THE HETEROKONTAE
| SYSTEMATIC BIOLOGY |
THE HETEROKONTAE |
||||
| HOME | SYLLABUS | WEEKLY ASSIGNMENTS | J. SYSTEMATIC BIOLOGY | TAXA OF LIFE | |
| PHYLUM OOMYCOTA | |||||
Oomycota (o-o-mi-KO-ta) is derived from two Greek roots that mean egg (oario -ωάριο) and fungus (mykes -μύκης). The reference is to the fungus-like habit of the organism and its typical oogamous sexual reproduction.
The water molds are among the most economically important organisms in the heterokonts. Although some like Saprolegnia (Figure A) are used for the commercial production of acetone and other small organics, most are important because of the damage that they do to agriculture. The most notable outcome of economic importance was the great potato famine of Ireland (1845-1850) caused by Phytophthora (Figure B). The famine began in September of 1845 with the leaves of potato plants appearing blackened. Soon, however, the feeding filaments (hyphae) had coursed through the plants, which turned to stinking, nauseous masses of slime. The spores released by the plants infected in the initial assault spread over the island of Ireland, and the crop of potatoes, the principle food crop, failed. The potato blight persisted through the next four years, a time called The great Hunger during which as many as 5 million may have died from starvation. Other members of the oomycotes cause the familiar downy mildew of Lilac.
Hyphochytridium (Figure B) is in a group that is reduced in form and resembles a chytrid, but only superficially so. Unlike other microbial eukaryotic filamentous organisms, these tend to be diploid with a haploid phase that is reduced or restricted to the gametes.
|
A. Oogonia of Saprolegnia. |
B. Labeled photomicrograph of Phytophthora. |
C. Hyphochytridium grown from a spore in 24 hrs from an infected pollen grain. |
| Images taken from: A: http://www.botany.utoronto.ca/ResearchLabs/MallochLab/Malloch/Moulds/Classification.html B: http://www.inspection.gc.ca/english/plaveg/pestrava/phyram/images/fig17.jpg C: http://www.bsu.edu/classes/ruch/msa/barr.html |
||
SYNOPTIC DESCRIPTION OF THE OOMYCOTA
| The following description comes from Margulis and Schwartz (1988 and 1998), Dick (1990), Fuller (1990), Barr and Desaulniers (1989), and Beakes (1989). |
|
I. SYNONYMS: Oomycetes, water molds, phycomycota. II. NUMBER: >800 species. III. PHYLUM CHARACTERISTICS:
|
Note that the Hyphochytrids (Pr-25 and Pr-21) and Oomycetes (Pr-27 and Pr-20) are treated as separate phyla by Margulis and Schwartz (1988 and 1998, respectively), Dick (1990) and Fuller (1990). Indeed, Fuller (1990) explores the reasons for lumping or separating them and concludes that the similarities in lysine synthesis and mitochondria cannot counterbalance differences in cell wall chemistry, mitosis, centrioles and rRNA. Perhaps, such differences do not really occur. Patterson (1999) places them together in the Stramenopiles (Heterokonts). Indeed, the tree of Sogin and Patterson (Tree of Life Project) has the two groups closely linked on the same clade within the heterokonts. Barr and Desaulniers (1989) further suggest that the similarities in their flagellar structures warrant their association. I have followed their lead and lumped both groups into a single phylum, Oomycota.
Beakes (1989) reports that the oomycetes have ultrastructural and biochemical affinities with the brown algae and eustigs as well as obvious (but perhaps misleading) morphological similarities with the xanthophyte, Vaucheria. Dick (1990) agrees that the siphonaceous form in the Oomycota is not only convergent with that of the Xanthophyta but highly advanced within the Oomycota. Still, they have obvious ties to other members of the heterokonts.
HIERARCHICAL CLASSIFICATION OF THE OOMYCOTA
| TAXONOMY OF THE PHYLUM OOMYCOTA. This system is a modification of Dick (1990), Barr and Desaulniers (1989), and Beakes (1989). |
|
CLASS OOMYCOTEA
CLASS HYPHOCHYTRIDEA
|
This page is maintained by Jack R. Holt. Last revised: 02/20/2008.
