Sterol Metabolism as a Basis for a Mutualistic Symbiosis

Abstract

MUTUALISM makes it possible for certain organisms with unusual nutritional requirements to survive and reproduce¹. Species of this kind may be invaluable in studies of metabolism if the aposymbiotic (symbiote-free) organisms can ultimately be raised easily on defined substrates in sterile conditions². During efforts to develop such a laboratory culture of the beetle Xyleborus ferruguineus, fungus-free females feeding on media containing cholesterol as the sterol source produced second-generation larvae that did not pupate³. Here we report evidence that the mutualistic fungus Fusarium solani associated with the beetle produced ergosterol as its only sterol in pure culture on a chemically defined substrate in sterile conditions. Ergosterol proved to be adequate as the sole sterol source for continued growth, development and reproduction of the fungus-free beetle. Subsequent tests of other sterols showed that 7-dehydrocholesteroi was similarly adequate.