Cataphylls of the Middle Triassic cycad Antarcticycas schopfii and new insights into cycad evolution

Disparity of cycad leaves dispels the living fossil metaphor

The living fossil metaphor is tightly linked with the cycads. This group of gymnosperms is supposed to be characterised by long-term morphological stasis, particularly after their peak of diversity and disparity in the Jurassic. However, no formal test of this hypothesis exists. Here, we use a recent phylogenetic framework and an improved character matrix to reconstruct the Disparity Through Time for cycad leaves using a Principal Coordinate Analysis and employing Pre-Ordination Ancestral State Reconstruction to test the impact of sampling on the results. Our analysis shows that the cycad leaf morsphospace expanded up to the present, with numerous shifts in its general positioning, independently of sampling biases. Moreover, they also show that Zamiaceae expanded rapidly in the Early Cretaceous and continued to expand up to the present, while now-extinct clades experienced a slow contraction from their peak in the Triassic. We also show that rates of evolution were constantly high up to the Early Cretaceous, and then experienced a slight decrease in the Paleogene, followed by a Neogene acceleration. These results show a much more dynamic history for cycads, and suggest that the 'living fossil' metaphor is actually a hindrance to our understanding of their macroevolution.

Nitrogen isotopes reveal independent origins of N2-fixing symbiosis in extant cycad lineages

Cycads are ancient seed plants (gymnosperms) that emerged by the early Permian. Although they were common understory flora and food for dinosaurs in the Mesozoic, their abundance declined markedly in the Cenozoic. Extant cycads persist in restricted populations in tropical and subtropical habitats and, with their conserved morphology, are often called 'living fossils.' All surviving taxa receive nitrogen from symbiotic N2-fixing cyanobacteria living in modified roots, suggesting an ancestral origin of this symbiosis. However, such an ancient acquisition is discordant with the abundance of cycads in Mesozoic fossil assemblages, as modern N2-fixing symbioses typically occur only in nutrient-poor habitats where advantageous for survival. Here, we use foliar nitrogen isotope ratios-a proxy for N2 fixation in modern plants-to probe the antiquity of the cycad-cyanobacterial symbiosis. We find that fossilized cycad leaves from two Cenozoic representatives of extant genera have nitrogen isotopic compositions consistent with microbial N2 fixation. In contrast, all extinct cycad genera have nitrogen isotope ratios that are indistinguishable from co-existing non-cycad plants and generally inconsistent with microbial N2 fixation, pointing to nitrogen assimilation from soils and not through symbiosis. This pattern indicates that, rather than being ancestral within cycads, N2-fixing symbiosis arose independently in the lineages leading to living cycads during or after the Jurassic. The preferential survival of these lineages may therefore reflect the effects of competition with angiosperms and Cenozoic climatic change.

Reconciling fossils with phylogenies reveals the origin and macroevolutionary processes explaining the global cycad biodiversity

The determinants of biodiversity patterns can be understood using macroevolutionary analyses. The integration of fossils into phylogenies offers a deeper understanding of processes underlying biodiversity patterns in deep time. Cycadales are considered a relict of a once more diverse and globally distributed group but are restricted to low latitudes today. We still know little about their origin and geographic range evolution. Combining molecular data for extant species and leaf morphological data for extant and fossil species, we study the origin of cycad global biodiversity patterns through Bayesian total-evidence dating analyses. We assess the ancestral geographic origin and trace the historical biogeography of cycads with a time-stratified process-based model. Cycads originated in the Carboniferous on the Laurasian landmass and expanded in Gondwana in the Jurassic. Through now-vanished continental connections, Antarctica and Greenland were crucial biogeographic crossroads for cycad biogeography. Vicariance is an essential speciation mode in the deep and recent past. Their latitudinal span increased in the Jurassic and restrained toward subtropical latitudes in the Neogene in line with biogeographic inferences of high-latitude extirpations. We show the benefits of integrating fossils into phylogenies to estimate ancestral areas of origin and to study evolutionary processes explaining the global distribution of present-day relict groups.

