Recent speciation and adaptation to aridity in the ecologically diverse Pilbara region of Australia enabled the native tobaccos (Nicotiana; Solanaceae) to colonize all Australian deserts.

Over the last 6 million years, the arid Australian Eremaean Zone (EZ) has remained as dry as it is today. A widely accepted hypothesis suggests that the flora and fauna of arid regions were more broadly distributed before aridification began. In Australia, this process started around 20 million years ago (Ma), leading to gradual speciation as the climate became increasingly arid. Here, we use genomic data to investigate the biogeography and timing of divergence of native allotetraploid tobaccos, Nicotiana section Suaveolentes (Solanaceae). The original allotetraploid migrants from South America were adapted to mesic areas of Australia and recently radiated in the EZ, including in sandy dune fields (only 1.2 Ma old), after developing drought adaptations. Coalescent and maximum likelihood analyses suggest that Nicotiana section Suaveolentes arrived on the continent around 6 Ma, with the ancestors of the Pilbara (Western Australian) lineages radiating there at the onset of extreme aridity 5 Ma by locally adapting to these various ancient, highly stable habitats. The Pilbara thus served as both a mesic refugium and cradle for adaptations to harsher conditions, due to its high topographical diversity, providing microhabitats with varying moisture levels and its proximity to the ocean, which buffers against extreme aridity. This enabled species like Nicotiana to survive in mesic refugia and subsequently adapt to more arid conditions. These results demonstrate that initially poorly adapted plant groups can develop novel adaptations in situ, permitting extensive and rapid dispersal despite the highly variable and unpredictable extreme conditions of the EZ.

Genomic insights into recent species divergence in Nicotiana benthamiana and natural variation in Rdr1 gene controlling viral susceptibility.

One of the most commonly encountered and frequently cited laboratory organisms worldwide is classified taxonomically as Nicotiana benthamiana (Solanaceae), an accession of which, typically referred to as LAB, is renowned for its unique susceptibility to a wide range of plant viruses and hence capacity to be transformed using a variety of methods. This susceptibility is the result of an insertion and consequent loss of function in the RNA-dependent RNA polymerase 1 (Rdr1) gene. However, the origin and age of LAB and the evolution of N. benthamiana across its wide distribution in Australia remain relatively underexplored. Here, we have used multispecies coalescent methods on genome-wide single nucleotide polymorphisms (SNPs) to assess species limits, phylogenetic relationships and divergence times within N. benthamiana. Our results show that the previous taxonomic concept of this species in fact comprises five geographically, morphologically and genetically distinct species, one of which includes LAB. We provide clear evidence that LAB is closely related to accessions collected further north in the Northern Territory; this species split much earlier, c. 1.1 million years ago, from their common ancestor than the other four in this clade and is morphologically the most distinctive. We also found that the Rdr1 gene insertion is variable among accessions from the northern portions of the Northern Territory. Furthermore, this long-isolated species typically grows in sheltered sites in subtropical/tropical monsoon areas of northern Australia, contradicting the previously advanced hypothesis that this species is an extremophile that has traded viral resistance for precocious development.

Macroevolutionary trends of the Neotropical genus Ameroglossum (Linderniaceae) in rocky outcrop environments.

Ameroglossum is a rare plant genus endemic to northeastern of Brazil, initially monospecific (A. pernambucense) and recently expanded by the description of eight new species and two related genera. The genus was initially placed in the family Scrophulariaceae, but this has never been phylogenetically tested. This group is ecologically restricted to rocky inselberg habitats that function as island-like systems (ILS) with spatial fragmentation, limited area, environmental heterogeneity, temporal isolation and low connectivity. Here we use a phylogenetic perspective to test the hypothesis that Ameroglossum diversification was related to island-like radiation in inselbergs. Our results support that Ameroglossum is monophyletic only with the inclusion of Catimbaua and Isabelcristinia (named here as Ameroglossum sensu lato) and this group was well-supported in the family Linderniaceae. Biogeographic analyses suggest that the ancestral of Ameroglossum and related genus arrived in South America c.a. 15 million years ago by long-distance dispersal, given the ancestral distribution of Linderniaceae in Africa. In rocky outcrop habitats, Ameroglossum s.l. developed floral morphological specialization associated with pollinating hummingbirds, compatible with an island-like model. However, no increase in speciation rate was detected, which may be related to high extinction rates and/or slow diversification rate in this ecologically restrictive environment. Altogether, in Ameroglossum key innovations involving flowers seem to have offered opportunities for evolution of greater phenotypic diversity and occupation of new niches in rocky outcrop environments.

