Inhibition Effects in Back-Mutation Tests with Mutants of Neurospora

Abstract

THE observation of Grigg¹ that large numbers of conidia of biochemically deficient mutant strains of Neurospora crassa strongly inhibit the growth of ‘wild-type’ conidia when plated on minimal medium containing 0.2 per cent sucrose and 0.4 per cent sorbose led him to question the validity of back-mutation tests. On the other hand, Kølmark and Westergaard² have reported that in comparable experiments (using an adenine mutant W 40) the substitution of a minimal medium containing 2 per cent glucose completely abolishes the growth inhibition. They suggest that the inhibition of ‘wild-type’ conidia on sorbose medium results from a removal by the large population of germinating mutant spores of the limited amount of sucrose available; they conclude, therefore, that the growth inhibition has no relation to back-mutation tests as usually conducted.