SYSTEMATIC BIOLOGY					THE CERCOZOAE
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PHYLUM PHYTOMYXA

INTRODUCTION TO THE PHYTOMYXA

Phytomyxa (fi-to-MIX-a) is derived from two Greek roots that mean plant (phyto -φυτό); and fungus (mykes -μύκης).  The name is derived from their being fungus-like internal parasites of plants.

Plasmodiophorids are important pathogens of plants and other eukaryotes.  The most well known member is Plasmodiophora brassicae, a species that causes club root in cabbages and their relatives.  See the life cycle of Plasmodiophora brassicae.  Resting spores in the soil can be infective for up to seven years.  If one were to germinate near the root of a cabbage (or a cabbage relative),  the biflagellate zoospore can invade a roothair cell and form a small plasmodium, which releases secondary zoospores into the soil.  There, compatible mating types pair and fuse back to back.  Note that the cells fuse, but the nuclei do not.  The resulting binucleate cell becomes amoeboid and invades the root epithelium again.  There, it works its way into the cortex of the root where it invades a cortical parenchyma cell.  Nuclear fusion (karyogamy) may occur at this point.  If so, the resulting diploid cell feeds on the parenchymal cell and becomes enlarged and multinucleate (secondary plasmodium; see Figure A).  The resulting tumorus growth of cells in the root cortex eventually prevent water and food from moving up and down the plant, which then dies.  At this point, the plasmodia in necrotic tissue likely undergo meiosis, cytokinesis, and spore formation.  

Other economically important members of this phylum include Spongospora subterranea, the cause of powdery scab in potato and crook root of watercress.  Polymyxa betae infects sugar beet and facilitates the entrance of a virus.  Together, they cause rhizomania of sugar beet.  Furthermore, Polymyxa betae seems to behave as a vector for viruses in potatoes, watercress, and some cereal crops.

SYNOPTIC DESCRIPTION OF THE PHYTOMYXA

THE SYSTEMATICS OF THE PHYTOMYXA

The systematics of this phylum has a varied  history.   Sleigh et al. (1984) placed the "endoparasitic slime molds" in the "sarcodina" as did Hunter (1985). They interpreted the plasmodial form as a structure indicated affinities with the Myxotista.  However, Dylewski (1990) believed the the ancestor was a coccidian (Apicomplexotista) because their zoospores had similar developmental stages. Patterson (1999) recognized the Plasmodiophorids as a natural taxon with no known sister groups.

For some time the affinities of the plasmodiophorids were a big question mark.  Archibald and Keeling (2004) supported their affiliation with other members of the Rhizaria.  In particular, their analysis suggested a sister taxon relationship with the foraminifera.

HIERARCHICAL CLASSIFICATION OF THE PHYTOMYXA

This page is maintained by Jack R. Holt.  Last revised: 02/25/2008.