A DOMINANT ROUND SPORE MUTANT IN NEUROSPORA TETRASPERMA AND ITS IMPLICATIONS FOR ASCOSPORE DEVELOPMENT

A new mutation (Rsp-I) in N. tetrasperma produces nearly all round spores when heterozygous in a cross. The gene maps 12 units from mating type in linkage group I and mutant cultures are female-sterile. Nongenetic reversal of dominance sometimes occurs, in that crosses heterozygous for round spores yield a few asci containing four spores of normal shapes. In the absence of crossovers between Rsp-1 and the centromere of its chromosome, these heterokaryotic spores give rise to self-fertile mycelia which produce perithecia that contain round-spored asci. One never observes asci that contain both round and normal spores. Therefore, the nuclei included within ascospores do not determine the shape of the spores. Rather, dominance effects are exerted on the ascus as a whole, as if its development were controlled by the zygote. The round spores of N. tetraspertna contain two germ pores, and these are unsystematically situated. Evidence is presented to show that the position of pores is directly related to the development of the striation pattern.