Summary: Production of oospores by in media supplemented with ten Δ-sterols, differing in the structure of their side chains, has been measured. Spore production increased with increasing size of the substituent at C-24: H < CH< C H. A 24α-methyl group conferred greater activity than a 24β-methyl, but there was less difference between Δ-sterols with 24α- and 24β-ethyl groups. There was a greater difference between Δ-sterols with 24α- and 24β-ethyl substituents. A double bond at C-22 had no effect on spore production when but reduced activity when

Four Δ-sterols have been compared with their corresponding Δ-sterols. Fewer oospores were produced at low concentrations (0.2 to 1 mg 1) of each Δ-sterol than with the Δ-sterol, but at higher concentrations (10 to 30 mg 1), the numbers with the Δ-sterol bore no consistent relation to the numbers with the Δ-sterol.

The difference between two sterols in promoting sexual reproduction in is not apparently related to differences in uptake into the mycelium, or to known effects on properties of membranes, but is believed to be due to differences in metabolism of the sterols, and to differences in the hormone activity of compounds derived from them.


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