylogeny of Carya. The phylogenetic positions of seven extinct genera of the Juglandaceae were inferred using morphological characters and the molecular phylogeny as a backbone constraint. Divergence times within Carya were estimated with relaxed Bayesian dating. Biogeographic analyses were performed in DIVA and LAGRANGE. Diversification rates were inferred by LASER and APE packages. Our results support two major clades within Carya, corresponding to the lineages of eastern Asia and eastern North America. The split between the two disjunct clades is estimated to be 21.58 (95% HPD 11.07-35.51) Ma. Genus-level DIVA and LAGRANGE analyses incorporating both extant and extinct genera of the Juglandaceae suggested that Carya originated in North America, and migrated to Eurasia during the early Tertiary via the North Atlantic land bridge. Fragmentation of the distribution caused by global cooling in the late Tertiary resulted in the current disjunction. The diversification rate of hickories in eastern North America appeared to be higher than that in eastern Asia, which is ascribed to greater ecological opportunities, key morphological innovations, and polyploidy.     

Timing the Eastern Asian–Eastern North American Floristic Disjunction: Molecular Clock Corroborates Paleontological Estimates

Sequence data of the chloroplast gene rbcL were used to estimate the time of the well-known eastern Asian–eastern North American floristic disjunction. Sequence divergence of rbcL was examined for 22 species of 11 genera (Campsis, Caulophyllum, Cornus, Decumaria, Liriodendron, Menispermum, Mitchella, Pachysandra, Penthorum, Podophyllum, and Phryma) representing a diverse array of flowering plants occurring disjunctly in eastern Asia and eastern North America. Divergence times of putative disjunct species pairs were estimated from synonymous substitutions, using rbcL molecular clocks calibrated for Cornus. Relative rate tests were performed to assess rate constancy of rbcL evolution among lineages. Corrections of estimates of divergence times for each species pair were made based on rate differences of rbcL between Cornus and other species pairs. Results of these analyses indicate that the time of divergence of species pairs examined ranges from 12.56 ± 4.30 million years to recent (<0.31 million years), with most within the last 10 million years (in the late Miocene and Pliocene). These results suggest that the isolation of most morphologically similar disjunct species in eastern Asia and eastern North America occurred during the global climatic cooling period that took place throughout the late Tertiary and Quaternary. This estimate is closely correlated with paleontological evidence and in agreement with the hypothesis that considers the eastern Asian–eastern North American floristic disjunction to be the result of the range restriction of a once more or less continuously distributed mixed mesophytic forest of the Northern Hemisphere that occurred during the late Tertiary and Quaternary. This implies that in most taxa the disjunction may have resulted from vicariance events. However, long-distance dispersal may explain the disjunct distribution of taxa with low divergence, such as Menispermum.     

Phylogeny and biogeography of Aralia sect. Aralia (Araliaceae)

Aralia sect. Aralia (Araliaceae) consists of approximately eight species disjunctly distributed in Asia and North America. Phylogenetic and biogeographic analyses were conducted using sequences of the internal transcribed spacer regions of the nuclear ribosomal DNA. Aralia racemosa from eastern North America was sister to A. californica from western North America. Aralia cordata from eastern Asia did not form a species-pair relationship with the eastern North American A. racemosa. The two subspecies of A. racemosa formed a monophyletic group. Biogeographic analyses showed a close area relationship between eastern North America and western North America. The Himalayas were cladistically basal and eastern Asia was placed between the Himalayas and North America. The biogeographic analysis supported the origin of the eastern Asian and eastern North American disjunct pattern in Aralia sect. Aralia via the Bering land bridges. Comparisons with results of phylogenetic analyses of other genera suggested that (1) the floristic connection between eastern North America and western North America may be stronger than previously thought; and (2) the biogeographic patterns in the Northern Hemisphere are complex. Furthermore, a lack of correlation between sequence divergence values and phylogenetic positions was observed, suggesting the importance of a phylogenetic framework in biogeographic analyses.