The extreme Beringian/Atlantic disjunction in Saxifraga rivularis (Saxifragaceae) has formed at least twice

Aim The oceanic Saxifraga rivularis L. presents one of the most extreme disjunctions known in the arctic flora: it has a small amphi‐Beringian range and a larger amphi‐Atlantic one. It was recently suggested to have had a single allopolyploid origin in Beringia at least one glacial cycle ago, followed by gradual expansion in a more humid period and differentiation into two allopatric subspecies (the Atlantic ssp. rivularis and the Beringian ssp. arctolitoralis). Here we explore the history of its extreme disjunction. Location The amphi‐Beringian and northern amphi‐Atlantic regions. Methods We obtained amplified fragment length polymorphisms (AFLPs) and chloroplast DNA sequences from 36 populations (287 individuals) and 13 populations (15 individuals), respectively. The data were analysed using principal coordinates analyses, Bayesian clustering methods, and analyses of molecular variance. Results Two distinctly divergent AFLP groups were observed, corresponding to the two described subspecies, but, surprisingly, four of the West Atlantic populations belonged to the supposedly Beringian endemic ssp. arctolitoralis. This was confirmed by re‐examination of their morphological characteristics. The overall AFLP diversity in the species was low (26.4% polymorphic markers), and there was no variation in the five investigated chloroplast DNA (cpDNA) regions. There was little geographic structuring of the AFLP diversity within each subspecies, even across the extreme disjunction in ssp. arctolitoralis, across the Bering Sea, and across the Atlantic Ocean, except that most plants from the arctic Svalbard archipelago formed a separate genetic group with relatively high diversity. Main conclusions The extreme disjunction in S. rivularis has evidently formed at least twice. The first expansion from Beringia was followed by allopatric differentiation into one Beringian and one Atlantic subspecies, which are distinctly divergent at AFLP loci but still harbour identical cpDNA haplotypes, suggesting that the expansion was quite recent but before the last glaciation. The next expansion from Beringia probably occurred by means of several long‐distance dispersals in the current interglacial, resulting in the colonization of the western Atlantic region by ssp. arctolitoralis. The poor geographic structuring within each subspecies suggests frequent long‐distance dispersals from two main Weichselian refugia, one Beringian and one western‐central European, but it is possible that the genetic group in Svalbard originates from an additional refugium.     

Intercontinental genetic structure in the amphi‐Pacific peatmoss Sphagnum miyabeanum (Bryophyta: Sphagnaceae)

Unlike seed plants where global biogeographical patterns typically involve interspecific phylogenetic history, spore‐producing bryophyte species often have intercontinental distributions that are best understood from a population genetic perspective. We sought to understand how reproductive processes, especially dispersal, have contributed to the intercontinental ‘Pacific Rim’ distribution of Sphagnum miyabeanum. In total, 295 gametophyte plants from western North America (California, Oregon, British Columbia, Alaska), Russia, Japan, and China were genotyped at 12 microsatellite loci. Nucleotide sequences were obtained for seven anonymous nuclear loci plus two plastid regions from 21 plants of S. miyabeanum and two outgroup species. We detected weak but significant genetic differentiation among plants from China, Japan, Alaska, British Columbia, and the western USA. Alaskan plants are genetically most similar to Asian plants, and British Columbian plants are most similar to those in the western USA. There is detectable migration between regions, with especially high levels between Alaska and Asia (China and Japan). Migration appears to be recent and/or ongoing, and more or less equivalent in both directions. There is weak (but significant) isolation‐by‐distance within geographical regions, and the slope of the regression of genetic on geographical distance differs for Asian versus North American plants. A distinctive Vancouver Island morphotype is very weakly differentiated, and does not appear to be reproductively isolated from plants of the normal morphotype. The intercontinental geographical range of S. miyabeanum reflects recent and probably ongoing migration, facilitated by the production of tiny spores capable of effective long distance dispersal. The results of the present study are consistent with Pleistocene survival of S. miyabeanum in unglaciated Beringia, although we cannot eliminate the possibility that the species recolonized Alaska from Asia more recently. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 17–37.     

