Heterocaryosis in Neurospora Crassa.

T IS possible by means of X-ray or ultraviolet treatment to produce mutant I strains of Neurospora crassa that are deficient in their abilities to synthesize specific vitamins, amino acids, or other essential substances (BEADLE and TATUM 1941; TATUM and BEADLE 1g4zb). It occurred to us that it might be possible to determine whether the mutant genes concerned in such haploid strains are recessive or dominant to their normal alleles by producing heterocaryons, the cells of which would contain haploid nuclei from two mutant strains, one differing from wild type by one gene and the other differing by a second gene. In our first attempt to produce a heterocaryon the mutant strains used were aminobenzoicless (unable to synthesize the vitamin para-aminobenzoic acid) and nicotinicless (unable to synthesize nicotinic acid). Growth rates were measured in special growth tubes (RYAN, BEADLE, and TATUM 1943) containing “minimal medium” (salts, sucrose, biotin, and agar, and containing neither p-aminobenzoic acid nor nicotinic acid). The two mutant strains were grown separately as controls. One experimental tube was inoculated with conidia of the two mutant strains of opposite mating types (sexes)-that is, the conidia were mixed a t one end of the tube. It was supposed that the conidia would germinate and hyphae of opposite sex fuse. If both mutant genes were recessive, the two types of nuclei present in the fusion hyphae would compIement each other, each type carrying the normal allele of the mutant gene in the other, and growth would occur. To determine the rate of any growth that might occur as a result of intercellular diffusion of complementary growth factors synthesized and excreted into the medium by separate hyphae of the two types, nicotinic acid by aminobenzoicless hyphae and p-aminobenzoic acid by nicotinicless hyphae, a tube was set up which was inoculated with conidia of the two mutants of the same mating type. Here no fusion was expected. However, the observed results immediately indicated that we were wrong in this expectation. The tubes in which like mating types were introduced gave growth rates similar to a wild type control, while growth was much slower in those in which different mating types were mixed. The control tubes in which single mutants were put showed little or no growth. Although we did not properly appreciate the fact a t the time these experiments were carried