Cretaceous pollen cone with three-dimensional preservation sheds light on the morphological evolution of cycads in deep time

The Cycadales are an ancient and charismatic group of seed plants. However, their morphological evolution in deep time is poorly understood. While molecular divergence time analyses estimate a Cretaceous origin for most major living cycad clades, much of the extant diversity is inferred to be a result of Neogene diversifications. This leads to long branches throughout the cycadalean phylogeny that, with few exceptions, have yet to be rectified by unequivocal fossil cycads. We report a permineralized pollen cone from the Campanian Holz Shale located in Silverado Canyon, CA, USA (c. 80 million yr ago). This fossil was studied via serial sectioning, SEM, 3D reconstruction and phylogenetic analyses. Microsporophyll and pollen morphology indicate this cone is assignable to Skyttegaardia, a recently described genus based on disarticulated lignitized microsporophylls from the Early Cretaceous of Denmark. Data from this new species, including a simple cone architecture, anatomical details and vasculature organization, indicate cycadalean affinities for Skyttegaardia. Phylogenetic analyses support this assignment and recover Skyttegaardia as crown-group Cycadales, nested within Zamiaceae. Our findings support a Cretaceous diversification for crown-group Zamiaceae, which included the evolution of morphological divergent extinct taxa with unique traits that have yet to be widely identified in the fossil record.

Not that young: combining plastid phylogenomic, plate tectonic and fossil evidence indicates a Palaeogene diversification of Cycadaceae

BACKGROUND AND AIMS

Previous molecular dating studies revealed historical mass extinctions and recent radiations of extant cycads, but debates still exist between palaeobotanists and evolutionary biologists regarding the origin and evolution of Cycadaceae.

METHODS

Using whole plastomic data, we revisited the phylogeny of this family and found the Palawan endemic Cycas clade was strongly related to all lineages from Southeast Eurasia, coinciding with a plate drift event occurring in the Early Oligocene. By integrating fossil and biogeographical calibrations as well as molecular data from protein-coding genes, we established different calibration schemes and tested competing evolutionary timelines of Cycadaceae.

KEY RESULTS

We found recent dispersal cannot explain the distribution of Palawan Cycas, yet the scenario including the tectonic calibration yielded a mean crown age of extant Cycadaceae of ~69-43 million years ago by different tree priors, consistent with multiple Palaeogene fossils assigned to this family. Biogeographical analyses incorporating fossil distributions revealed East Asia as the ancestral area of Cycadaceae.

CONCLUSIONS

Our findings challenge the previously proposed Middle-Late Miocene diversification of cycads and an Indochina origin for Cycadaceae and highlight the importance of combining phylogenetic clades, tectonic events and fossils for rebuilding the evolutionary history of lineages that have undergone massive extinctions.

Congruence between ocean-dispersal modelling and phylogeography explains recent evolutionary history of Cycas species with buoyant seeds

Ocean currents play a significant role in driving the long-distance dispersal (LDD), spatial distribution and phylogeographic patterns of many organisms. Integrating phylogeographic analyses and mechanistic ocean current modelling can provide novel insights into the evolutionary history of terrestrial littoral species but has been rarely applied in this context. We focused on a group of Cycas that have buoyant seeds and occupy coastal habitats. By integrating evidence from mechanistic simulations and whole plastomic data, we examined the role of ocean circulation in shaping the phylogeography of these Cycas species. Plastomes of the studied Cycas species showed extreme conservatism, following a post-Pleistocene divergence. Phylogenies revealed three subclades, corresponding to the Pacific Ocean, Sunda Shelf and Indian Ocean. The ocean modelling results indicate that hotspots of seed stranding coincide well with the contemporary distribution of the Cycas species and that drifting trajectories from the three subclades are largely confined to separate regions. These findings suggest that ocean current systems, by driving long-distance dispersal, have shaped the distribution and phylogeography for Cycas with buoyant seeds. This study highlights how the combination of genomic data and ocean drift modelling can help explain phylogeographic patterns and diversity in terrestrial littoral ecosystems.

