Genetically determined nonselective abortion of asci in Neurospora crassa

Abstract

In many crosses of laboratory strains of Neurospora crassa, a large proportion of complete asci abort shortly after ascospore delimitation. The aborted asci contain eight small, degenerate, vacuolated ascospores. We use the term "bubble asci" to signify this special form of ascus abortion. The frequency of bubble asci was scored in 8- to 10-day-old perithecia from crosses between closely related strains and between unrelated strains. When two unrelated or distantly related strains are mated, fewer than 10% of asci abort, whereas up to 70% or more of the asci abort when inbred strains are mated to each other. We conclude that the degree of genetic relatedness of the two haploid parents determines whether abortion occurs. Progeny tests indicate that several genes are involved; the simplest interpretation is that they are weak deleterious recessives whose effects must be combined to produce a high frequency of bubble asci. No ascospores survive in the aborted asci. Therefore, bubble asci cannot distort allele ratios of markers among surviving progeny. When a dominant Round-spore gene was present in one parent of a high bubble-ascus cross, none of the young bubble asci contained nonround spores. Therefore, bubble asci do not result from illegitimate fusion of two nuclei of the same mating type. Heterokaryon-incompatibility genes are not involved in producing bubble asci. No or little bubble-ascus abortion occurs in the homothallic or pseudohomothallic species of Neurospora.