Premeiotic and Meiotic Instability Generates Numerous b2 Mutation Derivatives in Ascobolus

We have studied the genetic characteristics of an unstable mutation located in the central region of the b2 gene of the fungus Ascobolus. In crosses to wild type, this spontaneous white ascospore mutation (G0) gives rise to a stable white spored derivative (G1) at a frequency of 5 × 10-3. G1 is a frameshift mutation and differs from G0 by its gene conversion pattern. In self crosses, G0 gives asci with colored spore derivatives at a frequency of 1 × 10-3. We isolated and analyzed genetically 97 independent colored derivatives ("G2" series). All but one are pseudorevertants. By the criteria of phenotype and gene conversion pattern with wild type and with G1, the pseudorevertants represent at least 13 distinct classes. Two of them are large silent deletion mutations. In crosses with wild type, some G2 derivatives, represented by G21, continue to exhibit instability, G21 yields white spored b2 mutant derivatives at a frequency of 2.6 × 10-3. In turn, some of these "G3" mutants are themselves unstable. All the derivatives lie at the same site within the b2 locus as the parental mutation G0 . Different mutations in the G series manifest their instability at different times in the Ascobolus life cycle. Derivatives of G0 arise premeiotically (leading to two derivative meiotic products among the four), while those of G21 arise during meiosis (leading to only one derivative out of four products). The characteristics of the G instability system are similar to those of unstable mutations in other eukaryotes which are due to insertion of mobile elements.