Estimating Divergence Times within the Charophyta: A Bayesian Approach Using Fossil and DNA Sequence Data

Abstract

AbstractMorphological and molecular studies have advanced our understanding of charophycean green algal relationships. A recent multigene phylogenetic analysis has identified the Charales as the closest living relatives of land plants with the Coleochaetales sister to the Charales/land plant lineage. With a robust phylogeny now in hand, it is possible to explore phylogeny‐dependent questions that were previously difficult to assess. Estimating the divergence time of the land plant lineage is one such question. The most recent time estimate for the colonization of land by plants is 1061 ± 109 million years ago and 703 ± 45 million years ago for the divergence of vascular plants and bryophytes, a result much older than that found in the fossil record (roughly 470 million years ago). Several problems are inherent in estimating divergence times using DNA sequence data; these include the assumption of a molecular clock and choosing from a sparse fossil record for an external calibration date. Unlike most algae, a rich fossil record exists for the Charales in the form of calcified oospores termed gyrogonites. Representative gyrogonites that can be attributed to all six of the extant genera in the Characeae have been identified with reasonable accuracy. These six calibration points, identified from a closely related lineage of land plants, can be used in conjunction with a multigene DNA dataset to estimate the time that the land plant and Charales lineages diverged from their common ancestor.