Down, then up: non-parallel genome size changes and a descending chromosome series in a recent radiation of the Australian allotetraploid plant species, Nicotiana section Suaveolentes (Solanaceae).

BACKGROUND AND AIMS: The extent to which genome size and chromosome numbers evolve in concert is little understood, particularly after polyploidy (whole-genome duplication), when a genome returns to a diploid-like condition (diploidization). We study this phenomenon in 46 species of allotetraploid Nicotiana section Suaveolentes (Solanaceae), which formed <6 million years ago and radiated in the arid centre of Australia. METHODS: We analysed newly assessed genome sizes and chromosome numbers within the context of a restriction site-associated nuclear DNA (RADseq) phylogenetic framework. KEY RESULTS: RADseq generated a well-supported phylogenetic tree, in which multiple accessions from each species formed unique genetic clusters. Chromosome numbers and genome sizes vary from n = 2x = 15 to 24 and 2.7 to 5.8 pg/1C nucleus, respectively. Decreases in both genome size and chromosome number occur, although neither consistently nor in parallel. Species with the lowest chromosome numbers (n = 15-18) do not possess the smallest genome sizes and, although N. heterantha has retained the ancestral chromosome complement, n = 2x = 24, it nonetheless has the smallest genome size, even smaller than that of the modern representatives of ancestral diploids. CONCLUSIONS: The results indicate that decreases in genome size and chromosome number occur in parallel down to a chromosome number threshold, n = 20, below which genome size increases, a phenomenon potentially explained by decreasing rates of recombination over fewer chromosomes. We hypothesize that, more generally in plants, major decreases in genome size post-polyploidization take place while chromosome numbers are still high because in these stages elimination of retrotransposons and other repetitive elements is more efficient. Once such major genome size change has been accomplished, then dysploid chromosome reductions take place to reorganize these smaller genomes, producing species with small genomes and low chromosome numbers such as those observed in many annual angiosperms, including Arabidopsis.

Is Santaniella a ranunculid? Reassessment of this enigmatic fossil angiosperm from the Lower Cretaceous (Aptian, Crato Konservat-Lagerstätte, Brazil) provides a new interpretation.

PREMISE: The Lower Cretaceous Crato Konservat-Lagerstätte (CKL) preserves a rich flora that includes early angiosperms from northern Gondwana. From this area, the recently described fossil genus Santaniella was interpreted as a ranunculid (presumably Ranunculaceae). However, based on our examination of an additional specimen and a new phylogenetic analysis, we offer an alternative interpretation. METHODS: The new fossil was collected from an active quarry for paving stones in the state of Ceará, northeastern Brazil. We assessed support for alternative phylogenetic hypotheses using a combined analysis of morphological data and DNA sequence data using Bayesian inference. We used a consensus network to visualize the posterior distribution of trees, and we used RoguePlot to illustrate the support for alternative positions on a scaffold tree. RESULTS: The new material includes a flower-like structure not present in the original material and also includes follicles preserved at early stages of development. The flower-like structure is a compact terminal cluster of elliptical sterile laminar organs surrounding internal filamentous structures that occur on flexuous axes. Phylogenetic analyses did not support the fossil placement among eudicots. Instead, Santaniella appears to belong in the magnoliid clade. CONCLUSIONS: The presence of seeds in a marginal-linear placentation and enclosed in a follicle supports the fossil as an angiosperm. However, even though most characters are clearly recognizable, its combination of characters does not provide strong support for a close relationship to any extant order of flowering plants. Its position in the magnoliid clade is intriguing and, based on plicate carpels, it is definitely a mesangiosperm.

Giant Fern Genomes Show Complex Evolution Patterns: A Comparative Analysis in Two Species of Tmesipteris (Psilotaceae).

