INTRAGENIC SUPPRESSION AT THE b 2 LOCUS IN

The present report is a study of GO intragenic suppressors located in the A or E group of the b2 spore-color locus of Ascobolus immersus. The frameshift nature of the suppressors was shown by 19 combinations of plus and minus suppressor mutations. The location of the mutation sites on the genetic map of group A was defined. The conversion pattern ofi the frameshift suppressors is discussed according to their plus or minus phase shift and their mutagenic origin. intragenic suppression groups ( A , E and P ) have been isolated in the TEEcospore-color locus of Ascobolus immersus, based on the analysis of reversion experiments (LEBLON and PAQUETTE 1978). Several facts indicate that the mutations in these groups are of the frameshift type. Critical support of this hypothesis would require the demonstration that the mutations within a group fall into two distinct classes (plus and minus). Any double mutant combining a plm with a minus mutation or any triple mutant combining three m i n u s or three plus mutations would show a wild-type or pseudowild-type phenotype, provided the sites are close enough and that no barrier lies between the sites. On the contrary, any combination of two plus or two minus mutations would fail to restore wild-type or pseudowild-type phenotype (CRICK& al. 1961). The present report contains available data concerning the phenotypes of all combinations of double and triple mutations we tested in group A in order to define the genetic map of group A, as well as a discussion of the conversion pattern of the mutations in groups A and E. The results concerning group F will be presented in the subsequent publication of this series (PAQUETTE, in preparation). MATERIALS A N D METHODS Culture media and crossing techniques: These have been previously described (RIZET et al. Mutations used: The primary mutations A 0 and EO, the secondary mutations (isolated as 1960; LISSOUBA et al. 1962; Yu-SUN 1964). Supported in part by a grant from the Laboratoire assxi6 11'86, Centre National de la Recherche Scientifique. Genetics 92: 1093-1106 August, 1979.