Unique chemistry associated with diversification in a tightly coupled cycad-thrips obligate pollination mutualism

Cycad cone thermogenesis and its associated volatiles are intimately involved in mediating the behavior of their obligate specialist pollinators. In eastern Australia, thrips in the Cycadothrips chadwicki species complex are the sole pollinators of many Macrozamia cycads. Further, they feed and reproduce entirely in the pollen cones. M. miquelii, found only in the northern range of this genus, is pollinated only by a C. chadwicki cryptic species that is the most distantly related to others in the complex. We examined the volatile profile from M. miquelii pollen and ovulate (receptive and non-receptive) cones to determine how this mediates pollination mechanistically, using GC-MS (gas chromatography-mass spectrometry) and behavioral tests. Monoterpenes comprise the bulk of M. miquelii volatile emissions, as in other Macrozamia species, but we also identified compounds not reported previously in any cycad, including three aliphatic esters (prenyl acetate and two of uncertain identity) and two aliphatic alcohols. The two unknown esters were confirmed as prenyl (3-methylbut-2-enyl) esters of butyric and crotonic ((E))-but-2-enoic) acids after chemical synthesis. Prenyl crotonate is a major component in emissions from pollen and receptive ovulate cones, is essentially absent from non-receptive cones, and has not been reported from any other natural source. In field bioassays, Cycadothrips were attracted only to those volatile treatments containing prenyl crotonate. We discuss M. miquelii cone odorants relative to those of other cycads, especially with respect to prenyl crotonate being a species-specific signal to this northern C. chadwicki cryptic species, and how this system may have diversified.

Evidence for an extinct lineage of angiosperms from the Early Cretaceous of Patagonia and implications for the early radiation of flowering plants

The pinnately lobed Aptian leaf fossil Mesodescolea plicata was originally described as a cycad, but new evidence from cuticle structure suggests that it is an angiosperm. Here we document the morphology and cuticle anatomy of Mesodescolea and explore its significance for early angiosperm evolution. We observed macrofossils and cuticles of Mesodescolea with light, scanning electron and transmission electron microscopy, and used phylogenetic methods to test its relationships among extant angiosperms. Mesodescolea has chloranthoid teeth and tertiary veins forming elongate areoles. Its cuticular morphology and ultrastructure reject cycadalean affinities, whereas its guard cell shape and stomatal ledges are angiospermous. It shares variable stomatal complexes and epidermal oil cells with angiosperm leaves from the lower Potomac Group. Phylogenetic analyses and hypothesis testing support its placement within the basal ANITA grade, most likely in Austrobaileyales, but it diverges markedly in leaf form and venation. Although many Early Cretaceous angiosperms fall within the morphological range of extant taxa, Mesodescolea reveals unexpected early morphological and ecophysiological trends. Its similarity to other Early Cretaceous lobate leaves, many identified previously as eudicots but in some cases pre-dating the appearance of tricolpate pollen, may indicate that Mesodescolea is part of a larger extinct lineage of angiosperms.

An ancient push-pull pollination mechanism in cycads

Most cycads engage in brood-site pollination mutualisms, yet the mechanism by which the Cycadales entice pollination services from diverse insect mutualists remains unknown. Here, we characterize a push-pull pollination mechanism between a New World cycad and its weevil pollinators that mirrors the mechanism between a distantly related Old World cycad and its thrips pollinators. The behavioral convergence between weevils and thrips, combined with molecular phylogenetic dating and a meta-analysis of thermogenesis and coordinated patterns of volatile attraction and repulsion suggest that a push-pull pollination mutualism strategy is ancestral in this ancient, dioecious plant group. Hence, it may represent one of the earliest insect/plant pollination mechanisms, arising long before the evolution of visual floral signaling commonly used by flowering plants.

First cycad seedling foliage from the fossil record and inferences for the Cenozoic evolution of cycads

The morphology of the early ontogenetic stages of cycad foliage may help resolve the relationships between extinct to extant cycad lineages. However, prior to this study, fossil evidence of cycad seedlings was not known. We describe a compression fossil of cycad eophylls with co-occurring fully developed leaves of adult specimens from the early Palaeocene (ca 63.8 Ma) Castle Rock flora from the Denver Basin, CO, USA and assign it to the fossil genus Dioonopsis (Cycadales) based on leaf morphology and anatomy. The new fossil seedling foliage is particularly important because fully differentiated pinnate leaves of adult plants and the eophylls belong to the same species based on shared epidermal micromorphology, therefore, increasing the number of morphological characteristics that can be used to place Dioonopsis phylogenetically. Significantly, the seedling fossil has a basic foliage structure that is very similar to seedlings of extant cycads, which is consistent with a cycadalean affinity of Dioonopsis. Nevertheless, the set of morphological characters in the seedling and adult specimens of Dioonopsis suggests a distant relationship between Dioonopsis and extant Dioon. This indicates that extinct lineages of cycads were present and widespread during the early Cenozoic (Palaeogene) coupled with the subordinate role of extant genera in the Palaeogene fossil record of cycads.

