Neurospora crassa

SIGNIFICANCE: Neurospora crassa is a bread mold with a relatively small genome, allowing this organism to be studied by causing mutations in its genes and observing the effects of these mutations. Such studies are important to the understanding of genetics and genetically related disease, particularly because N. crassa is eukaryotic and more similar to human DNA than it is to bacteria and viruses.

The Beginning of Biochemical Genetics

Neurospora crassa was first used in genetic experiments by Carl Lindegren in the 1930s. He was able to isolate several morphological mutant strains and create the first “linkage maps” showing where genes are located on chromosomes. This research determined some of the basic principles of “crossing over” during meiosis. Crossing over is the exchange of genes between chromosome pairs by the breaking and reunion of the chromosome. Lindegren was able to show that occurs before the separation of the homologous pair, between the second and fourth chromatids. Neurospora crassa was used as a model organism in the investigation of crossing-over mechanisms because the four products of (later duplicated by to produce eight spores) are arranged in the organism’s saclike in a way that exactly reflects the orientation of the four chromatids of each at the plate in the first meiotic division. The products of meiosis line up in order and therefore are more easily studied in this organism.

One Gene, One Enzyme

In 1941, George Beadle and Edward Tatum published a paper establishing biochemical genetics as an experimental science. They introduced a procedure for isolating an important class of lethal mutations in an organism, namely, those for blocking the synthesis of essential biological substances. These were expressed in the organism as new nutritional requirements.

By supplying a variety of compounds in the nutrient medium and seeing which allowed various mutant strains to grow and which did not, Beadle and Tatum saw that they could deduce the sequence of biochemical reactions in cells that make necessary compounds, such as amino acids. They concluded that the function of a gene is to direct the formation of a particular enzyme that regulates a chemical event. A mutation can alter a gene so that it no longer produces the normal enzyme, resulting in a physical symptom, such as the need for nutritional supplements. Beadle and Tatum proposed that, in general, each gene directs the formation of one enzyme.

These mutation studies promoted understanding of the biochemistry of and promoted the use of fungi in genetic experiments. In 1958, Beadle and Tatum were awarded the Nobel Prize in Physiology or Medicine for their discovery that the characteristic function of the gene was to control the synthesis of a particular enzyme.

The Organism

The orange bread mold Neurospora crassa, a multicellular lower eukaryote, is the best characterized of the filamentous fungi. Filamentous fungi are a group of fungi with a microscopic, stalklike structure called the mycelium. They grow on substances of plant or animal origins and reproduce via spores. This group of organisms has importance in agriculture, medicine, and the environment because they are so abundant and are able to proliferate very quickly. It is therefore easy and cheap to reproduce them rapidly. Moreover, the widespread availability of Neurospora crassa in nature makes genetic population studies more feasible. Because it can be grown in large quantity, experiments are easier to conduct, and their results are more easily analyzed.

Neurospora crassa is a filamentous ascomycete that has asci; an ascus is a saclike structure inside which four or eight ascospores develop during reproduction. In the N. crassa asci, one round of mitosis usually follows meiosis and leaves eight nuclei (new daughter cells). These nuclei eventually become eight ascospores (sexual spores produced by ascomycetes). After the ascospores are formed within the ascus, they are released and germinate to form a new mycelium.

A Model Organism

Geneticists use a variety of organisms in their research. Because it is haploid (containing half the chromosomal material of the parent cell), genotypic changes in N. crassa (mutations in genes) are directly observed through the changes in the (physical characteristics), because only one gene determines physical characteristics. The small size of the genome is a result of a unique feature of N. crassa: It has very little repeated DNA. The lack of repetitive DNA is also valuable to researchers when parts of the genome are amplified or sequenced.

Neurospora crassa has been extensively used for genetic research, resulting in hundreds of published articles. They include research on gene expression and effects of external factors, metabolic studies, and genomal mapping experiments. A large number of mutants have been characterized, providing the foundation for many genetic experiments.

