Ascus development in two temperature-sensitive Four-spore mutants of Neurospora crassa

Abstract

Two nonallelic Four-spore mutants are known in which ascospore walls enclose the four immediate products of meiosis rather than the normal eight products of a postmeiotic mitosis. Expression depends on temperature. The Four-spore phenotype is expressed when the developing asci are subjected either to high temperatures (25–30 °C) for Fsp-1 or to low temperatures (15–20 °C) for Fsp-2. Heterozygous Fsp-1 × Fsp-1⁺ crosses make eight-spored asci at 15–20 °C but produce many four-spored asci at 25 °C and mostly four-spored asci at 30 °C. Homozygous Fsp-1 × Fsp-1 crosses respond similarly to increasing temperature but make 40–50% four-spored asci even at 20 °C. Heterozygous Fsp-2 × Fsp-2⁺ crosses produce almost exclusively four-spored asci at 15 °C but a mixture of four- and eight-spored asci at 20, 25, and 30 °C. Homozygous Fsp-2 × Fsp-2 crosses make all four-spored asci at 15 and 20 °C and a mixture of four- and eight-spored asci at 25 and 30 °C. When both Fsp-1 and Fsp-2 are present in a cross, either homozygous or heterozygous, no asci contain more than four ascospores at any temperature. Limited temperature-shift experiments with Fsp-1 and Fsp-2 show that the sensitive period for Four-spore expression is sometime after meiotic prophase, possibly at interphase II.Key words: Neurospora, temperature sensitive, Four-spore mutants, large ascospores.