Genetic studies of ad-8 mutants in Neurospora crassa. II. Interallelic complementation at the ad-8 locus.

N earlier studies with Neurospora, it was believed that with biochemical I mutants the heterocaryon test was a satisfactory test for allelism, based on the expectation that only heterocaryons between nonallelic mutants should be capable of growth on nonsupplemented media (BEADLE and COONRADT 1944). However, experiments by MITCHELL and MITCHELL (1956) suggested that a positive heterocaryon test did not constitute acceptable evidence of nonallelism, since some combinations of their allelic mutants at the pyr-3 locus were capable of complementation. Recently it has become evident that interallelic complementation is a phenomenon of common occurrence, not only at various loci of Neurospora (GILES 1958; CATCHESIDE 1960) but also in other organisms such as Salmonella ( HARTMAN, HARTMAN and SERMAN 1960), yeast (MEGNET 1959), Aspergillus ( PONTECORVO 1958) and Drosophila ( CARLSON 1959; FAHMY and FAHMY 1959; BROSSEAU 1960). In the present study, evidence will be presented for the occurrence of complementation between adenine-requiring mutants at the ad. 8 locus in Neurospora. A comparison of the complementation map of the locus with the genetic map established in previous studies (ISHIKAWA 1962) provides the basis for a discussion of the structure and function of the a d 4 gene.