Pleistocene diversification in an ancient lineage: a role for glacial cycles in the evolutionary history of Dioon Lindl. (Zamiaceae)

PREMISE OF THE STUDY

Recent estimates of crown ages for cycad genera (Late Miocene) challenge us to consider what processes have produced the extant diversity of this ancient group in such relatively little time. Pleistocene climate change has driven major shifts in species distributions in Mexico and may have led to speciation in the genus Dioon by forcing populations to migrate up in elevation, thereby becoming separated by topography.

METHODS

We inferred orthologs from transcriptomes of five species and sequenced these in 42 individuals representing all Dioon species. From these data and published plastid sequences, we inferred dated species trees and lineage-specific diversification rates.

KEY RESULTS

Analyses of 84 newly sequenced nuclear orthologs and published plastid data confirm four major clades within Dioon, all of Pleistocene age. Gene tree analysis, divergence dates, and an increase in diversification rate support very recent and rapid divergence of extant taxa.

CONCLUSIONS

This study confirms the Pleistocene age of Dioon species and implicates Pleistocene climate change and established topography in lineage spitting. These results add to our understanding of the cycads as evolutionarily dynamic lineages, not relicts or evolutionary dead ends. We also find that well-supported secondary calibration points can be reliable in the absence of fossils. Our hypothesis of lineage splitting mediated by habitat shifts may be applicable to other taxa that are restricted to elevation specific ecotones.

Historical Biogeography of endemic seed plant genera in the Caribbean: Did GAARlandia play a role?

The Caribbean archipelago is a region with an extremely complex geological history and an outstanding plant diversity with high levels of endemism. The aim of this study was to better understand the historical assembly and evolution of endemic seed plant genera in the Caribbean, by first determining divergence times of endemic genera to test whether the hypothesized Greater Antilles and Aves Ridge (GAARlandia) land bridge played a role in the archipelago colonization and second by testing South America as the main colonization source as expected by the position of landmasses and recent evidence of an asymmetrical biotic interchange. We reconstructed a dated molecular phylogenetic tree for 625 seed plants including 32 Caribbean endemic genera using Bayesian inference and ten calibrations. To estimate the geographic range of the ancestors of endemic genera, we performed a model selection between a null and two complex biogeographic models that included timeframes based on geological information, dispersal probabilities, and directionality among regions. Crown ages for endemic genera ranged from Early Eocene (53.1 Ma) to Late Pliocene (3.4 Ma). Confidence intervals for divergence times (crown and/or stem ages) of 22 endemic genera occurred within the GAARlandia time frame. Contrary to expectations, the Antilles appears as the main ancestral area for endemic seed plant genera and only five genera had a South American origin. In contrast to patterns shown for vertebrates and other organisms and based on our sampling, we conclude that GAARlandia did not act as a colonization route for plants between South America and the Antilles. Further studies on Caribbean plant dispersal at the species and population levels will be required to reveal finer-scale biogeographic patterns and mechanisms.

Eobowenia gen. nov. from the Early Cretaceous of Patagonia: indication for an early divergence of Bowenia?

BACKGROUND

Even if they are considered the quintessential "living fossils", the fossil record of the extant genera of the Cycadales is quite poor, and only extends as far back as the Cenozoic. This lack of data represents a huge hindrance for the reconstruction of the recent history of this important group. Among extant genera, Bowenia (or cuticles resembling those of extant Bowenia) has been recorded in sediments from the Late Cretaceous and the Eocene of Australia, but its phylogenetic placement and the inference from molecular dating still imply a long ghost lineage for this genus.

RESULTS

We re-examine the fossil foliage Almargemia incrassata from the Lower Cretaceous Anfiteatro de Ticó Formation in Patagonia, Argentina, in the light of a comparative cuticular analysis of extant Zamiaceae. We identify important differences with the other member of the genus, viz. A. dentata, and bring to light some interesting characters shared exclusively between A. incrassata and extant Bowenia. We interpret our results to necessitate the erection of the new genus Eobowenia to accommodate the fossil leaf earlier assigned as Almargemia incrassata. We then perfom phylogenetic analyses, including the first combined morphological and molecular analysis of the Cycadales, that indicate that the newly erected genus could be related to extant Bowenia.

