Molecular divergence between disjunct taxa in eastern Asia and eastern North America

Eastern Asian-eastern North American disjuncts in four genera were examined for allozyme divergence and sequence divergence of the internal transcribed spacers (ITS) of nuclear ribosomal DNA. The disjunct pairs of taxa include Caulophyllum robustum-C. thalictroid.es, Menispermum dauricum-M. canadense, Penthorum chinense-P. sedoides, and Phryma leptostachya var. asiatica-P. leptostachya var. leptostachya. Allozyme divergence was comparable in Caulophyllum and Penthorum (genetic identities of 0.534 and 0.546) and was considerably higher than between pairs of taxa in Menispermum (0.273) and Phryma (0.291). Caulophyllum and Penthorum, which have the highest genetic identities at allozyme loci, also have low ITS sequence divergences (1.30 and 1.65%, respectively). Phryma, which has low isozyme identity, also has the highest ITS sequence divergence (4.46%). The two taxa of Menispermum have low ITS sequence divergence (0.93%) despite having a low identity (0.273) at allozyme loci. The results suggest that divergence between the taxa in the four genera are not the result of a single historical event. Estimated divergence times are reasonably consistent with a late Miocene disjunction for Caulophyllum and Penthorum, whereas the age of the Phryma disjunction is calculated at over 20 million years. The nonconcordant divergences between allozymes and ITS sequences in Menispermum may be caused by concerted evolution in the latter or possibly longer generation time in the woody plants. Additional molecular data are needed to clarify the situation.     

The region effect on mesoscale plant species richness between eastern Asia and eastern North America

The greater number of plant species in temperate eastern Asia compared to eastern North America has been ascribed to both local environment and regional characteristics, but the relative contributions of each have not been resolved. In this analysis, we related species richness of flowering plants in mesoscale floras (<10⁴ km²) dominated by temperate forest vegetation to area, elevation, latitude, and several climate variables. When analyses were conducted separately within each region, area and, in eastern Asia, elevation, were the primary determinants of species richness. It appears that the number of species in mesic temperate floras within these regions is largely unrelated to the relatively narrow range of local climate factors associated with these floras. Analysis of covariance of the logarithm of species richness with the logarithm of area (b=0.148) and climate measurements as independent variables revealed a region effect, with species richness in eastern Asia exceeding that in eastern North America by 0.294 log₁₀ units, or a factor of 2.0. Similar regional differences in species richness were apparent in floras compiled from larger areas. Understanding differences in plant species richness between regions requires consideration of regional influences, whose effects should be tested in comparative analyses based on floristic surveys of ecologically characterized small areas.     

Beta diversity of angiosperms in temperate floras of eastern Asia and eastern North America

The diversity of a region reflects both local diversity and the turnover of species (beta diversity) between areas. The angiosperm flora of eastern Asia (EAS) is roughly twice as rich as that of eastern North America (ENA), in spite of similar area and climate. Using province/state‐level angiosperm species floras, we calculated beta diversity as the slope of the relationship between the log of species similarity (S ) and either geographic distance or difference in climate. Distance‐based beta diversity was 2.6 times greater in the north–south direction in EAS than in ENA and 3.3 times greater in the east–west direction. When ln S was related to distance and climate difference in multiple regressions, both distance and climate PC1 were significant effects in the north–south direction, but only geographic distance had a significant, unique influence in the east–west direction. The general predominance of distance over environment in beta diversity suggests that history and geography have had a strong influence on the regional diversity of these temperate floras.     

Phylogeny and biogeography of Sassafras (Lauraceae) disjunct between eastern Asia and eastern North America

Sassafras (Lauraceae) consists of three species disjunct between eastern Asia (S. tzumu and S. randaiense) and eastern North America (S. albidum). Phylogenetic analysis based on sequences of nuclear ribosomal ITS and three chloroplast non-coding regions (rpl16, trnL-F, and psbA-trnH) showed that Sassafras is monophyletic and that the eastern North American S. albidum is sister to the clade of its two eastern Asian counterparts. Their intercontinental divergence was estimated to be 13.80 ± 2.29−16.69 ± 2.52 million years ago (mya) using the penalized likelihood method with the ITS and three chloroplast markers. Biogeographic analyses combined with fossil evidence suggest that Sassafras has a relict distribution in the Northern Hemisphere without a Gondwanan link. The divergence time of the two eastern Asian species (the continental Chinese Sassafras tzumu and S. randaiense endemic to Taiwan) is estimated to be 0.61 ± 0.75−2.23 ± 0.76 mya. Sassafras randaiense from Taiwan was most likely derived from an ancestor from continental China.     

