Neurospora from natural populations: Toward the population biology of a haploid eukaryote

Abstract Natural populations of the ascomycete Neurospora have been sampled systematically throughout much of the world, and the haploid strains from colonies in nature have been characterized genetically in the laboratory. Our findings are described in the context of a broader review of wild-collected strains, their uses, and their significance for population genetics. Visible Neurospora colonies found on recently burned vegetation are usually unique in genotype. More than three-fourths are pure strains originating from a single ascospore; the remainder can be purified. Thus, despite the potential for clonal propagation, these colonies provide effective population samples comparable to those collected for higher plants and animals. Over 3900 isolates from burned substrates have been analyzed from over 500 collection sites, mostly from tropical and subtropical regions. These strains have been assigned to five species—four heterothallic species with eight-spored asci and one pseudohomothallic species with four-spored asci. Each species has a unique pattern of distribution, but each overlaps with all the others in one or another part of its range. All of these species are similar in vegetative morphology, with orange or yellow-orange conidia. All have two homologous mating types, but the different species are reproductively isolated from one another. Fertility in crosses with reference strains has provided a reliable and convenient criterion for species classification of heterothallic strains. The species of a newly obtained haploid strain is determined by finding a tester strain with which it is fully fertile and produces predominantly viable ascospores. Viable ascospores are extremely rare for most interspecific combinations, but genes can nevertheless be transferred by matings among all but one of the nonhomothallic species. Abundant but mostly inviable ascopores are produced by some interspecific combinations. Karyotypes, karyogamy, and meiotic chromosome behavior are similar for all the known Neurospora species. There are seven chromosomes and a single terminal nucleolus organizer. This pattern also applies to the five eight-spored homothallic Neurospora lines that were designated by their discoverers as different species on the basis of ascospore morphology. These homothallic lines all lack orange pigment and are devoid of conidia. They were obtained by enrichment from soil samples and would not have been obtained by our collecting methods, which rely on visibility in the field. Examination of wild-collected strains of N. crassa and N. intermedia has revealed a wealth of intraspecific genetic variation. Genetic polymorphism of isozymes in local populations is comparable to that in outbreeding higher animals and plants. DNA restriction fragment length polymorphisms are also abundant, as are differences at vegetative (heterokaryon) incompatibility loci and recessive genes that adversely affect one or more stages of the sexual diplophase. Chromosomally located factors, called Spore killer, act in the sexual phase to produced meiotic drive. The four Spore-killer-sensitive ascospores in every ascus are killed in crosses of sensitive × killer, but all eight ascospores remain viable in crosses of killer × killer and sensitive × sensitive. Mitochondrial genomes of wild strains differ in both length mutations and nucleotide substitutions. Many isolates contain mitochondrial plasmids. A few strains have been found to undergo senescence following insertion of a foreign element into mitochondrial DNA.