A mutation designated modD, which was selected on the basis of the suppression of selflysis, produced a decrease in the density of protoperithecia and a deficiency in ascospore outgrowth at 26 "C. Strains carrying the mutation were wild-type at 32 "C. Incubation of mutant ascospores revealed two abnormalities. First, the induction of ascospore outgrowth occurred only under a restricted range of conditions. Outgrowth of wild-type spores was normal down to 8 "C in a standard germination medium and could also be initiated in a non-germinating medium by the addition of actinomycin D or 5-fluorouracil. Mutant ascospores did not exhibit outgrowth in the presence of these two drugs or in a standard germination medium below 20 "C. The second defect was the death of SO./, of mutant ascospores when they were incubated in germination medium at 26 "C. This killing was suppressed by /?-phenylpyruvic acid, a protease inhibitor, and by a mutation (rnodC) which suppresses the lytic proteases associated with protoplasmic incompatibility and self-lysis. The killing of mutant spores was also suppressed by cycloheximide but was unaffected by actinomycin D and 5-fluorouracil.
I N T R O D U C T I O NMutations affecting the female sexual cycle in fungi have been described previously, especially in the ascomycete Neurospora crassa (Dodge, 1935 ;Horowitz et al., 1960;Johnson, 1978;Mylik & Threlkeld, 1974;Srb et al., 1973;Tan & Ho, 1970). Mutations inhibiting protoperithecium formation have also been reported in Podospora anserina (Esser, 1968 ;Belcour, 1971).We have investigated mutations in P. anserina which were selected on the basis of the suppression of self-lysis and which also restrict female organ formation (Boucherie & Bernet, 1974). These mutations inhibit the formation of the specific proteases associated with cell destruction (Boucherie et al., 1976a; Labarkre & Bernet, 1977, 1978. In the present paper, we describe a thermosensitive mutation, selected on the same physiological basis, which affects both protoperithecium formation and ascospore germination.
METHODSOrganism. Podospora anserina is a heterothallic ascomycete whose life cycle closely resembles that of Neurospora crassa. Asci contain four binucleate spores that include the two products of a half tetrad. As a result of post-reduction these spores provide the heterokaryotic strains used for dominance and complementation tests. In about 5 % of asci, one binucleate spore is replaced by two uninucleate spores. These give rise to the homokaryotic self-sterile (+ cr -) strains used to perform appropriate crosses and for F1 genetic analysis. For details, see Esser (1974).Origin of the modD mutation. Mod mutations are selected by growth restoration of 'self-lysing' strains. In self-lysing strains, growth stops early in development and cells lyse as a result of either non-allelic incompatibility genes (for instance C and E or Vand R) or a recessive mutation in the genes ZysA or ZysB (Bernet, 1965; 0022-1287/79/oooO-8221 $02.00 @ 1979 SGM