Foster, Charles , Sauquet, Herve , van der Merwe, Marlien , McPherson, Hannah , Rossetto, Maurizio , Ho, Simon .
Evaluating the impact of genomic data and priors on Bayesian estimates of the angiosperm evolutionary timescale.
The evolutionary timescale of angiosperms has long been a key question in biology. The oldest crown-group fossils date to the early Cretaceous (~140 Ma), yet fossils attributed to the angiosperm stem lineage date back to 247.2–242.0 Ma. Molecular estimates of the angiosperm evolutionary timescale have shown considerable variation, being influenced by differences in taxon sampling, gene sampling, fossil calibrations, evolutionary models, and choices of priors. However, a common theme in modern molecular dating studies is that the angiosperm crown age far predates the oldest fossils by a non-trivial amount of time, even by up to ~100 Ma. Did crown-group angiosperms really arise in the Triassic? Or are the substantially older molecular dating estimates a product of methodological biases? Here, we analyse a data set comprising 76 protein-coding genes from the chloroplast genomes of 195 taxa spanning 86 families, including novel genome sequences for 11 taxa, to evaluate the impact of models, priors, and gene sampling on Bayesian estimates of the angiosperm evolutionary timescale. Using a Bayesian relaxed molecular-clock method, with a core set of 35 minimum and two maximum fossil constraints, we estimated that crown angiosperms arose 221 (251–192) million years ago during the Triassic. Based on a range of additional sensitivity and subsampling analyses, we found that our date estimates were generally robust to large changes in the parameters of the birth-death tree prior and of the model of rate variation across branches. We found an exception to this when we implemented fossil calibrations in the form of highly informative gamma priors rather than as uniform priors on node ages. Under all other calibration schemes, including trials of seven maximum age constraints, we consistently found that the earliest divergences of angiosperm clades substantially predate the oldest fossils that can be assigned unequivocally to their crown group. Our results also suggest that increases in gene sampling are unlikely to produce substantial changes in estimates of the angiosperm evolutionary timescale. Instead, our understanding of this important question is likely to improve through increased taxon sampling, significant methodological changes, and/or new information from the fossil record.
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1 - University of Sydney, School of Life and Environmental Sciences, Edgeworth David Building A11, University of Sydney, Sydney, NSW, 2006, Australia
2 - Universite Paris-Sud, CNRS UMR 8079, Bat. 360, Orsay, N/A, 91405, France
3 - National Herbarium of New South Wales, Sydney, NSW, 2000, Australia
4 - National Herbarium Of NSW, Mrs Macquaries Road, Sydney, NSW, N/A, 2000, Australia
5 - University of Sydney, Edgeworth David Building A11, University of Sydney, Sydney, NSW, 2006, Australia
Presentation Type: Oral Paper
Session: 1, Phylogenomics I
Location: 101/Savannah International Trade and Convention Center
Date: Monday, August 1st, 2016
Time: 11:00 AM
Candidate for Awards:Margaret Menzel Award