CONCLUSION

Eobowenia incrassata could represent an important clue for the understanding of evolution and biogeography of the extant genus Bowenia, as the presence of Eobowenia in Patagonia is yet another piece of the biogeographic puzzle that links southern South America with Australasia.

Green Web or megabiased clock? Plant fossils from Gondwanan Patagonia speak on evolutionary radiations

Evolutionary divergence-age estimates derived from molecular 'clocks' are frequently correlated with paleogeographic, paleoclimatic and extinction events. One prominent hypothesis based on molecular data states that the dominant pattern of Southern Hemisphere biogeography is post-Gondwanan clade origins and subsequent dispersal across the oceans in a metaphoric 'Green Web'. We tested this idea against well-dated Patagonian fossils of 19 plant lineages, representing organisms that actually lived on Gondwana. Most of these occurrences are substantially older than their respective, often post-Gondwanan molecular dates. The Green Web interpretation probably results from directional bias in molecular results. Gondwanan history remains fundamental to understanding Southern Hemisphere plant radiations, and we urge significantly greater caution when using molecular dating to interpret the biological impacts of geological events.

Origin and diversification of living cycads: a cautionary tale on the impact of the branching process prior in Bayesian molecular dating

Background

Bayesian relaxed-clock dating has significantly influenced our understanding of the timeline of biotic evolution. This approach requires the use of priors on the branching process, yet little is known about their impact on divergence time estimates. We investigated the effect of branching priors using the iconic cycads. We conducted phylogenetic estimations for 237 cycad species using three genes and two calibration strategies incorporating up to six fossil constraints to (i) test the impact of two different branching process priors on age estimates, (ii) assess which branching prior better fits the data, (iii) investigate branching prior impacts on diversification analyses, and (iv) provide insights into the diversification history of cycads.

Results

Using Bayes factors, we compared divergence time estimates and the inferred dynamics of diversification when using Yule versus birth-death priors. Bayes factors were calculated with marginal likelihood estimated with stepping-stone sampling. We found striking differences in age estimates and diversification dynamics depending on prior choice. Dating with the Yule prior suggested that extant cycad genera diversified in the Paleogene and with two diversification rate shifts. In contrast, dating with the birth-death prior yielded Neogene diversifications, and four rate shifts, one for each of the four richest genera. Nonetheless, dating with the two priors provided similar age estimates for the divergence of cycads from Ginkgo (Carboniferous) and their crown age (Permian). Of these, Bayes factors clearly supported the birth-death prior.

Conclusions

These results suggest the choice of the branching process prior can have a drastic influence on our understanding of evolutionary radiations. Therefore, all dating analyses must involve a model selection process using Bayes factors to select between a Yule or birth-death prior, in particular on ancient clades with a potential pattern of high extinction. We also provide new insights into the history of cycad diversification because we found (i) periods of extinction along the long branches of the genera consistent with fossil data, and (ii) high diversification rates within the Miocene genus radiations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0347-8) contains supplementary material, which is available to authorized users.

The historical demography and genetic variation of the endangered Cycas multipinnata (Cycadaceae) in the red river region, examined by chloroplast DNA sequences and microsatellite markers

Cycas multipinnata C.J. Chen & S.Y. Yang is a cycad endemic to the Red River drainage region that occurs under evergreen forest on steep limestone slopes in Southwest China and northern Vietnam. It is listed as endangered due to habitat loss and over-collecting for the ornamental plant trade, and only several populations remain. In this study, we assess the genetic variation, population structure, and phylogeography of C. multipinnata populations to help develop strategies for the conservation of the species. 60 individuals from six populations were used for chloroplast DNA (cpDNA) sequencing and 100 individuals from five populations were genotyped using 17 nuclear microsatellites. High genetic differentiation among populations was detected, suggesting that pollen or seed dispersal was restricted within populations. Two main genetic clusters were observed in both the cpDNA and microsatellite loci, corresponding to Yunnan China and northern Vietnam. These clusters indicated low levels of gene flow between the regions since their divergence in the late Pleistocene, which was inferred from both Bayesian and coalescent analysis. In addition, the result of a Bayesian skyline plot based on cpDNA portrayed a long history of constant population size followed by a decline in the last 50,000 years of C. multipinnata that was perhaps affected by the Quaternary glaciations, a finding that was also supported by the Garza-Williamson index calculated from the microsatellite data. The genetic consequences produced by climatic oscillations and anthropogenic disturbances are considered key pressures on C. multipinnata. To establish a conservation management plan, each population of C. multipinnata should be recognized as a Management Unit (MU). In situ and ex situ actions, such as controlling overexploitation and creating a germplasm bank with high genetic diversity, should be urgently implemented to preserve this species.