Vascular plant diversity in eastern Asia and North America: historical and ecological explanations

Taxonomic diversity of vascular plants (ferns, gymnosperms and angiosperms) was compared between eastern Asia and North America. Eastern Asia has significantly higher species richness in all three classes but the difference was greatest in ferns and least in angiosperms. Differences in taxonomic treatments between the two continents are not likely contributors to these patterns. The relationship of regional to global species richness across the three plant classes suggested that diversity patterns were relatively homogeneous at three taxonomic levels. Thus, differences in species richness are established at the family level and are therefore relatively old. The previously noted fact that eastern Asia has a higher proportion of primitive taxa was shown by analyses both among and within plant classes. Diversity patterns across three taxonomic levels (i.e. family, genus and species) of the three classes may reflect the relative historical positions of the two continents (following continental drift) to the centre(s) of their origin, neighbouring land masses, differential speciation/extinction rates, and switches in dominance levels associated with climate change (including glaciation), as well as reproductive/dispersal mechanisms of the three plant classes.     

Floristic analysis of vascular plant genera of North America north of Mexico: characteristics of phytogeography

The phytogeographical characteristics of North America north of Mexico (NANM) were examined based on an analysis of the 1904 indigenous genera of the vascular plants known to the area. According to the worldwide distribution patterns of the genera, the 1904 genera were grouped into ten phytogeographical elements (categories). About 67.3% of the genera found in the native flora of NANM also occur in other areas of the world; of these, 71.4% occur in the Old World. About 32.7% of the 1904 genera are endemic to North America. The origins of the floristic relationships between North America and the rest of the world and the origin of the present-day flora of North America are discussed. The current floristic patterns of the NANM vascular plant genera resulted in part from palaeogeological events such as the unification of tectonic plates into a single supercontinent, Pangaea, and then the breakup of the united supercontinent into Laurasia and Gondwana followed by the fragmentation of Laurasia and Gondwana into individual continents as they are today due to the movement of plate tectonics. Many of the endemic genera of North America likely originated in the North American continent during the Tertiary, particularly during the uplifting of Rocky Mountains.     

Molecular divergence in the eastern Asia– eastern North America disjunct section Rytidospermum of Magnolia (Magnoliaceae)

Molecular divergence in the eastern Asia—eastern North American disjunct section Rytidospermum of Magnolia was investigated by allozyme electrophoresis, chloroplast DNA (cpDNA) restriction site analysis, and gene sequencing. We calculated Nei's genetic identities between two Asian species, M. officinalis var. biloba and M. hypoleuca, and three American species, M. tripetala, M. fraseri var. fraseri, and M. macrophylla var. macrophylla, by using gene frequency data from 17 nuclear-encoded allozyme loci in 67 populations. We then estimated cpDNA sequence divergence between the five species by examining restriction site variation for ten endonucleases over the entire genome. Finally, nucleotide sequences of the chloroplast gene rbcL were compared between M. hypoleuca, M. tripetala, and M. macrophylla var. macrophylla. All three methods consistently yielded low divergence values between the American species M. tripetala and its Asian sister taxa, M. officinalis var. biloba and M. hypoleuca (Nei's I = 0.712 and 0.809, respectively; D-cpDNA = 0.083% for both pairs; D-rbcL = 0.000% between M. tripetala and M. hypoleuca). The other two American species, M. fraseri var. fraseri and M. macrophylla var. macrophylla, neither of which is sister to the Asian taxa, exhibited much higher divergence from the Asian taxa. We interpreted the low divergence between M. tripetala and its Asian sister taxa as a result of recent separation (the late Miocene to early Pliocene), based on time estimates from molecular data as well as geological and paleoclimatic evidence. A comparison of our results with those of the earlier studies revealed a diverse array of levels of divergence between several eastern Asian and eastern North American species pairs. Though different extinction patterns and variation in molecular evolutionary rates may be partly responsible, this heterogeneous pattern of divergence is best explained by different times of disjunction in different taxa, which in turn suggests that the floristic similarity between the two continents was most likely attained by multiple migrations via both Bering and North Atlantic land bridges, or possibly even with involvement of dispersal.     