Giant genomes are rare across the plant kingdom and their study has focused almost exclusively on angiosperms and gymnosperms. The scarce genetic data that are available for ferns, however, indicate differences in their genome organization and a lower dynamism compared to other plant groups. Tmesipteris is a small genus of mainly epiphytic ferns that occur in Oceania and several Pacific Islands. So far, only two species with giant genomes have been reported in the genus, T. tannensis (1C = 73.19 Gbp) and T. obliqua (1C = 147.29 Gbp). Low-coverage genome skimming sequence data were generated in these two species and analyzed using the RepeatExplorer2 pipeline to identify and quantify the repetitive DNA fraction of these genomes. We found that both species share a similar genomic composition, with high repeat diversity compared to taxa with small (1C < 10 Gbp) genomes. We also found that, in general, characterized repetitive elements have relatively high heterogeneity scores, indicating ancient diverging evolutionary trajectories. Our results suggest that a whole genome multiplication event, accumulation of repetitive elements, and recent activation of those repeats have all played a role in shaping these genomes. It will be informative to compare these data in the future with data from the giant genome of the angiosperm Paris japonica, to determine if the structures observed here are an emergent property of massive genomic inflation or derived from lineage specific processes.

Biogeography and genome size evolution of the oldest extant vascular plant genus, Equisetum (Equisetaceae).

BACKGROUND AND AIMS: Extant plant groups with a long fossil history are key elements in understanding vascular plant evolution. Horsetails (Equisetum, Equisetaceae) have a nearly continuous fossil record dating back to the Carboniferous, but their phylogenetic and biogeographic patterns are still poorly understood. We use here the most extensive phylogenetic analysis to date as a framework to evaluate their age, biogeography and genome size evolution. METHODS: DNA sequences of four plastid loci were used to estimate divergence times and investigate the biogeographic history of all extant species of Equisetum. Flow cytometry was used to study genome size evolution against the framework of phylogenetic relationships in Equisetum. KEY RESULTS: On a well-supported phylogenetic tree including all extant Equisetum species, a molecular clock calibrated with multiple fossils places the node at which the outgroup and Equisetum diverged at 343 Mya (Early Carboniferous), with the first major split among extant species occurring 170 Mya (Middle Jurassic). These dates are older than those reported in some other recent molecular clock studies but are largely in agreement with a timeline established by fossil appearance in the geological record. Representatives of evergreen subgenus Hippochaete have much larger genome sizes than those of deciduous subgenus Equisetum, despite their shared conserved chromosome number. Subgenus Paramochaete has an intermediate genome size and maintains the same number of chromosomes. CONCLUSIONS: The first divergences among extant members of the genus coincided with the break-up of Pangaea and the resulting more humid, warmer climate. Subsequent tectonic activity most likely involved vicariance events that led to species divergences combined with some more recent, long-distance dispersal events. We hypothesize that differences in genome size between subgenera may be related to the number of sperm flagellae.

Plastid phylogenomic insights into the evolution of Caryophyllales.

The Caryophyllales includes 40 families and 12,500 species, representing a large and diverse clade of angiosperms. Collectively, members of the clade grow on all continents and in all terrestrial biomes and often occupy extreme habitats (e.g., xeric, salty). The order is characterized by many taxa with unusual adaptations including carnivory, halophytism, and multiple origins of C4 photosynthesis. However, deep phylogenetic relationships within the order have long been problematic due to putative rapid divergence. To resolve the deep-level relationships of Caryophyllales, we performed phylogenomic analyses of all 40 families of Caryophyllales. We time-calibrated the molecular phylogeny of this clade, and evaluated putative correlations among plastid structural changes and rates of molecular substitution. We recovered a well-resolved and well-supported phylogeny of the Caryophyllales that was largely congruent with previous estimates of this order. Our results provide improved support for the phylogenetic position of several key families within this clade. The crown age of Caryophyllales was estimated at ca. 114.4 million years ago (Ma), with periods of rapid divergence in the mid-Cretaceous. A strong, positive correlation between nucleotide substitution rate and plastid structural changes was detected. Our study highlights the importance of broad taxon sampling in phylogenomic inference and provides a firm basis for future investigations of molecular, morphological, and ecophysiological evolution in Caryophyllales.

Trends and concepts in fern classification.

