Evidence for metabolic and functional discrimination of sterols by Phytophthora cactorum

When fed 10 ppm of one of the following sterols: cholesterol (cholest-5-en-3beta-ol), wingsterol (21-isopentylcholesterol), desmosterol [cholesta-5,24(25)-dien-3beta-ol], 24-methylenecholesterol [ergosta-5,24(28)-dien-3beta-ol], or fucosterol [stigmasta-5,24(28)-dien-3beta-ol], the pathogenic fungus Phytophthora cactorum, which is naturally unable to epoxidize squalene, accumulated each of the test compounds to similar levels. Fucosterol, the only sterol metabolized, was reduced to yield 24-ethylcholesterol. All the sterols tested induced the formation of sex structures. Fertilization and subsequent maturation of oospores capable of germination occurred only with the naturally occurring sterols. Wingsterol treatments resulted in aborted oospores. None of the sterols tested was inhibitory to growth, measured as changes in the 21-day mycelial dry weight. The results are consistent with the view that the accumulated sterol functions to regulate the life cycle of P. cactorum. However, the metabolism and kinds of recognition of the sterol molecule, in terms of uptake and effects on growth and induction of the various sexual events, contrast sharply with what is known for other oomycetous fungi such as Achlya and Saprolegnia. This implies that the evolutionary histories of the Oomycetes may be different.