MCCOURT, RICHARD M.1*, JUN PARK2, DAVID A. HEWITT3, and KENNETH G. KAROL4. 1Department of Botany, Academy of Natural Sciences, Philadelphia, PA 19103; 2Department of Biology, University of Pennsylvania, Philadelphia, PA 19104; 3Farlow Herbarium, Harvard University, Cambridge, MA 02138; 4Cell Biology & Molecular Genetics, University of Maryland, College Park, MD 20742. - Phylogeny of the conjugating green algae (Zygnematales and Desmidiales) based on plastid and mitochondrial gene sequences.
The conjugating green algae (Order Zygnematales and Desmidiales) are
charophyceans, that is, they are among the green algae most closely
related to land plants. Compared to the five other groups of
charophyceans (Mesostigma, Klebsormidiales, Chlorokybales,
Coleochaetales, Charales), the conjugating greens are by far the most
diverse taxonomically (6 families, ~55 genera, 3-4,000 species) and
very widespread in distribution (including common species such as
Spirogyra, Zygnema, Cosmarium, and Staurastrum). Thallus shape ranges
from simple, oblong unicells (saccoderm desmids, or Mesotaeniaceae),
to unbranched filaments of cells with smooth walls (Zygnemataceae), to
elaborately ornate and deeply incised unicells or filaments made of
such cells (placoderm desmids, in four families). Prior analyses of
sequences of the chloroplast-encoded gene rbcL (1,354 bp, or 95% of
the gene) sampled from all 6 families showed that two of the
traditional families (saccoderms and filamentous forms) are
paraphyletic, i.e., they do not form two independent clades, although
the component genera may together constitute a monophyletic group. In
contrast, three families of placoderm desmids formed three
monophyletic groups within a larger clade. New analyses using partial
sequences from the mitochondrial gene coxIII (608 bp, or 76% of the
gene) for 21 species of conjugating green algae from all 6 families
supported the conclusions of the rbcL analysis, although bootstrap
support for each gene along was weak for many branches. CoxIII
sequences are more variable than rbcL and have proportionately more
parsimony-informative sites (43% vs. 34% respectively). Combining the
two genes improved the resolution of the resulting tree: 1) bootstrap
support increased for all branches, particularly in regards
relationships of placoderm desmids, and 2) monophyly of a clade
containing filamentous and saccoderm desmids was strongly supported.
We conclude that phylogeny of conjugating green algae will be much
improved by sampling from additional genes.
Key words: Charophyceae, chloroplast and mitochondrial genes, conjugating green algae, evolution, phylogeny