Phylogeny of the cycads based on multiple single-copy nuclear genes: congruence of concatenated parsimony, likelihood and species tree inference methods

BACKGROUND AND AIMS

Despite a recent new classification, a stable phylogeny for the cycads has been elusive, particularly regarding resolution of Bowenia, Stangeria and Dioon. In this study, five single-copy nuclear genes (SCNGs) are applied to the phylogeny of the order Cycadales. The specific aim is to evaluate several gene tree-species tree reconciliation approaches for developing an accurate phylogeny of the order, to contrast them with concatenated parsimony analysis and to resolve the erstwhile problematic phylogenetic position of these three genera.

METHODS

DNA sequences of five SCNGs were obtained for 20 cycad species representing all ten genera of Cycadales. These were analysed with parsimony, maximum likelihood (ML) and three Bayesian methods of gene tree-species tree reconciliation, using Cycas as the outgroup. A calibrated date estimation was developed with Bayesian methods, and biogeographic analysis was also conducted.

KEY RESULTS

Concatenated parsimony, ML and three species tree inference methods resolve exactly the same tree topology with high support at most nodes. Dioon and Bowenia are the first and second branches of Cycadales after Cycas, respectively, followed by an encephalartoid clade (Macrozamia-Lepidozamia-Encephalartos), which is sister to a zamioid clade, of which Ceratozamia is the first branch, and in which Stangeria is sister to Microcycas and Zamia.

CONCLUSIONS

A single, well-supported phylogenetic hypothesis of the generic relationships of the Cycadales is presented. However, massive extinction events inferred from the fossil record that eliminated broader ancestral distributions within Zamiaceae compromise accurate optimization of ancestral biogeographical areas for that hypothesis. While major lineages of Cycadales are ancient, crown ages of all modern genera are no older than 12 million years, supporting a recent hypothesis of mostly Miocene radiations. This phylogeny can contribute to an accurate infrafamilial classification of Zamiaceae.

Cenozoic extinctions account for the low diversity of extant gymnosperms compared with angiosperms

We test the widely held notion that living gymnosperms are 'ancient' and 'living fossils' by comparing them with their sister group, the angiosperms. This perception derives partly from the lack of gross morphological differences between some Mesozoic gymnosperm fossils and their living relatives (e.g. Ginkgo, cycads and dawn redwood), suggesting that the rate of evolution of gymnosperms has been slow. We estimated the ages and diversification rates of gymnosperm lineages using Bayesian relaxed molecular clock dating calibrated with 21 fossils, based on the phylogenetic analysis of alignments of matK chloroplast DNA (cpDNA) and 26S nuclear ribosomal DNA (nrDNA) sequences, and compared these with published estimates for angiosperms. Gymnosperm crown groups of Cenozoic age are significantly younger than their angiosperm counterparts (median age: 32 Ma vs 50 Ma) and have long unbranched stems, indicating major extinctions in the Cenozoic, in contrast with angiosperms. Surviving gymnosperm genera have diversified more slowly than angiosperms during the Neogene as a result of their higher extinction rate. Compared with angiosperms, living gymnosperm groups are not ancient. The fossil record also indicates that gymnosperms suffered major extinctions when climate changed in the Oligocene and Miocene. Extant gymnosperm groups occupy diverse habitats and some probably survived after making adaptive shifts.

