Genetic alterations of ascus development in Neurospora tetrasperma.

Abstract

ascus of Neurosopra has proved to be a favorable system for developTE:ntal genetic studies. One aspect of its experimental utility is that the genotype of the zygote contributes to ascus shape in a straightforward fashion such that standard relationships of dominance and recessiveness apply. In N. crassa, MURRAY and SRB (1959, 1962) reported a mutant gene, peak-2 (pk-2), which when homoallelic in a cross produces abnormal asci. Asci initiated by -I-/pk-2 zygotes are phenotypically normal. At present, abnormal ascus mutants with similar effects have been found to map to at least seven different sites in the genome of N. crassa. The mutants fall into complementation groups, and dominants as well as recessives have been found (SRB and BASL 1969). Comparative cytological studies of the mutant and of normal asci have enlightened the basis of their differences and revealed some of the potential for variability in the sexual reproductive apparatus of the species (PINCHEIRA and SRB 1969). Additional insight into the genetic control of the development of the ascus could be expected to emerge from studies of mutants affecting a system in which the sexual reproductive apparatus is homologous to that of N. crassa but differs from it. Such a system is available in the pseudohomothallic species N. tetrasperma. The typical ascus of this species contains four bisexual spores, in contrast to the eight unisexual spores normally found in the ascus of N. crassa. The functional differences in the two kinds of ascospores and the difference in spore numbers are demonstrably related to the orientation of meiotic and mitotic spindles in the developing ascus (DODGE 1927). Thus the two species have evolved a marked difference in their sexual reproductive apparatus. PINCHEIRA and SRB (1969) have studied the effects of mutant pk-2 in N. tetrasperma after hybridizing and backcrossing were used to transfer the gene from N. crasssa. The effect of pk-2 on ascus development in N. tetrasperma is not simply recessive. Although the effects of homozygous pk-2 are similar in the two species, heterozygotes in N. tetrasperma do not show simple dominance of the wild-type allele and do follow a unique developmental pattern. The lack of predictability for the action of a mutation transferred from one species to another emphasizes the desirability of a study of ascus mutants obtained directly in N. tetrasperma. This report concerns such a study.