The differential maturation of asci and its relevance to recombination studies ofNeurospora, Sordariaand similar Ascomycetes

Abstract

Four ascospore-pigmentation mutants were crossed with their wild-types by methods giving a considerable degree of synchrony in perithecial development; segregation patterns were scored from perithecia at different stages of maturity. The observed second-division segregation frequencies forascoinN. crassawere at a maximum when little or no ascal dehiscence had occurred but decreased markedly as dehiscence proceeded. No significant change in this frequency with maturity occurred fortan sporeinNeurosporabut inS. fimicolathe observed second-division segregation frequencies forgrayandhyalinerose with increasing perithecial maturity. As similar changes in this frequency were observed when asci were mounted in water instead of the 2msucrose solution normally used, it was concluded that changes in this frequency with maturity generally resulted from a differential maturation and bursting of asci with different spore arrangements, rather than from changes in crossover frequency in successive meioses. Some evidence of the latter phenomenon was found in two of theSordariacrosses.In a cross ofasco×rib-1 inNeurospora, the observed frequency of recombination between the two loci was significantly higher in dispersed spores collected from young perithecia than in those from more mature perithecia: this effect was probably an artifact resulting from the differential maturation and bursting of asci with different divisions of segregation forasco. In a cross ofcrisp× + inNeurospora, an excess ofcrispprogeny was obtained from dispersed spores and an excess of +-progeny from spores from harvested perithecia. These deviations from the expected 1:1 ratio forcr: + were thought to result from a differential maturation of asci with different segregation patterns forcrispand the failure of many dehiscing asci to expel all eight spores.Hypotheses to explain the various phenomena observed were discussed. Suggestions were made concerning the avoidance of bias from the differential maturation and bursting of asci in experimental procedures used in the study of recombination and gene conversion. This bias may be responsible for some phenomena previously attributed to events at meiosis.