© 1979 by The Society for Integrative and Comparative Biology
Electron Microscopy and The Phylogeny of Green Algae and Land Plants
Department of Molecular, Cellular and Developmental Biology, University of Colorado Boulder, Colorado 80309
Electron microscopy of dividing green algal cells has demonstrated that there are two quite separate phyletic lines in the Ulotrichales and Chaetophorales, groups that are considered classically to have provided the progenitors of the green land plants. These groups are distinguished by whether their mitotic spindle is "open" or "closed," and whether their cytokinetic apparatus at telophase contains a persistent interzonal spindle in which the microtubules are oriented perpendicular to the plane of cytokinesis (including the phragmoplast), or a "phycoplast," in which the microtubules are coplanar with the plane of cytokinesis. The majority of genera of these two groups are related and have the closed spindle and phycoplast, and so they are allied with the Volvocales, Chlorococcales, and the Oedogoniales. A few genera from each group have the open spindle and the persistent interzonal telophase spindle or the phragmoplast (the derivative of the persistent spindle), as do the Conjugales (Zygnematales) and the Charales. That these latter plants are related phyletically to the land plants is clearly demonstrated by the structure of their motile cells (where formed): these are intrinsically asymmetric and possess a single band of cytoskeletal microtubules, just as do the sperm cells of bryophytes, ferns, cycads, and the gyninosperni tree, Gingko. In contrast, the motile cells of genera which have the phycoplast are derived from a much more symmetrical (Chlamydomonad) cell-type with 2–4 bands of cytoskeletal microlubules emanating from 2–4 basal bodies.