Trends in Ecology & Evolution
Charophyte algae and land plant origins
Section snippets
Chlorophyte and charophyte algae
The close evolutionary relationship of green plants to green algae (Chlorophyta sensu lato) has been long accepted, but the diversity of form (unicells to multicellular) and life cycles (with one or two free-living stages in the life cycle) among green algae led to diverse hypotheses about which taxa gave rise to land-dwelling descendants. Frederick O. Bower's [3] early 20th-century book about the evolution of land plants and their life cycles framed the debate about the origin of a land flora
Genes and genomes unravel charophyte algal phylogeny
Recent molecular phylogenetic studies support the monophyly of individual charophyte algal groups and of most of the charophyte algal groups plus land plants (the exception being Mesostigma, which is discussed in detail below). However, the branching order of groups within the phylogeny has been uncertain 13, 14, 15, 16, 17, 18, 19, 20, 21. Hypotheses of relationships among charophyte algae have developed in a piecemeal fashion, as subsets of taxa from several groups were sampled one gene at a
From simple to complex?
Despite the uncertainty of some of the branches in Figure 1, some inferences can be drawn about the derivation of characters that might have played a role in the morphological elaboration of embryophytes. Groups near the base of the tree are either unicellular (e.g. Mesostigmatales) or consist of packets of a few cells (e.g. Chlorokybus). Later diverging groups are multicellular and exhibit various types of branching through cell division (Box 1). Similarly, sexual reproduction is absent or
The fossil record and timing of divergence
The fossil record of most charophyte algae is scarce and provides few clues to phylogeny, and minimum dates for some groups might be greatly underestimated [37]. However, fossils are abundant for Charales and several related extinct groups. Calcified oogonia of these algae (gyrogonites) first occur in deposits of the lower Silurian [425 million years ago (Mya)] and extinct taxa are more diverse than extant forms 24, 38. Fossils of other charophyte algal groups are known only from rocks younger
The enigma of Mesostigma
The placement of the unicellular Mesostigma viride, the sole known member of Mesostigmatales, has been the focus of several studies that have yielded conflicting results. This scaly flagellate unicell was initially classified in a paraphyletic assemblage of unicellular algae (Class Prasinophyceae) [5]. Later study of ultrastructural features led to its placement in the lineage of charophyte algae and land plants [51], which meant it might resemble the unicellular ancestor of the group.
Acknowledgements
We thank Kyle Luckenbill for the artwork and preparation of Figure 1, Scott Kroken for advice on identifying the moss photograph, and several anonymous reviewers for criticisms of the article. This material is based upon work supported by the National Science Foundation under Grant No. DEB 9978117.
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