Sequence Analysis of the Small Subunit Ribosomal Rnas of Three Zoosporic Fungi and Implications for Fungal Evolution

ABSTRACT

The small-subunit ribosomal RNA gene sequences of the chytridiomycete Blastocladiella emersomi and the oomycetes Lagenidium giganteum and Phytophthora megasperma f. sp. glycinea were determined and compared to published fungal sequences of Achlya bisexualis, Saccharomyces cerevisiae, and Neurospora crassa and those of other eukaryotic organisms. The gene phylogeny that was constructed showed two distinct fungal evolutionary lineages. Oomycetes together with chrysophytes and diatoms formed one lineage. Oomycetes appeared to be monophyletic and derived from heterokont photosynthetic algae. On a different phylogenetic branch, chytridiomycetes and ascomycetes were found. “Higher” fungi and chytridiomycetes appeared to share a relatively recent common ancestor. These two fungal evolutionary lines were unrelated to the higher plant lineage. It is evident that the fungi do not represent a natural taxonomic group of eukaryotic organisms.

Key Words:

• Small-subunit ribosomal RNA sequences
• Blastocladiella emersomi
• Lagenidium giganteum
• Phytophthora megasperma f. sp
• glycinea
• fungi
• evolution