A cytological study on Kelloggia (Rubiaceae), an intercontinental disjunct genus between eastern Asia and western North America

A cytological study was carried out for the first time on four populations of the only two species of Kelloggia (Rubiaceae), which occur disjunctly in eastern Asia (K. chinensis Franch.) and western North America (K. galioides Torr.). The consistent mitotic prophase chromosome condensation pattern and interphase nuclei type were determined for both species. The chromosome base number of the genus is suggested to be x=11. The karyotype of 2n=22=2x=16m+6sm was examined for both species. The karyotypical asymmetry of 1A and 2A was found in K. chinensis and K. galioides, respectively. According to the predominant evolutionary direction of karyotype asymmetry in angiosperms, K. galioides (2A) seems slightly more evolved than K. chinensis (1A). Our finding is consistent with the hypothesis of the Old World origin of Kelloggia based on molecular study.     

A comparison of the taxonomic richness of temperate plants in East Asia and North America

The taxonomic richness of seed plants at different taxonomic levels was compared between temperate East Asia and North America at both continental and semi-continental scales. In each comparison, land area and latitude range were adjusted to a comparable level between the two continental regions. East Asia is significantly more diverse than North America. In general, differences in taxonomic diversity arise at and below the genus level. At the continental scale, East Asia has 1.3 and 1.5 times as many genera and species, respectively, as North America. The northern part of East Asia has 1.1 times as many species as the northern part of North America. At the genus level, the northern part of East Asia is less diverse than the northern part of North America by a factor of 0.94. This pattern indicates that the diversity bias between the two continental regions results from the flora of southern East Asia. The diversity differences between East Asia and North America are not homogenously distributed across different plant groups. At the species level, East Asia had significantly more species than expected in magnoliids, alismatids, Liliidae, ranunculids, and rosids and had significantly less species in the Commelinidae, Caryophyllidae, and euasterids than North America.     

Endemicity and evolutionary value: a study of Chilean endemic vascular plant genera

This study uses phylogeny‐based measures of evolutionary potential (phylogenetic diversity and community structure) to evaluate the evolutionary value of vascular plant genera endemic to Chile. Endemicity is regarded as a very important consideration for conservation purposes. Taxa that are endemic to a single country are valuable conservation targets, as their protection depends upon a single government policy. This is especially relevant in developing countries in which conservation is not always a high resource allocation priority. Phylogeny‐based measures of evolutionary potential such as phylogenetic diversity (PD) have been regarded as meaningful measures of the “value” of taxa and ecosystems, as they are able to account for the attributes that could allow taxa to recover from environmental changes. Chile is an area of remarkable endemism, harboring a flora that shows the highest number of endemic genera in South America. We studied PD and community structure of this flora using a previously available supertree at the genus level, to which we added DNA sequences of 53 genera endemic to Chile. Using discrepancy values and a null model approach, we decoupled PD from taxon richness, in order to compare their geographic distribution over a one‐degree grid. An interesting pattern was observed in which areas to the southwest appear to harbor more PD than expected by their generic richness than those areas to the north of the country. In addition, some southern areas showed more PD than expected by chance, as calculated with the null model approach. Geological history as documented by the study of ancient floras as well as glacial refuges in the coastal range of southern Chile during the quaternary seem to be consistent with the observed pattern, highlighting the importance of this area for conservation purposes.     

Regional differences in rates of plant speciation and molecular evolution: a comparison between eastern Asia and eastern North America

The eastern Asian (EAS)-eastern North American (ENA) floristic disjunction is one of the best-known biogeographic patterns in the Northern Hemisphere. Recent paleontological and molecular analyses have illuminated the origins of the biogeographic pattern, but subsequent diversification and evolution of the disjunct floras in each of the two continents after isolation remains poorly understood. Although similar in climate and floristic composition, EAS has twice as many species as ENA in genera occurring in both regions. Explaining such differences in species diversity between regions with similar environmental conditions (diversity anomalies) is an important goal of the study of the global patterns of biodiversity. We used a phylogenetic approach to compare rates of net speciation and molecular evolution between the two regions. We first identified EAS-ENA disjunct sister clades from ten genera (Asarum, Buckleya, Carpinus, Carya, Cornus, Hamamelis, Illicium, Panax, Stewartia, and Styrax) that represent diverse angiosperm lineages using phylogenetic analyses of ITS (internal transcribed spacer of nuclear ribosomal DNA) sequence data. Species richness and substitution rate of ITS between sister clades were compared. The results revealed a pattern of greater species diversity in the EAS counterparts. A positive relationship between species diversity and ITS substitution rate was also documented. These results suggest greater net speciation and accelerated molecular evolution in EAS. The data support the idea that a regional difference in net speciation rate related to topographic heterogeneity contributes to the diversity anomaly between EAS and ENA. The close relationship between rates of ITS evolution and species richness further suggests that species production may be directly linked to rate of nucleotide substitution.     

