Phylogenetic relationships between chlorophytes, chrysophytes, and oomycetes.

Abstract

The phylogenetic relationships among the chlorophyte Chlamydomnonas reinhardtii, the chrysophyte Ochromonas danica, and the oomycete Achlya bisexualis were explored by comparing the sequences of their small-subunit ribosomal RNA coding regions. Comparisons of similarity values or inspection of phylogenetic trees constructed by distance matrix methods reveal a very close relationship between oomycetes and chrysophytes. The separation of chrysophytes from chlorophytes is comparable to that of plants from animals, and both separations are far antedated by the divergence of a number of other protist groups.Traditional analyses based on phenotypic criteria frequently depict chlorophytes (green algae) and chrysophytes (goldenbrown algae) as representatives of lineages that diverged soon after the appearance of the earliest protists (1, 2). Comparisons of numerous characteristics such as features of nuclear division, chloroplast structure and pigment types, kinetid ultrastructure, the nature of the cell wall, and mitochondrial crista structure suggest a long and separate evolutionary history for these algal lineages. The presence of tubular mitochondrial cristae in chrysophytes and lamellar mitochondria cristae in chlorophytes (3) may be particularly significant. Since no recognizably monophyletic protistan groups are split with respect to crista type, and related groups seem to be connected by this characteristic, fundamental differences in mitochondrial structure are thought to reflect ancient divergences and to be of significance at a high taxonomic level.Certain colorless protistan lineages may be more closely related to the chrysophytes than to the chlorophytes. Among these groups are the "lower" fungi. These forms are widely regarded as a polyphyletic assemblage of uncertain relationships to "higher" fungi (4). For example, the oomycetes have traditionally been grouped with the "true" fungi but comparisons of several biochemical and morphological characteristics suggest a close relationship to xanthophytes and chrysophytes (4-7). Taxonomic placement of the oomycetes is still controversial; in some schemes they are grouped with the higher fungi (8), and in others they are grouped with the chrysophytes and their relatives (9, 10).Since there is little agreement about which phenotypic characteristics are most reliable for inferring evolutionary relationships, a consensus phylogeny for protists has never emerged. A phylogenetic tree, which reflects true genotypic similarity, can be inferred from comparisons of macromolecular sequences. Ribosomal RNAs have been used extensively for measuring evolutionary distances (11, 12) and these can be converted into phylogenetic trees by parsimony or distance matrix analyses. Because of their large size (relative to that of 5S and 5.8S rRNAs) and the existence of highly conserved and partially conserved sequence elements, the 16S-like or small-subunit rRNAs have been particularly useful for measuring both close and distant phylogenetic relationships (13,14). As part of a...

Phylogenetic analysis of five medically important Candida species as deduced on the basis of small ribosomal subunit RNA sequences

“…Analysis of srRNA sequences has already been successfully applied to studies on the evolution of bacteria (Woese, 1987) and protists (Gunderson et al, 1987;Sogin et al, 1989). The results presented in this work, though still limited, also show the potential of srRNA analysis for the deduction of evolutionary relationships among yeast species.…”

Phylogeny and Molecular Evolution of the Green Algae

“…Analyses of DNA sequence data took a start in the mid 1980's with the first phylogenies of green plants inferred from 5.8S nrDNA sequences (Hori et al, 1985;Hori & Osawa, 1987), soon followed by 18S and 28S nrDNA sequence analyses (Gunderson et al, 1987;Perasso et al, 1989;Buchheim et al, 1990;Zechman et al, 1990;Chapman et al, 1991;Mishler et al, 1992;Chapman et al, 1998). Ribosomal sequences were chosen at the time because enough RNA could be obtained for direct sequencing and (later) because regions of the gene were conserved enough to make universal primers.…”

Global phylogeography in the cosmopolitan speciesCladophora vagabunda(Chlorophyta) based on nuclear rDNA internal transcribed spacer sequences

“…Termini of the coding regions were determined by alignment with Chlamydomonas reinhardtii Dangeard coding regions (Gunderson et al, 1987) and Volvox carteri Stein (Rausch et al, 1989). Final alignment adjustments were done manually.…”