Addition of exogenous cholesterol to Pythium initially increases the growth rate, but the final dry weight yield is reduced. Cholesterol induces an overall increase in lipid synthesis after the initial period of rapid growth. The lipid content of cholesterol-grown mycelium becomes about double that of mycelium grown without cholesterol. The proportion of phosphatidyl serine relative to other phospholipids is reduced by half in mycelia grown with cholesterol. The major phospholipids are phosphatidyl ethanolamine, phosphatidyl serine, and phosphatidyl choline. Minor phospholipids identified are phosphatidyl inositol, lysophosphatidyl choline, lysophosphatidyl ethanolamine, phosphatidyl glycerol, and cardiolipin. No significant differences were noted in fatty acid composition.
A study of the vesicular and tubular bodies observed in cells of Poria monticola was done with particular regard to their form, origin, and membrane structure. The membranes of lomasomes and pinocytotic vesicles display the sometimes asymmetric, distinctly trilaminar structure characteristic of the plasma membrane. Lomasomes are abundant and vary in appearance depending upon the method of fixation used. Nonlamellar associations of vesicles and short tubules probably represent the true structure of lomasomes in this organism. These lomasomes originate from the plasma membrane and do not appear to be associated exclusively with wall formation.
The young oogonium and young antheridium in Pythium acanthicum Drechsler are densely and randomly packed with numerous mitochondria, dictyosomes, nuclei, interlocking vacuoles of several types, some of which contain a dense storage body, a variety of vesicles, endoplasmic reticulum, and cytoplasmic ribosomes. Wall vesicles, evenly distributed next to the plasma-lemma in rapidly growing oogonia, become localized in groups at points where they appear to initiate the hyphal tip-like development of the oogonial spines. They are also found on both sides of the antheridium−oogonium contact zone. Spine development starts shortly after antheridium−oogonial contact is made and ceases with entry of antheridial material into the oogonium. Excess nuclei, mitochondria, and various organelles are abandoned in the periplasmic space, where they normally quickly disintegrate when the oospore is formed. The periplasmic space is invaded frequently by vegetative hyphae originating outside of the oogonium.
Nuclear division in P. monticola is in general similar to mitosis in higher organisms. Synchronous division of the nuclei in the dikaryon progresses with clamp development. Mitosis begins with the movement of the centriolar plaques into and under the forming clamp. The pull of the centriolar plaque on the attached nucleolus forms a long strand of nucleolar material. Chromosomes now appear as dense granules at the end of the nucleus proximal to the clamp. At this time the nucleolus moves adjacent to the centriolar plaque and contracted chromosomes. The nuclear membrane at least partially disintegrates, and the nucleolus is released into the cytoplasm where it may persist through telophase. A faintly staining spindle is often observed, and it produces a “double bridge” effect in separating chromatin. Somatic chromosomes are attached together forming strings that appear double and at least partially separated before metaphase.
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