Repeat-Induced Point (RIP) Mutations

By using methods, researchers can study N. crassa using a technique known as repeat-induced point (RIP) mutations, the creation of point mutations of a single base pair in specific genes. RIP detects duplications of gene-sized segments and creates repeated point mutations. RIP specifically changes a GC (guanine-cytosine) pair to an AT (adenine-thymine) pair. Repeated sequences are heavily mutated by RIP in the period between fertilization (the time when the sperm comes into contact with the egg) and karyogamy (fusion of the haploid cells to form cells). After the mutation, the altered sequence is methylated (a CH₃, or methyl, group is attached). The methyl group serves as a tag so the mutations can be easily identified. RIP mutations usually indicate a crossing over during meiosis. RIP mutations cause inactivations of duplicate genes, whose functions are then more easily detected.

Sequencing and Linkage

Large-scale sequencing of the N. crassa genome has been initiated for several linkage groups (genes that are located on the same chromosomes). Early in the sequencing of the N. crassa genome, it became apparent that its genome contains many unique genes. These genes and others have been sorted into linkage groups. There are many maps available for N. crassa. The largest group is that at the Whitehead Institute Center for Genome Research under the Fungal Genome Initiative. Restriction fragment length polymorphism (RFLP) maps show the restriction site for a particular restriction endonuclease. Linkage maps show the distribution and linkage of genes throughout the N. crassa genome. These maps are particularly important when a researcher is interested in recombinant DNA research.

Key terms

• ascomycetesorganisms of the phylum Ascomycota, a group of fungi known as the sac fungi, which are characterized by a saclike structure, the ascus
• auxotrophic straina mutant strain of an organism that cannot synthesize a substance required for growth and therefore must have the substance supplied in the growth medium
• cytogeneticsthe study of normal and mutated chromosomes and their behavior
• diploid cella cell that contains two copies of each chromosome
• haploid cella cell that contains one copy of each chromosome
• minimal mediuman environment that contains the simplest set of ingredients that the microorganism can use to produce all the substances required for reproduction and growth
• model organisman organism well suited for genetic research because it has a well-known genetic history, a short life cycle (allowing the production of several generations in a short space of time), and genetic variation between individuals in the population

Bibliography

Beadle, G. W., and E. L. Tatum. “Genetic Control of Biochemical Reactions in Neurospora.” Proceedings of the National Academy of Sciences 27 (1941): 499–506. Print.

Davis, Rowland H. Neurospora: Contributions of a Model Organism. New York: Oxford UP, 2000. Print.

De la Pena, Mariana Villalba, Pauliina A. M. Summanen, Martta Kiukkonen, and Ilkka Kronholm. " Chromatin Structure Influences Rate and Spectrum of Spontaneous Mutations in Neurospora crassa." Genome Research, vol. 33, 2023, pp. 599-611, doi:10.1101/gr.276992.122. Accessed 5 Sept. 2024.

Hong, Christian I. "Circadian Rhythms Synchronize Mitosis in Neurospora crassa." Proceedings of the National Academy of Sciences in the United States of America 111.4 (2014): 1397–402. Print.

Horowitz, N. H. “Fifty Years Ago: The Neurospora Revolution.” Genetics 127 (1991): 631–36. Print.

Kinsey, John A., and Philip W. Garrett-Engele. “The Neurospora Transposon Tad Is Sensitive to Repeat-Induced Point Mutation (RIP).” Genetics 138.3 (November, 1994): 657–64. Print.

Mertens, Thomas Robert, and Robert L. Hammersmith. Genetics: Laboratory Investigations. 14th ed. San Francisco: Cummings, 2013. Print.

Perkins, David D., Alan Radford, and Matthew S. Sachs. The Neurospora Compendium: Chromosomal Loci. San Diego: Academic Press, 2001. Print.

Ramakrishnan, Mukund. "A Factor in a Wild Isolated Neurospora crassa Strain Enables a Chromosome Segment Duplication to Suppress Repeat-Induced Point Mutation." Journ. of Biosciences 36.5 (2011): 817–21. Print.

Thancker, Paul D. “Understanding Fungi Through Their Genomes.” Bioscience 53.1 (2003): 10–15. Print.