Unstable ascospore color mutants of Ascobolus immersus

Summary New unstable mutants of Ascobolus immersus involving the color or size of ascospores were sought among spontaneous mutants. Among the 34 unstable mutants isolated, 31 had white spores, 2 had pink spores and 1 had a large sized spores. The unstable mutants involve 11 loci whose mutation leads to white spores and 2 loci whose mutations give pink spores, among the 19 loci known to be implicated in this character; 1 locus is defined by only one large spore mutant. All these genes are localized on at least 7 different chromosomes. Unstable mutants of the same locus may correspond to several different sites, but the number of these sites is very limited.The frequency of unstable mutations was estimated: among 36 mutants belonging to 8 different genes, 20 were stable and 16 were unstable. This high frequency of unstable mutants is undoubtedly underestimated. The moment of reversion of 23 of these new mutants was also sought: 15 of them revert as does mutant B, previously studied, in the very young mycelium, at high temperature and with a reversion frequency of 0.004 to 0.34, according to the mutant; 5 of them revert as mutant 301, also previously studied, during the development of the fruit-body and with a frequency of 0.009 to 0.035; two of these mutants revert very early in the ascospore as soon as the first mitoses or in the very young mycelium at 22° C, with a very high reversion frequency that may reach 1.0; finally, the last mutant studied reverts in the fruit-body with a frequency reaching 0.40, but with modalities different from mutant 301. The mutants of the same locus may revert with different modalities. The same modality may correspond to different sites of the same gene.In unstable double-mutant strains involving two different genes, the reversion of one is independent of the reversion of the other, whether or not the reversion modailities of each mutant are identical.Results indicate the existence of inducers common to several unstable mutants which present the same modalities of reversion.These data support the previously formulated hypothesis of transposable elements.