‘Out‐of‐Africa’ dispersal of tropical floras during the Miocene climatic optimum: evidence from Uvaria (Annonaceae)

Aim African–Asian disjunctions are common in palaeotropical taxa, and are typically explained by reference to three competing hypotheses: (1) ‘rafting’ on the Indian tectonic plate, enabling Africa‐to‐Asia dispersal; (2) migration via Eocene boreotropical forests; and (3) transoceanic long‐distance dispersal. These hypotheses are tested using Uvaria (Annonaceae), which is distributed in tropical regions of Africa, Asia and Australasia. Recent phylogenetic reconstructions of the genus show a clear correlation with geographical provenance, indicating a probable origin in Africa and subsequent dispersal to Asia and then Australasia. Ancestral areas and migration routes are inferred and compared with estimates of divergence times in order to distinguish between the prevailing dispersal hypotheses. Location Palaeotropics. Methods Divergence times in Uvaria are estimated by analysing the sequences of four DNA regions (matK, psbA–trnH spacer, rbcL and trnL–F) from 59 Uvaria species and 77 outgroup species, using a Bayesian uncorrelated lognormal (UCLD) relaxed molecular clock. The ancestral area of Uvaria and subsequent dispersal routes are inferred using statistical dispersal–vicariance analysis (s‐diva ). Results Uvaria is estimated to have originated in continental Africa 31.6 Ma [95% highest posterior density (HPD): 38.4–25.1 Ma] between the Middle Eocene and Late Oligocene. Two main migration events during the Miocene are identified: dispersal into Madagascar around 17.0 Ma (95% HPD: 22.3–12.3 Ma); and dispersal into Asia between 21.4 Ma (95% HPD: 26.7–16.7 Ma) and 16.1 Ma (95% HPD: 20.1–12.1 Ma). Main conclusions Uvaria fruits are widely reported to be consumed by primates, and are therefore unlikely candidates for successful long‐distance transoceanic dispersal. The other biogeographical hypotheses, involving rafting on the Indian tectonic plate, and dispersal via the European boreotropical forests associated with the Eocene thermal maximum, can be discounted due to incongruence with the divergence time estimates. An alternative scenario is suggested, involving dispersal across Arabia and central Asia via the tropical forests that developed during the late Middle Miocene thermal maximum (17–15 Ma), associated with the ‘out‐of‐Africa’ dispersal of primates. The probable route and mechanism of overland dispersal between Africa and Asia for tropical plant groups during the Miocene climatic optimum are clarified based on the Uvaria data.     

Lauraceae fossils from a volcanic Palaeocene oceanic island,Ninetyeast Ridge,Indian Ocean: ancient long‐distance dispersal?

Aim Geological and fossil records are critical for historical biogeography studies. A plant fossil assemblage from a small, well‐dated, transient late Palaeocene island was re‐investigated with regard to regional geology and vicariance versus dispersal hypotheses. Location Deep Sea Drilling Program Leg 22, Site 214 on the Ninetyeast Ridge (NER) in the mid‐Indian Ocean region. Methods Leaf cuticular material was recovered from residues from a previous palynofloral study of Site 214 sediments during the 1970s and identified. The palynoflora was reassessed. Results The only leaf cuticular material recovered with stomata can be placed in crown‐group Lauraceae. It is confirmed that the palynoflora reflects the presence of a low‐diversity island flora in the late Palaeocene, comprising ferns and mostly herbaceous angiosperms with readily dispersible propagules, and perhaps austral podocarps. Other pollen taxa of almost certain local origin were arecoid palms and taxa related to Chloranthaceae. The strong overall similarity of the palynoflora to Australo‐Antarctic and New Zealand assemblages is also confirmed. Main conclusions Foliar fossils of Lauraceae demonstrate the occurrence of one of the world’s largest, most widely distributed woody plant families on a late Palaeocene island. The presence of plants on this island could be explained by vicariance via a vegetated Upper Cretaceous Kerguelen Plateau, in part because crown‐group Lauraceae may be at least this old. However, there are records of other taxa in the Kerguelen region that are anomalous with vicariance, plus evidence for a catastrophic biotic extinction event centred in the area in the latest Cretaceous. Plants were therefore most likely to have reached the island by means of dispersal. This suggests either the presence of presently unknown vegetated land nearby in the Kerguelen region in the late Palaeocene, or long‐distance dispersal, probably from the Australian region. The dispersal of viable seeds could have been facilitated by birds or perhaps by ocean‐surface drift with or without the assistance of ocean‐going animals. The fossils allow that even small, short‐lived islands could have acted as ‘stepping stones’ for biotic interchange between Australia and Africa, and perhaps other regions.     