Phylogenetic diversity of eastern Asia–eastern North America disjunct plants is mainly associated with divergence time

The underlying causes of biodiversity disparities among geographic regions have long been a fundamental theme in ecology and evolution. However, the patterns of phylogenetic diversity (PD) and phylogenetic beta diversity (PBD) of congeners that are disjunctly distributed between eastern Asia–eastern North America (EA−ENA disjuncts) and their associated factors remain unknown. Here we investigated the standardized effect size of PD (SES-PD), PBD, and potentially associated factors in 11 natural mixed forest sites (five in EA and six in ENA) where abundant EA−ENA disjuncts occur. We found that the disjuncts in ENA possessed higher SES-PD than those in EA at the continental scale (1.96 vs −1.12), even though the number of disjunct species in ENA is much lower than in EA (128 vs 263). SES-PD of the EA−ENA disjuncts tended to decrease with increasing latitude in 11 sites. The latitudinal diversity gradient of SES-PD was stronger in EA sites than in ENA sites. Based on the unweighted unique fraction metric (UniFrac) distance and the phylogenetic community dissimilarity, PBD showed that the two northern sites in EA were more similar to the six-site ENA group than to the remaining southern EA sites. Based on the standardized effect size of mean pairwise distances (SES-MPD), nine of eleven studied sites showed a neutral community structure (−1.96 ≤ SES-MPD ≤ 1.96). Both Pearson''s r and structural equation modeling suggested that SES-PD of the EA–ENA disjuncts was mostly associated with mean divergence time. Moreover, SES-PD of the EA–ENA disjuncts was positively correlated with temperature-related climatic factors, although negatively correlated with mean diversification rate and community structure. By applying approaches from phylogenetics and community ecology, our work sheds light on historical patterns of the EA−ENA disjunction and paves the way for further research.     

Bryophyte and vascular plant species richness in boreo-nemoral moist forests and mires

We compare species richness of bryophytes and vascular plants in Estonian moist forests and mires. The material was collected from two wetland nature reserves. Bryophyte and vascular plant species were recorded in 338 homogeneous stands of approximately 1 ha in nine forest and two mire types. Regional species pools for bryophytes and vascular plants were significantly correlated. The correlations between the species richnesses of bryophytes and vascular plants per stand were positive in all community types. The relative richnesses (local richness divided by the regional species pool size) were similar for bryophyte species and for vascular plant species. This shows that on larger scales, conservation of the communities rich in species of one taxonomic plant group, maintains also the species richness of the other. The minimum number of stands needed for the maintenance of the regional species pool of typical species for the every community type was calculated using the species richness accumulation curves. Less stands are needed to maintain the bryophyte species pools (300–5300 for bryophytes and 400–35 000 for vascular plants).     

Molecular phylogeny and biogeographic diversification of Parthenocissus (Vitaceae) disjunct between Asia and North America

? Premise of the study: Parthenocissus is a genus of the grape family Vitaceae and has a disjunct distribution in Asia and North America with members in both tropical and temperate regions. The monophyly of Parthenocissus has not yet been tested, and the species relationships and the evolution of its intercontinental disjunction have not been investigated with extensive sampling and molecular phylogenetic methods. ? Methods: Plastid (trnL-F, rps16, and atpB-rbcL) and nuclear GAI1 sequences of 56 accessions representing all 12 Parthenocissus species were analyzed with parsimony, likelihood, and Bayesian inference. Divergence times of disjunct lineages were estimated with relaxed Bayesian dating. Evolution of the leaflet number was assessed by tracing this character onto Bayesian trees using the Trace Character Over Trees option in the program Mesquite. ? Key results: Parthenocissus is monophyletic and sister to the newly described segregate genus Yua. Two major clades within Parthenocissus are recognizable corresponding to their distribution in Asia and North America. The disjunction between the two continents is estimated to be at 21.64 (95% higher posterior densities 10.23-34.89) million years ago. ? Conclusions: Parthenocissus is likely to have derived from the Eocene boreotropical element. Its current Asian-North American disjunction is dated to the early Miocene, congruent with fossil and paleoclimatic evidence. The tropical species is nested within the temperate clade and is inferred to have dispersed from the adjacent temperate regions. Parthenocissus and Yua are best treated as distinct genera. Leaflet number in this genus has a complex history and cannot be used as a character for infrageneric classification.     