BACKGROUND AND AIMS: Throughout the history of fern classification, familial and generic concepts have been highly labile. Many classifications and evolutionary schemes have been proposed during the last two centuries, reflecting different interpretations of the available evidence. Knowledge of fern structure and life histories has increased through time, providing more evidence on which to base ideas of possible relationships, and classification has changed accordingly. This paper reviews previous classifications of ferns and presents ideas on how to achieve a more stable consensus. SCOPE: An historical overview is provided from the first to the most recent fern classifications, from which conclusions are drawn on past changes and future trends. The problematic concept of family in ferns is discussed, with a particular focus on how this has changed over time. The history of molecular studies and the most recent findings are also presented. KEY RESULTS: Fern classification generally shows a trend from highly artificial, based on an interpretation of a few extrinsic characters, via natural classifications derived from a multitude of intrinsic characters, towards more evolutionary circumscriptions of groups that do not in general align well with the distribution of these previously used characters. It also shows a progression from a few broad family concepts to systems that recognized many more narrowly and highly controversially circumscribed families; currently, the number of families recognized is stabilizing somewhere between these extremes. Placement of many genera was uncertain until the arrival of molecular phylogenetics, which has rapidly been improving our understanding of fern relationships. As a collective category, the so-called 'fern allies' (e.g. Lycopodiales, Psilotaceae, Equisetaceae) were unsurprisingly found to be polyphyletic, and the term should be abandoned. Lycopodiaceae, Selaginellaceae and Isoëtaceae form a clade (the lycopods) that is sister to all other vascular plants, whereas the whisk ferns (Psilotaceae), often included in the lycopods or believed to be associated with the first vascular plants, are sister to Ophioglossaceae and thus belong to the fern clade. The horsetails (Equisetaceae) are also members of the fern clade (sometimes inappropriately called 'monilophytes'), but, within that clade, their placement is still uncertain. Leptosporangiate ferns are better understood, although deep relationships within this group are still unresolved. Earlier, almost all leptosporangiate ferns were placed in a single family (Polypodiaceae or Dennstaedtiaceae), but these families have been redefined to narrower more natural entities. CONCLUSIONS: Concluding this paper, a classification is presented based on our current understanding of relationships of fern and lycopod clades. Major changes in our understanding of these families are highlighted, illustrating issues of classification in relation to convergent evolution and false homologies. Problems with the current classification and groups that still need study are pointed out. A summary phylogenetic tree is also presented. A new classification in which Aspleniaceae, Cyatheaceae, Polypodiaceae and Schizaeaceae are expanded in comparison with the most recent classifications is presented, which is a modification of those proposed by Smith et al. (2006, 2008) and Christenhusz et al. (2011). These classifications are now finding a wider acceptance and use, and even though a few amendments are made based on recently published results from molecular analyses, we have aimed for a stable family and generic classification of ferns.

The genome sequence of field madder, Sherardia arvensis L., 1753 (Rubiaceae).

We present a genome assembly from an individual Sherardia arvensis (field madder; Tracheophyta; Magnoliopsida; Gentianales; Rubiaceae). The genome sequence is 440.9 megabases in span. Most of the assembly is scaffolded into 11 chromosomal pseudomolecules. The mitochondrial and plastid genome assemblies have lengths of 203.98 kilobases and 152.73 kilobases in length, respectively.

The genome sequence of black poplar, Populus nigra subsp. betulifolia L., 1753 (Salicaceae).

We present a genome assembly from an individual Populus nigra subsp. betulifola (black poplar; Tracheophyta; Malpighiales; Salicaceae). The genome sequence is 413.2 megabases in span. Most of the assembly (99.73%) is scaffolded into 19 chromosomal pseudomolecules. Mitochondrial and plastid genomes were also assembled. Three mitochondrial assemblies have lengths of 281.85, 335.57 and 186.15 kilobases, and the plastid genome has a length of 156.37 kilobases.

The genome sequence of the English holly, Ilex aquifolium L. (Aquifoliaceae).

We present a genome assembly from an individual Ilex aquifolium (the English holly; Eudicot; Magnoliopsida; Aquifoliales; Aquifoliaceae). The genome sequence is 800.0 megabases in span. Most of the assembly is scaffolded into 20 chromosomal pseudomolecules. The assembled mitochondrial and plastid genomes have lengths of 538.43 kilobases and 157.52 kilobases in length, respectively.

The genome sequence of common knotgrass, Polygonum aviculare L. (Polygonaceae).

We present a genome assembly from an individual Polygonum aviculare (common knotgrass; Eudicot; Magnoliopsida; Caryophyllales; Polygonaceae). The genome sequence is 351.6 megabases in span. Most of the assembly is scaffolded into 10 chromosomal pseudomolecules. The mitochondrial and plastid genome assemblies have lengths of 333.39 kilobases and 163.28 kilobases in length, respectively.

The genome sequence of the Lesser Skullcap, Scutellaria minor Huds., 1762 (Lamiaceae).

