from Shin Hatakeyama writing in Neurospora: Genomics and Molecular Biology:
George Wells Beadle and Edward Lawrie Tatum chose Neurospora crassa as a model organism to uncover the role of the gene in biochemical synthesis pathways. Their studies led them to propose the famous, one gene-one enzyme hypothesis, by generating Neurospora mutants by the treatment with the mutagen (X-ray irradiation) to asexual spores (Beadle and Tatum, 1941; Beadle and Tatum, 1945). This artificial mutagenesis was originally reported by Hermann Joseph Muller, who discovered the generation of Drosophila mutants by irradiation with X-rays. Artificial mutagenesis has since become a great tool to facilitate progress in the study of gene function.
Further reading: Neurospora: Genomics and Molecular Biology