Neurospora as an object for cytogenetic research : chromosome rearrangements, crossing over, duplications, meiosis, meiotic drive, recombination

Abstract

Several lines of research have been stimulated because the direct results of Mendelian segregation are apparent in Neurospora asci. Spore killer genes provide an example. When across is heterozygous for Spore killer, four ascospores in each ascus are white and inviable. Surviving ascospores contain the killer allele, which thus shows meiotic drive. Meiotic and postmeiotic divisions are completed before killing occurs.---Chromosome rearrangements are frequent; many have been characterized. Rearrangements are detected initially because they produce white deficiency ascospores. Provisional diagnosis is by visual inspection of asci: the frequencies and patterns of white ascospores are characteristically different for different rearrangement types. When insertional or terminal translocations are crossed by normal sequence, meiotic recombination results in progeny that contain a nontandem duplication. The duplications can provide information on map sequence, dominance, nuclear autonomy, vegetative incompatibility, nucleolus-organizer behavior, mitotic recombination, chromosome stability, and chromosome organization.---Some mutants that impair meiosis produce no ascospores; others produce hypoploid ascospores that are inviable.---Light microscopy has been effective in detailing pachytene chromosome morphology, identifying rearranged chromosomes, and describing the behavior of chromosomes and organelles during meiosis and ascus development in normal and mutant genotypes. Studies of the synaptonemal complex are favored by the small genome size of Neurospora.---Crossing over in Neurospora resembles that in higher organisms, with positive chiasma interference. When gene conversion occurs, flanking markers are recombined with a probability less than 50 percent, suggesting a constraint on random isomerization in molecular recombination models. Recombination frequencies in specific local regions are precisely controlled by a system of regulatory genes.---Natural populations of Neurospora are readily sampled. Chromosomal polymorphisms have not been detected, although genic polymorphisms are abundant. Karyotypes are similar in all known species.---Beginnings have been made using Neurospora in molecular cytogenetics and for research with recombinant DNA.