The gene abnormal oocyte (abo) is one of a cluster of maternal effect genes. Its unusual phenotypes are moderated by specific regions in heterochromatin. Analysis of the abo mutation has led to the suggestion that the gene may regulate aspects of heterochromatic function, heterochromatic structure, and the replication of heterochromatic sequences. Copies of the gene may also reside in the heterochromatic DNAs and be functional in contexts when other sequences are not. Such times may include gamete and early embryo formation. Analysis of the gene has rested on a single mutant allele. Collection of new mutations has been difficult because the gene lies in a poorly saturated and studied region of the second chromosome, and because the existing hypomorph produces no visable phenotype when homozygous. The proposal describes a new method of screening for mutations at the abo function, and the several approaches that will be taken to characterize new alleles. The identification of new alleles will allow us to determine if the effects of abo are a general property of the gene's normal function, and to distinguish between several aspects of models which describe the process producing the known abo phenotypes. There are many genetic studies on the function and structure of euchromatic chromatin. There are few studies or even possible approaches on the structure and function of heterochromatin, or that part of genomes which has been found to contain very few functional genes. The abo mutation promises to allow approaches to be devised which could change that situation. However, more alleles must first be found for the system to be optimal.
