KAROL, KENNETH G.1*, RICHARD M. MCCOURT2, and CHARLES F. DELWICHE1. 1Department of Cell Biology and Molecular Genetics, University of Maryland - College Park, College Park, MD, 20742; 2Department of Botany, Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA, 19103. - Estimating divergence times within the Charophyta: a Bayesian approach using fossil and DNA sequence data.
Morphological and molecular studies have advanced our understanding of
charophycean green algal relationships. A recent multi-gene
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-dependant 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 1,061 ± 109 million years
ago (Ma) and 703 ± 45 Ma for the divergence of vascular plants and
bryophytes, a result much older than that found in the fossil record
(roughly 470 Ma). 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 multi-gene DNA data set to estimate the time that the land
plant and Charales lineages diverged from their common ancestor.
Key words: Bayesian, Charales, Charophyta, divergence time, fossils, land plants