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

Land plants comprise the bryophytes and the polysporangiophytes. All extant polysporangiophytes are vascular plants (tracheophytes), but to date, some basalmost polysporangiophytes (also called protracheophytes) are considered non-vascular. Protracheophytes include the Horneophytopsida and Aglaophyton/Teruelia. They are most generally considered phylogenetically intermediate between bryophytes and vascular plants and are therefore essential to elucidate the origins of current vascular floras. Here, we propose an alternative evolutionary framework for the earliest tracheophytes. The supporting evidence comes from the study of the Rhynie chert historical slides from the Natural History Museum of Lille (France). From this, we emphasize that Horneophyton has a particular type of tracheid characterized by narrow, irregular, annular and/or, possibly spiral wall thickenings of putative secondary origin, and hence that it cannot be considered non-vascular anymore. Accordingly, our phylogenetic analysis resolves Horneophyton and allies (i.e. Horneophytopsida) within tracheophytes, but as sister to eutracheophytes (i.e. extant vascular plants). Together, horneophytes and eutracheophytes form a new clade called herein supereutracheophytes. The thin, irregular, annular to helical thickenings of Horneophyton clearly point to a sequential acquisition of the characters of water-conducting cells. Because of their simple conducting cells and morphology, the horneophytophytes may be seen as the precursors of all extant vascular plant biodiversity. □ Cladistics, Horneophyton, lower Devonian, Rhynie chert, tracheophyte.

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An alternative model for the earliest evolution of vascular plants

Abstract The lignophytes (Embryophytes that possess a bifacial cambium) evolved during the Devonian period and include seed plants. Their advent was a major event in the history of life and had a profound impact on terrestrial environments. Recent reinvestigations of a Devonian locality, Dechra Aït Abdallah in Central Morocco, led to the discovery of a rich assemblage of fossil plants and Tentaculita. This paper focuses on a single specimen of the lignophyte Rellimia Leclercq & Bonamo. Rellimia (Aneurophytales) is a monospecific genus reported from a large number of Middle Devonian localities from western Europe (Belgium, Czech Republic, Germany and Scotland) and America. Its fertile organs are highly distinctive and borne helically on branches. They consist of a basal stalk that dichotomizes once near the base, the resulting branches dividing pinnately and bearing elongated sporangia terminally on ultimate divisions. According to the late Emsian age indicated by our sample of Tentaculita, this Moroccan specimen is to date the earliest representative of both the genus and the lignophytes. If confirmed, this occurrence suggests a possible origin of the Aneurophytalean lignophytes in Gondwana and their rapid and widespread colonization in the Middle Devonian towards Laurussia.

Philippe Gerrienne

Papers

An alternative model for the earliest evolution of vascular plants

First record of Rellimia Leclercq & Bonamo (Aneurophytales) from Gondwana, with comments on the earliest lignophytes

Les fossiles végétaux du Dévonieninférieur de Marchin (bord nord du Synclinorium de Dinant, Belgique). III. Psilophyton parvulum nov. sp

The proto-ovule Runcaria heinzelinii Stockmans 1968 emend. Gerrienne et al., 2004 (mid-Givetian, Belgium): Concept and epitypification.

A Middle Devonian Callixylon (Archaeopteridales) from Ronquières, Belgium