Studies on the Mutation Abnormal Oocyte and Its Interaction with the Ribosomal Dna of Drosophila Melanogasteri

SANDLER (1970) suggested that mutation, abnormal oocyte (abo:2-38), may influence the function of the ribosomal RNA cistrons. We have examined the abo mutation and its interaction with the ribosomal D N A of Drosophila melanogaster. We observed that the expression of the abo phenotype is unstable under the appropriate conditions, a behavior which paralleled changes in the phenotypic expression of bobbed mutations during the magnification of the ribosomal DNA. The change in the expression of the abo phenotype is correlated with an increase in the redundancy of the ribosomal cistrons. further suggesting a functional interaction of the abo and bobbed regions. HE DNA from which ribosomal RNA is transcribed has been shown to be Tredundant and clustered in the nucleolus organizer regions of the Drosophita melanogaster X and Y chromosomes (RITOSSA and SPIEGELMAN 1965). Studies of the stability of the ribosomal RNA cistrons have led to the conclusion that mechanisms are present which function to maintain the redundancy of the ribosomal DNA’s. For instance, compensation for the absence of a nucleolus organizer occurs such that flies bearing a single organizer region contain as much ribosomal DNA as flies having two such regions (TARTOF 1971). In larval tissue the compensation events are limited to polytene cells (SPEAR and GALL 1973) and probably reflect regulatory mechanisms influencing the replication of the ribosomal DNA during the growth of the polytene chromosome. Transcriptional controls have also been indicated from biochemical, autoradiographic, and cytological observations in D. melanogaster ( MOHAN and RITOSSA 1970; KRIDER and PLAUT 1972a,b). SANDLER (1970) has suggested that the chromosomal region characterized by the mutation, abnormal oocyte, may influence the expression of the ribosomal RNA cistrons. We have initiated a study of the interaction of the mutation and the nucleolus organizer region as a means to identify elements in the regulation of transcription and replication of the ribosomal DNA. The recessive mutation, abnormal oocyte (abo), is located on the left arm of the second chromosome of Drosophila melanogaster. The phenotype of abo is a pronounced excess 09 female progeny when abo homozygous females are mated This work has been supported by grants from the University of Cormecticut Research Foundation, the National Science Foundatlon (GB-43817) and the National Cancer Institute (CA-14733). Genetics 81: 501-513 November, 1975 502 H. M. KRIDER A N D B. I. L E V I N E to attached-XY males. SANDLER (1970) observed that duplication of the proximal part of the X chromosome in either the maternal parent or the male progeny decreased the seventy of the sex ratio phenotype of abo. Deletions had the opposite effect. The only DNA’s of known function in the proximal region of the X chromosome are the ribosomal RNA cistrons. suggesting the interaction of the nucleolus organizer region and the mutant abo allele. While alternative interpretations were not excluded by the observations, we felt the interaction of the abo region with the proximal part of X warranted a closer examination. Our observations show that the expression of the abo phenotype is unstable (under the appropriate conditions). We have observed both the loss of the capacity to express the abnormal sex ratio effect in abo homozygotes, and the progressive return of the phenotype in populations maintained in the heterozygous condition. These observations had a number of formal similarities to the change in expression associated with deletions of the ribosomal DNA (RITOSSA 1968). Therefore, we examined the influence of abo on the redundancy of the ribosomal DNA of adults. The loss of the capacity to generate an abnormal sex ratio in abo homozygotes is correlated with a significant increase in the redundancy of the ribsomal RNA cistrons, further suggesting a functional interaction of the abo region and the ribosomal DNA’s. MATERIALS A N D METHODS