The role of long‐distance seed dispersal in the local population dynamics of an invasive plant species

Aim Long‐distance dispersal is important for plant population dynamics at larger spatial scales, but our understanding of this phenomenon is mostly based on computer modelling rather than field data. This paper, by combining field data and a simulation model, quantifies the fraction of the seed of the alien species Heracleum mantegazzianum that needs to disperse over a long distance for successful invasion. Location Central Europe, Czech Republic. Methods To assess the role of random dispersal in long‐term population dynamics of the studied species, we combined longitudinal data covering 50 years of the invasion of this plant from its very start, inferred from a series of aerial photographs of 60‐ha plots, with data on population dynamics at a fine scale of 10‐m² plots. Results A simulation model based on field data indicates that the fraction of seed that is dispersed from source plants not described by the short‐distance dispersal kernel ranges from 0.1 to 7.5% of the total seed set. The fraction of long‐distance dispersed seed that provides the best prediction of the observed spread was significantly negatively correlated with the percentage of habitats suitable for invasion. Main conclusions Our results indicate that the fraction of seeds that needed to be dispersed over long distances to account for the observed invasion dynamics decreased with increasing proportion of invasible habitats, indicating that the spatial pattern of propagule pressure differs in landscapes prone to invasion. Long‐distance dispersal is an important component of the population dynamics of an invasive species even at relatively small scales.     

Trans‐Atlantic,trans‐Pacific and trans‐Indian Ocean dispersal in the small Gondwanan Laurales family Hernandiaceae

Aim To investigate the historical biogeography of the pantropical flowering plant family Hernandiaceae (Laurales), which today comprises 62 species in five genera. Location Hernandiaceae occur in Africa (9 species), Madagascar (4), the Neotropics (25), Australia (3), southern China, Indochina, Malesia, and on numerous Pacific Islands (32). These numbers include two widespread species, Hernandia nymphaeifolia, which ranges from East Africa to the Ogasawara Islands and New Caledonia, and Gyrocarpus americanus, thought to have a pantropical range. Methods We sampled 37 species from all genera, the widespread ones with multiple accessions, for a chloroplast DNA matrix of 2210 aligned nucleotides, and used maximum likelihood to infer species relationships. Divergence time estimation relied on an uncorrelated‐rates relaxed molecular clock calibrated with outgroup fossils of Lauraceae and Monimiaceae. Results The deepest split in the family is between a predominantly African–Madagascan–Malesian lineage comprising Hazomalania, Hernandia and Illigera, and an African–Neotropical lineage comprising Gyrocarpus and Sparattanthelium; this split may be 122 (110–134) Myr old. The stem lineages of the five genera date back at least to the Palaeocene, but six splits associated with transoceanic range disjunctions date only to the Oligocene and Miocene, implying long‐distance dispersal. It is inferred that Hernandia beninensis reached the West African islands of São Tomé and Bioko from the West Indies or the Guianas; Hernandia dispersed across the Pacific; and Illigera madagascariensis reached Madagascar from across the Indian Ocean. Main conclusions The disjunct ranges and divergence times of sister clades in the Hernandiaceae are partly congruent with the break‐up of West Gondwana, but mostly with later transoceanic dispersal. An exceptional ability to establish following prolonged oceanic dispersal may be largely responsible for the evolutionary persistence of this small clade.     

Vicariance or long‐distance dispersal: historical biogeography of the pantropical subfamily Chrysophylloideae (Sapotaceae)

Aim Continental disjunctions in pantropical taxa have been explained by vicariance or long‐distance dispersal. The relative importance of these explanations in shaping current distributions may vary, depending on historical backgrounds or biological characteristics of particular taxa. We aimed to determine the geographical origin of the pantropical subfamily Chrysophylloideae (Sapotaceae) and the roles vicariance and dispersal have played in shaping its modern distribution. Location Tropical areas of Africa, Australasia and South America. Methods We utilized a recently published, comprehensive data set including 66 species and nine molecular markers. Bayesian phylogenetic trees were generated and dated using five fossils and the penalized likelihood approach. Distributional ranges of nodes were estimated using maximum likelihood and parsimony analyses. In both biogeographical and molecular dating analyses, phylogenetic and branch length uncertainty was taken into account by averaging the results over 2000 trees extracted from the Bayesian stationary sample. Results Our results indicate that the earliest diversification of Chrysophylloideae was in the Campanian of Africa c. 73–83 Ma. A narrow time interval for colonization from Africa to the Neotropics (one to three dispersals) and Australasia (a single migration) indicates a relatively rapid radiation of this subfamily in the latest Cretaceous to the earliest Palaeocene (c. 62–72 Ma). A single dispersal event from the Neotropics back to Africa during the Neogene was inferred. Long‐distance dispersal between Australia and New Caledonia occurred at least four times, and between Africa and Madagascar on multiple occasions. Main conclusions Long‐distance dispersal has been the dominant mechanism for range expansion in the subfamily Chrysophylloideae. Vicariance could explain South American–Australian disjunction via Antarctica, but not the exchanges between Africa and South America and between New Caledonia and Australia, or the presence of the subfamily in Madagascar. We find low support for the hypothesis that the North Atlantic land bridge facilitated range expansions at the Palaeocene/Eocene boundary.