MAGALLóN, SUSANA A.*, MICHAEL J. SANDERSON, JAMES A. DOYLE, AND MARTIN F. WOJCIECHOWSKI. Section of Evolution and Ecology, University of California, Davis, CA 95616. - Estimate of the age of the angiosperm crown group derived from integrated analysis of molecular and paleontological data.
Previous estimates for the age of the angiosperms derived from
molecular clock models, although in conflict with one another, have
generally implied that angiosperms originated long before their first
appearance in the stratigraphic record (Early Cretaceous). We estimate
the age of the angiosperms by integrating new data and new methods of
analysis. Taxa were sampled from all major clades of extant land
plants. Sequences of two highly conserved chloroplast genes,
psaA and psbB, were used as primary data. A data set of
combined psaA and psbB sequences for 54 genera of land
plants was used to obtain hypotheses of relationships through
parsimony analysis. Gnetales are resolved as the sister taxon of all
other seed plants if codon positions are weighted equally (Tree 1), or
alternatively, as the sister taxon to Pinaceae within a monophyletic
gymnosperm clade if 1st and 2nd codon positions are given greater
weight (Tree 2), in accordance with their much lower substitution
rates. Substitution models for synonymous and non-synonymous sites
were obtained through a likelihood approach in which a molecular clock
was enforced. Temporal calibration was achieved by fixing the time of
divergence between Marchantia and all other land plants at 450
Ma (Late Ordovician). Based on data from non-synonymous sites, the
angiosperm crown group is dated as 144 Ma, according to Tree 1, and as
133 Ma, according to Tree 2. These preliminary estimates are closer to
ages derived from the fossil record than published estimates, but
confidence intervals are broad enough to include some previous dates.
Subsequent analyses will incorporate methods for estimating divergence
times assuming heterogeneous rates of molecular evolution, and
temporal calibrations that include minimum ages for several nodes,
obtained from reliably identified fossil taxa. These analytical
approaches are expected to provide a narrower estimate for the age of
the angiosperm crown group.
Key words: age, angiosperm origin, evolutionary rates, fossil record, seed plant phylogeny, temporal calibration