Latest approaches in establishing early land plant phylogeny and a timescale for plant evolution
Jennifer Morris1, Mark Puttick1,2, Silvia Pressel2, Paul Kenrick2,
Charles Wellman3, Ziheng Yang4, Sandra Arndt1, Dianne Edwards5,
Phil Donoghue1, Harald Schneider6
1. University of Bristol
2. Natural History Museum, UK
3. University of Sheffield
4. University College London
5. Cardiff University
6. Sun Yat-sen University
Thecolonisation of plant life on land was one of the most significant evolutionary events to transform the planet. Major anatomical, biochemical and morphological innovations in the early evolution of land plants (e.g. complex body plans, leaves, rooting systems, symbioses with fungi) led not only to the evolutionary success of the lineage, but also tofundamentalchanges to global biogeochemical cycles, such asincreased rates of continental weathering and sequestration of carbon over geological timescales. However, the nature and timing of the early evolutionary history of land plants and their interactions with the Earth is far from substantiated. There is currently no consensus on the fundamental phylogenetic relationships between living bryophytes (liverworts, mosses and hornworts) and tracheophytes, with conflicting results from numerousanalyses using either molecular or morphological data. Support can be found for most hypotheses, including monophyletic bryophytes, each solution implying very different scenarios of the sequence and tempo of the origin of land plant characters. These conflicts are a consequence of a dramatic increase in the availability of molecular data and the development of more realistic models of molecular evolution. However, these models have yet to be applied to genome-scale data. Secondly, living bryophytes possess a mosaic of anatomical characters that are difficult to determine if derived from convergent or common descent.
Our project aims totest the competing hypotheses of living plant phylogeny using a combination of phenotypic datawith genome-scale sequence alignments with the latest computational models of molecular evolution.Genomic data is obtained from published sequences. Phenotypic data is in the form of a morphologicalcharacter matrix of representative taxa across the plant kingdom, but with particular emphasis on bryophytes and ‘lower’ land plants.However, living taxa only represent a fraction of the diversity and disparity of early land plants that we known from the fossil record, thusfossil taxa are included in the phenotypic character matrix to understand their phylogenetic positions among their living relatives. One assemblage of particular importancein understanding early embryophyte evolutionis a group of charcoalified fossils fromrocks of Early Devonian age (~415 Ma), Shropshire, UK, that possessboth bryophytic and tracheophytic characteristics. These rare fossils are preserved in three-dimensionsto subcellular level by charcoalification. Traditionally they have been sectioned and characterised using Scanning Electron Microscopy. However, by using the latest techniques in synchrotron radiation X-ray Tomographic Microscopy, they can be characterised in a non-invasive and non-destructive method.
Finally, after establishing the evolutionary relationships of living bryophytes and tracheophytes and the phylogenetic position of fossil taxa, we willdetermine the timescale of early land plant evolution. This will be achieved by applying the latest developments in molecular clock methodology, using molecular sequence data, models of evolution and fossil age constraints, with consideration of facies biases that can severely impact upon the fossil record. By determining the nature and timing of early land plant evolution it will then be possible to test hypotheses on the co-evolution of plants and the planet, particularly the far-reaching impacts on global biogeochemical cycles.