The bread mold Neurospora crassa is a simple fungal eukaryote which has been used extensively as a model organism to elucidate many of the principles of genetics of higher organisms. It is relatively easy to cultivate in the laboratory. Neurospora are eukaryotic organisms; that is, they organize their genes onto chromosomes . They may exist as either diploid cells (two copies of gene and chromosome) or haploid (one copy of each gene and chromosome). Neurospora has both a sexual and an asexual reproductive cycle which allows exploration of genetic processes more complex than those found in bacteria .
The asexual cycle consists of a filamentous growth of haploid mycelia. This stage is the vegetative stage. While the nuclei in this stage are indeed haploid, the tubular filaments contain multiple nuclei often without the distinction of individual cells. Under conditions of sparse food resources, the filaments (called hyphae ) become segmented producing bright orange colored macroconidia, asexual spores that can become detached and are more readily dispersed throughout the environment. Asexual spores can develop again into multicellular hyphae, completing the cycle. Asexual spores can also function as male gametes in the sexual reproductive cycle.
The sexual part of the life cycle begins with the mature fruiting body called the perithecium. These are sacs of sexual spores (ascospores) resulting from meiotic division. The sexual spores are discharged from the perithecium and can germinate into haploid cultures or fuse with conidia of complementary mating types. There are two genetically distinct mating types A and a. Neurospora cannot self fertilize, rather haploid sexual spores of opposite mating types must be joined at fertilization. Nuclear fusion of the male and female gametes occurs setting the stage for meiotic division to form ascospores. The diploid stage is brief as nuclear fusion quickly gives way to two meiotic divisions that produce eight ascospores. Ascospores are normally black and shaped like a football. The physical position of the ascospores is linear and corresponds to the physical position of the individual chromosomes during meiosis. In the absence of crossing over, the four a-mating type ascospores are next to each other followed by the four A-mating type ascospores.
The existence of a large collection of distinct mutant strains of Neurospora and the linear array of the products of meiosis makes Neurospora an ideal organism for studying mutation, chromosomal rearrangements, and recombination . As a relatively simple eukaryote, Neurospora has permitted study of the interactions of nuclear genes with mitochondrial genes. Neurospora also exhibits a normal circadian rhythm in response to light in the environment, and much of the fundamental genetics and biology of circadian clock cycles (chronobiology) have been elucidated through the careful study of mutant cells which exhibit altered circadian cycles.
See also Microbial genetics