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.2010 Mar 30;107(13):5897-902.
doi: 10.1073/pnas.1001225107. Epub 2010 Mar 16.

An uncorrelated relaxed-clock analysis suggests an earlier origin for flowering plants

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An uncorrelated relaxed-clock analysis suggests an earlier origin for flowering plants

Stephen A Smith et al. Proc Natl Acad Sci U S A..

Abstract

We present molecular dating analyses for land plants that incorporate 33 fossil calibrations, permit rates of molecular evolution to be uncorrelated across the tree, and take into account uncertainties in phylogenetic relationships and the fossil record. We attached a prior probability to each fossil-based minimum age, and explored the effects of relying on the first appearance of tricolpate pollen grains as a lower bound for the age of eudicots. Many of our divergence-time estimates for major clades coincide well with both the known fossil record and with previous estimates. However, our estimates for the origin of crown-clade angiosperms, which center on the Late Triassic, are considerably older than the unequivocal fossil record of flowering plants or than the molecular dates presented in recent studies. Nevertheless, we argue that our older estimates should be taken into account in studying the causes and consequences of the angiosperm radiation in relation to other major events, including the diversification of holometabolous insects. Although the methods used here do help to correct for lineage-specific heterogeneity in rates of molecular evolution (associated, for example, with evolutionary shifts in life history), we remain concerned that some such effects (e.g., the early radiation of herbaceous clades within angiosperms) may still be biasing our inferences.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Phylogenetic tree and divergence time estimates for land plants. (A) MrBayes consensus tree from a three-gene (atpB,rbcL, and 18S) analysis (seeFig S1 for taxon names). Branch lengths represent average substitutions per site. Shaded bars representAngiospermae (black),Acrogymnospermae (dark gray), and the rest of the land plants (light gray). (B) The maximum clade credibility tree from the divergence time analysis of the same three-gene dataset as inA. Studies focused on the root of the land plants (e.g., ref. 81), including outgroups, place the root along the liverwort branch (“bryophytes” paraphyletic). Nodes marked by an asterisk (*) are supported by <0.95 posterior probability. The 95% highest posterior density (HPD) estimates for each well-supported clade are represented by bars. Numbers at nodes correspond to the fossil calibrations inTable S2. (C) Map with localities for the first tricolpate pollen records. Clade names follow Cantino et al. (47): ACR,Acrogymnospermae; ANA, ANITA grade; BRY, bryophytes; CER,Ceratophyllum; EUD,Eudicotyledonae; LYC,Lycopodiophyta; MAG,Magnoliidae; MOL,Monilophyta; MON,Monocotyledonae.
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