Establishing a time-scale for plant evolution

• Plants have utterly transformed the planet, but testing hypotheses of causality requires a reliable time-scale for plant evolution. While clock methods have been extensively developed, less attention has been paid to the correct interpretation and appropriate implementation of fossil data. • We constructed 17 calibrations, consisting of minimum constraints and soft maximum constraints, for divergences between model representatives of the major land plant lineages. Using a data set of seven plastid genes, we performed a cross-validation analysis to determine the consistency of the calibrations. Six molecular clock analyses were then conducted, one with the original calibrations, and others exploring the impact on divergence estimates of changing maxima at basal nodes, and prior probability densities within calibrations. • Cross-validation highlighted Tracheophyta and Euphyllophyta calibrations as inconsistent, either because their soft maxima were overly conservative or because of undetected rate variation. Molecular clock analyses yielded estimates ranging from 568-815 million yr before present (Ma) for crown embryophytes and from 175-240 Ma for crown angiosperms. • We reject both a post-Jurassic origin of angiosperms and a post-Cambrian origin of land plants. Our analyses also suggest that the establishment of the major embryophyte lineages occurred at a much slower tempo than suggested in most previous studies. These conclusions are entirely compatible with current palaeobotanical data, although not necessarily with their interpretation by palaeobotanists.

Beetle pollination of the fruit-scented cones of the South African cycad Stangeria eriopus

There has been considerable uncertainty about the importance of wind vs. insects in cycad pollination, but recent studies in several cycad genera have indicated that these are pollinated primarily, if not exclusively, by insects. Stangeria represents an isolated southern African cycad lineage previously thought to be wind-pollinated. Unlike in most other cycads, there is no evidence of cone thermogenesis in Stangeria. We found that the scent of both male and female Stangeria cones mimics that of fermented fruit, the main volatiles being esters of acetic acid, ketones, and aldehydes. We found a large variety of insect visitors on the cones, the most common ones being sap and rove beetles (Coleoptera: Nitidulidae, Staphylinidae) and fruit flies (Diptera: Drosophilidae). Of these, only sap beetles (Nitidulidae) were able to effect pollination under experimental conditions. Because sap beetles are also pollinators of Cycas and members of several ancient angiosperm families, their role in the pollination of Stangeria adds interesting details to the role this group of insects has played in the history of plant-pollinator interactions.

DNA barcoding in the cycadales: testing the potential of proposed barcoding markers for species identification of cycads

Barcodes are short segments of DNA that can be used to uniquely identify an unknown specimen to species, particularly when diagnostic morphological features are absent. These sequences could offer a new forensic tool in plant and animal conservation-especially for endangered species such as members of the Cycadales. Ideally, barcodes could be used to positively identify illegally obtained material even in cases where diagnostic features have been purposefully removed or to release confiscated organisms into the proper breeding population. In order to be useful, a DNA barcode sequence must not only easily PCR amplify with universal or near-universal reaction conditions and primers, but also contain enough variation to generate unique identifiers at either the species or population levels. Chloroplast regions suggested by the Plant Working Group of the Consortium for the Barcode of Life (CBoL), and two alternatives, the chloroplast psbA-trnH intergenic spacer and the nuclear ribosomal internal transcribed spacer (nrITS), were tested for their utility in generating unique identifiers for members of the Cycadales. Ease of amplification and sequence generation with universal primers and reaction conditions was determined for each of the seven proposed markers. While none of the proposed markers provided unique identifiers for all species tested, nrITS showed the most promise in terms of variability, although sequencing difficulties remain a drawback. We suggest a workflow for DNA barcoding, including database generation and management, which will ultimately be necessary if we are to succeed in establishing a universal DNA barcode for plants.

A method for constraining the age of origination of derived characters

Fossils are the physical records of the history of morphological character evolution on Earth and can provide valuable information concerning the sequence and timing of origination of derived characters. Knowledge of the timing of origination of synapomorphies makes it possible to estimate when unobserved character changes occurred in the geological past. Here we present a method for estimating the temporal interval during which synapomorphies evolved. The method requires either direct inclusion of fossil taxa (with or without extant taxa) in cladistic analyses based on morphological or combined data, or indirectly using the "molecular scaffold approach." Second, characters of interest are mapped on a most parsimonious tree and "minimum age node mapping" is used to place minimum ages on the nodes of the tree. Finally, characters of interest are evaluated for younger and/or older temporal constraints on the time of their origination; application of the older bound assumes ancestry of fossil terminals included in the tree. A key is provided herein describing the method. Among other applications, this approach has the potential to provide a powerful test of purported evolutionary cause-effect relationships. For example, the method has the ability to discover that derived characters of suggested adaptational significance may considerably pre-date the cause(s) that are hypothesized to have favored their establishment.