Genetic studies of ad-8 mutants in Neurospora crassa. I. Genetic fine structure of the ad-8 locus.

recent years, evidence has accumulated which indicates that genetic re'2mbination occurs not only between different loci but also between alleles within one locus (PONTECORVO 1958). A precise genetic analysis of allelic relationships within one locus is certainly an essential approach to understanding genes, since studies of allelism can help to elucidate both the structure and function of a particular gene. The present study will describe the genetic fine structure of the adenine-8 (ad-8) locus in Neurospora crassa, a locus at which a large number of mutants of independent origin are available. Some of the mutants are of spontaneous origin and others have been induced by various physical and chemical mutagens. Biochemical studies indicate that the ad-8 locus controls one reaction in the biosynthetic pathway to adenine, i.e., the conversion of inosine monophosphate to adenosine monophosphate succinate and that all ad-8 mutants lack activity for a single enzyme, adenylosuccinate synthetase. A genetic map has been established for the ad-8 locus as the result of a series of over 1,000 interallelic crosses. This map indicates the existence of numerous distinct mutational sites within the ad-8 locus. The map has proved especially useful in studies of interallelic complementation at this locus ( ISHIKAWA 1960). In addition, evidence concerning genetic fine structure has also been helpful in elucidating the nature of forward and reverse mutational events at this locus, especially in relation to the types of mutagens used.