The origin and early evolution of land plants in the mid-Palaeozoic era, between about 480 and 360 million years ago, was an important event in the history of life, with far-reaching consequences for the evolution of terrestrial organisms and global environments. A recent surge of interest, catalysed by palaeobotanical discoveries and advances in the systematics of living plants, provides a revised perspective on the evolution of early land plants and suggests new directions for future research.

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The origin and early evolution of plants on land

Reconstructing the origin and evolution of land plants and their algal relatives is a fundamental problem in plant phylogenetics, and is essential for understanding how critical adaptations arose, including the embryo, vascular tissue, seeds, and flowers. Despite advances in molecular systematics, some hypotheses of relationships remain weakly resolved. Inferring deep phylogenies with bouts of rapid diversification can be problematic; however, genome-scale data should significantly increase the number of informative characters for analyses. Recent phylogenomic reconstructions focused on the major divergences of plants have resulted in promising but inconsistent results. One limitation is sparse taxon sampling, likely resulting from the difficulty and cost of data generation. To address this limitation, transcriptome data for 92 streptophyte taxa were generated and analyzed along with 11 published plant genome sequences. Phylogenetic reconstructions were conducted using up to 852 nuclear genes and 1,701,170 aligned sites. Sixty-nine analyses were performed to test the robustness of phylogenetic inferences to permutations of the data matrix or to phylogenetic method, including supermatrix, supertree, and coalescent-based approaches, maximum-likelihood and Bayesian methods, partitioned and unpartitioned analyses, and amino acid versus DNA alignments. Among other results, we find robust support for a sister-group relationship between land plants and one group of streptophyte green algae, the Zygnematophyceae. Strong and robust support for a clade comprising liverworts and mosses is inconsistent with a widely accepted view of early land plant evolution, and suggests that phylogenetic hypotheses used to understand the evolution of fundamental plant traits should be reevaluated.

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Phylotranscriptomic analysis of the origin and early diversification of land plants

http://www.noahwhiteman.org/uploads/7/1/3/1/7131048/trplsc952.pdf

Evolution of jasmonate and salicylate signal crosstalk.

Although macroscopic plants, animals, and fungi are the most familiar eukaryotes, the bulk of eukaryotic diversity is microbial. Elucidating the timing of diversification among the more than 70 lineages is key to understanding the evolution of eukaryotes. Here, we use taxon-rich multigene data combined with diverse fossils and a relaxed molecular clock framework to estimate the timing of the last common ancestor of extant eukaryotes and the divergence of major clades. Overall, these analyses suggest that the last common ancestor lived between 1866 and 1679 Ma, consistent with the earliest microfossils interpreted with confidence as eukaryotic. During this interval, the Earth's surface differed markedly from today; for example, the oceans were incompletely ventilated, with ferruginous and, after about 1800 Ma, sulfidic water masses commonly lying beneath moderately oxygenated surface waters. Our time estimates also indicate that the major clades of eukaryotes diverged before 1000 Ma, with most or all probably diverging before 1200 Ma. Fossils, however, suggest that diversity within major extant clades expanded later, beginning about 800 Ma, when the oceans began their transition to a more modern chemical state. In combination, paleontological and molecular approaches indicate that long stems preceded diversification in the major eukaryotic lineages.

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Estimating the timing of early eukaryotic diversification with multigene molecular clocks

The East African rift system

Agaristoxylon garennicum Gerrienne et al., gen. et sp. nov., an arborescent Ericaceae from the Belgian Upper Paleocene: palaeoenvironmental implications