We present a genome assembly from an individual Scutellaria minor (Tracheophyta; Magnoliopsida; Lamiales; Lamiaceae). The genome sequence is 341.8 megabases in span. Most of the assembly is scaffolded into 14 chromosomal pseudomolecules. The mitochondrial and plastid genome assemblies have lengths of 376.64 kilobases and 152.59 kilobases in length, respectively.

The genome sequence of the Annual Mercury, Mercurialis annua L., 1753 (Euphorbiaceae).

We present a genome assembly from a diploid female Mercurialis annua (the Annual Mercury; Tracheophyta; Magnoliopsida; Malpighiales; Euphorbiaceae). The genome sequence is 453.2 megabases in span. Most of the assembly is scaffolded into 8 chromosomal pseudomolecules, including the X chromosome. The organelle genomes have also been assembled, and the mitochondrial genome is 435.28 kilobases in length, while the plastid genome is 169.65 kilobases in length.

The genome sequence of rosebay willowherb Chamaenerion angustifolium (L.) Scop., 1771 (syn. Epilobium angustifolium L., 1753) (Onagraceae).

We present a genome assembly from an individual Chamaenerion angustifolium (fireweed; Tracheophyta; Magnoliopsida; Myrtales; Onagraceae). The genome sequence is 655.9 megabases in span. Most of the assembly is scaffolded into 18 chromosomal pseudomolecules. The mitochondrial and plastid genome assemblies have lengths of 495.18 kilobases and 160.41 kilobases in length, respectively.

A 160 Gbp fork fern genome shatters size record for eukaryotes.

Vascular plants are exceptional among eukaryotes due to their outstanding genome size diversity which ranges ∼2,400-fold, including the largest genome so far recorded in the angiosperm Paris japonica (148.89 Gbp/1C). Despite available data showing that giant genomes are restricted across the Tree of Life, the biological limits to genome size expansion remain to be established. Here, we report the discovery of an even larger eukaryotic genome in Tmesipteris oblanceolata, a New Caledonian fork fern. At 160.45 Gbp/1C, this record-breaking genome challenges current understanding and opens new avenues to explore the evolutionary dynamics of genomic gigantism.

The genome sequence of lesser trefoil or Irish shamrock, Trifolium dubium Sibth. (Fabaceae).

We present a genome assembly from an individual Trifolium dubium (lesser trefoil; Tracheophyta; Magnoliopsida; Fabales; Fabaceae) as part of a collaboration between the Darwin Tree of Life and the European Reference Genome Atlas. The genome sequence is 679.1 megabases in span. Most of the assembly is scaffolded into 15 chromosomal pseudomolecules. The two mitochondrial genomes have lengths of 133.86 kb and 182.32 kb, and the plastid genome assembly has a length of 126.22 kilobases.

Genomic incongruence accompanies the evolution of flower symmetry in Eudicots: a case study in the poppy family (Papaveraceae, Ranunculales).

Reconstructing evolutionary trajectories and transitions that have shaped floral diversity relies heavily on the phylogenetic framework on which traits are modelled. In this study, we focus on the angiosperm order Ranunculales, sister to all other eudicots, to unravel higher-level relationships, especially those tied to evolutionary transitions in flower symmetry within the family Papaveraceae. This family presents an astonishing array of floral diversity, with actinomorphic, disymmetric (two perpendicular symmetry axes), and zygomorphic flowers. We generated nuclear and plastid datasets using the Angiosperms353 universal probe set for target capture sequencing (of 353 single-copy nuclear ortholog genes), together with publicly available transcriptome and plastome data mined from open-access online repositories. We relied on the fossil record of the order Ranunculales to date our phylogenies and to establish a timeline of events. Our phylogenomic workflow shows that nuclear-plastid incongruence accompanies topological uncertainties in Ranunculales. A cocktail of incomplete lineage sorting, post-hybridization introgression, and extinction following rapid speciation most likely explain the observed knots in the topology. These knots coincide with major floral symmetry transitions and thus obscure the order of evolutionary events.

The genome sequence of bittersweet, Solanum dulcamara L. (Solanaceae).

We present a genome assembly from an individual Solanum dulcamara (bittersweet; Eudicot; Magnoliopsida; Solanales; Solanaceae). The genome sequence is 946.3 megabases in span. Most of the assembly is scaffolded into 12 chromosomal pseudomolecules. The mitochondrial and plastid genomes have also been assembled, with lengths of 459.22 kilobases and 161.98 kilobases respectively.