GENETIC-CONTROL OF THE SEXUAL CYCLE IN NEUROSPORA

Over 200 mutants that affect sexual development have been isolated and analysed in Neurospora crassa. Mutational, recombinational, and molecular analyses on the mating type region show that the mating type genes A and a specify both the mating function and a vegetative (heterokaryon) incompatibility function. The two functions are inseparable by recombination but separable by a mutational event, as in a(m33). An unlinked tol mutation suppresses the vegetative incompatibility associated with mating type genes A and a without affecting their mating type function. Mating type mutants (except a(m33)), as well as male- and female-sterile mutants, are usually defective in haploid sexual functions or in early peritheical development. Many mutants that affect post-fertilization events produce normal-sized perithecia that are barren. Some mutants show distinct blocks during ascus development from karyogamy through ascospore formation, leading to complete or partial abortion of asci. Other mutants are variable in their expression, with development blocked at different stages from ascus to ascus, and a few asci capable of producing viable ascospores. Another category shows dramatic effects on ascus development without drastically reducing fertility. These mutations affect ascus or ascospore morphology, the size and number of ascospores, or ascospore pigmentation. Spore killer strains show meiotic drive, causing the death of ascospores not carrying the Spore killer allele. Although many applications have been found for specific mutants, as described in the text, their use for analysing the molecular mechanisms that underlie morphogenesis and development remain at best rudimentary. Judging from the recent history of developmental genetics in organisms such as Drosophila, the wide array of developmental mutations described for Neurospora in this review should be a valuable resource for future research into the control of sexual development.