Deux Psilophyton recoltes dans l'Emsien inferieur (Devonien inferieur) du bord nord du Synclinorium de Dinant (Belgique) sont decrits. Chez ces deux plantes, l'axe principal, divise ou non par dichotomie isotome, porte des systemes d'axes (unites laterales) issus de divisions anisotomes. Les sporanges se trouvent a l'extremite des axes de ces unites laterales. Ils sont fusiformes, portes par paires et, dans chaque paire, enroules l'un autour de l'autre dans le sens anti-horaire. Psilophyton cf. crenulatum Doran, 1980 provient de la carriere "Les Forges" a Marchin. L'axe principal de l'unique specimen est divise plusieurs fois par dichotomies isotomes et anisotomes, ce qui donne a la plante un aspect buissonneux. Tous les axes sont stries longitudinalement et portent sur toute leur surface de nombreux petits ornements hemi-spheriques. Psilophyton forbesii Andrews, Kasper & Mencher, 1968 a ete recolte a Marchin et a Dave (Nouvelles Carrieres). Les axes de cette plante sont depourvus d'emergence et montrent une striation longitudinale. Un systeme d'axes vegetatifs attribue a cette espece a egalement ete decouvert. Le statut des especes Psilophyton arcuatum, P. forbesii et Dawsonites arcuatus est discute.

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Le genre Psilophyton dans le Dévonien inférieur de Belgique: p. cf. Crenulatum Doran et p. forbesii Andrews, Kasper & Mencher

Etude comparative au microscope électronique à balayage du xylème secondaire de Gymnospermes et Angiospermes actuelles avant et après carbonisation contrôlée. Résultats préliminaires.

Together with Elkinsia and Condrusia, Moresnetia is one of the earliest hydrasperman Devonian seed. In contrast with most Devonian seed plants, our collection of Moresnetiafrom Belgium includes a large number of articulated specimens. Before this work, only the distal fertile parts of the plant had been described in detail, but our specimens have allowed us to study the branching pattern and three dimensional organisation of the plant. The first part of this study is based on ten specimens selected for their large dimensions and extensive branching. Classical camera lucida reconstructions have been attempted on these specimens. Furthermore, the size of the sample allowed us to do a numerical reconstruction of the plant. Several parameters have been measured such as the length and width of the axes and inner and outer angles of the ramifications. The modelling is still under progress, but preliminary results are presented here. Anatomy and morphology of the vegetative parts of the plant are still unknown, but we have obtained accurate anatomical information from the stem and the fertile axes preserved as goethite/limonite mineralisations. The stem shows a three-ribbed protostele with mesarch protoxylem strands. C-shaped vascular bundles of the Lyginorachis-type are produced at the tips of the ribs, according to a helical pattern. These bundles divide up to 9 times dichotomously, but not cruciately. Distalmost branches of the fertile frond show a single terete vascular strand. This anatomy is consistent with that observed earlier for Elkinsia polymorpha and Laceya hibernica. (Resume d'auteur)

The seed plant Moresnetia : morphology and anatomy of the stem and fertile frond

This paper discusses the main steps leading to the acquisition, during the Palaeozoic, of the seed habit. The earliest spermatophytes originated in the Late Devonian and produced small cupulate ovules characterized by an elaborated sporangial apex involved in pollination. The diversification of ovules during the Carboniferous led to a wider range of sizes and shapes, and an increasing importance of the integument. The earliest seeds containing embryos are reported in Carboniferous coniferophytes.

Letters to the twenty-first century botanist. Second series: “what is a seed?” – 3. How did we get there? Palaeobotany sheds light on the emergence of seed

Philippe Gerrienne

Papers

Agaristoxylon garennicum Gerrienne et al., gen. et sp. nov., an arborescent Ericaceae from the Belgian Upper Paleocene: palaeoenvironmental implications

Le genre Psilophyton dans le Dévonien inférieur de Belgique: p. cf. Crenulatum Doran et p. forbesii Andrews, Kasper & Mencher

Etude comparative au microscope électronique à balayage du xylème secondaire de Gymnospermes et Angiospermes actuelles avant et après carbonisation contrôlée. Résultats préliminaires.

The seed plant Moresnetia : morphology and anatomy of the stem and fertile frond

Letters to the twenty-first century botanist. Second series: “what is a seed?” – 3. How did we get there? Palaeobotany sheds light on the emergence of seed