Vascular plant species richness along latitudinal and altitudinal gradients: a contribution from New Zealand temperate rainforests

Vascular plant species richness is known to often decrease with both increasing latitude and increasing altitude. However, a number of studies have shown the reverse trend and the primary cause of these gradients remains unknown. In the present work, generalized linear models were used to assess the relative importance of latitude and altitude as well as of a number of other factors (mean annual precipita-tion, slope, substrate and forest type) on species richness in temperate rainforests of New Zealand. The effect of Southern beech ( Nothofagus spp.) as dominant canopy species on total species richness was shown to be much smaller than postulated in most previous studies. Within the region studied, altitude had by far the strongest effect on species richness. This effect was independent of latitude and was significant for woody but not for herbaceous vegetation.     

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.     

Factors influencing vascular plant diversity on 22 islands off the coast of eastern North America

Aim The influence of physiographic and historical factors on species richness of native and non‐native vascular plants on 22 coastal islands was examined. Location Islands off the coast of north‐eastern USA and south‐eastern Canada between 41° and 45° N latitude were studied. Island size ranges from 3 to 26,668 ha. All islands were deglaciated between 15,000 and 11,000 yr bp ; all but the four New Brunswick islands were attached to the mainland until rising sea level isolated them between 14,000 and 3800 yr bp . Methods Island species richness was determined from floras compiled or revised since 1969. Simple and multiple regression and rank correlation analysis were employed to assess the relative influence of independent variables on species richness. Potential predictors included island area, latitude, elevation, distance from the mainland, distance from the nearest larger island, number of soil types, years since isolation, years since deglaciation, and human population density. Results Native vascular plant species richness for the 22 islands in this study is influenced most strongly by island area, latitude, and distance from the nearest larger island; richness increases with island area, but decreases with latitude and distance from the nearest larger island as hypothesized. That a similar model employing distance from the mainland does not meet the critical value of P confirms the importance of the stepping‐stone effect. Habitat diversity as measured by number of soil types is also an important predictor of native plant species richness, but at least half of its influence can be attributed to island area, with which it is correlated. Two historical factors, years since deglaciation and years since isolation, also appear to be highly correlated with native species richness, but their influence cannot be separated from that of latitude for the present sample size. Non‐native vascular plant species richness is influenced primarily by island area and present‐day human population density, although human population density may be a surrogate for the cumulative effect of several centuries of anthropogenic impacts related to agriculture, hunting, fishing, whaling, tourism, and residential development. Very high densities of ground‐nesting pelagic birds may account for the high percentage of non‐native species on several small northern islands. Main conclusions Many of the principles of island biogeography that have been applied to oceanic islands apply equally to the 22 islands in this study. Native vascular plant species richness for these islands is strongly influenced by physiographic factors. Influence of two historical factors, years since deglaciation and years since isolation, cannot be assessed with the present sample size. Non‐native vascular plant species richness is influenced by island area as well as by human population density; human population density may be a surrogate for other anthropogenic impacts.     

Distributions of exotic plants in eastern Asia and North America

Although some plant traits have been linked to invasion success, the possible effects of regional factors, such as diversity, habitat suitability, and human activity are not well understood. Each of these mechanisms predicts a different pattern of distribution at the regional scale. Thus, where climate and soils are similar, predictions based on regional hypotheses for invasion success can be tested by comparisons of distributions in the source and receiving regions. Here, we analyse the native and alien geographic ranges of all 1567 plant species that have been introduced between eastern Asia and North America or have been introduced to both regions from elsewhere. The results reveal correlations between the spread of exotics and both the native species richness and transportation networks of recipient regions. This suggests that both species interactions and human-aided dispersal influence exotic distributions, although further work on the relative importance of these processes is needed.