Neurospora crassa is a type of red bread mold of the phylum Ascomycota. The genus name, meaning “nerve spore” refers to the characteristic striations on the spores that resemble axons.
BIOLOGY
Neurospora crassa produces two kinds of spores:
Conidia are spores produced by asexual reproduction, by mitosis of the haploid nuclei of the active, growing fungus.
Ascospores, on the other hand, are formed following sexual reproduction. If two different mating types are allowed to grow together, they will fuse to form a diploid zygote. Meiosis of this zygote then gives rise to the haploid ascospores.
Specialized hyphae, called conidialanastomosistubes (CATs), are produced by all types of conidia and by conidial germ tubes of Neurosporacrassa. The CAT is shown to be a cellular element that is morphologically and physiologically distinct from a germ tube and under separate genetic control. In contrast to germ tubes, CATs are thinner, shorter, lack branches, exhibit determinate growth, and home toward each other.
SPECIAL FEATURES
N. crassa is used as a model organism because it is easy to grow and has a haploid life cycle that makes genetic analysis simple since recessive traits will immediately show up in the offspring. Its entire genome of seven chromosomes has been sequenced. Neurospora was used by Edward Tatum and George Wells Beadle in their experiments for which they won the Nobel Prize in Physiology or Medicine in 1958
Neurospora is particularly well suited for genetic studies because:
- It can be grown quickly on simple culture medium.
- It spends most of its life cycle in the haploid condition so any recessive mutations will show up in its phenotype.
- When the diploid zygote undergoes meiosis, the nuclei produced by Meiosis I, Followed by Meiosis II, followed by one mitotic division are confined to a narrow tube, the ascus.
- The nuclei cannot slip past one another, if the zygote nucleus is heterozygous for a gene and no crossing over near that locus occurs during meiosis I, the ascus will finally have four spores at one end containing one allele and four spores at the other end containing the other alleles.
The filamentous fungus Neurospora crassa, which has played an important role in the development of modern genetics, has several unique genome-defense mechanisms, including a process called repeat-induced point mutation. The draft genome sequence has revealed several unusual features, which suggest that the evolution of N. crassa has been greatly influenced by these defense mechanisms.
In 1941 Beadle and Edward L. Tatum decided to examine step by step the chemical reactions in a pathway. They used Neurospora crassa as an experimental organism. It had a short life-cycle and was easily grown. Since it is haploid for much of its life cycle, mutations would be immediately expressed and the meiotic products could be easily inspected. Chromosome mapping studies on the organism facilitated their work.
Neurospora can be grown on a minimal medium and it’s nutritional requirements could be studied by its ability to metabolize sugars and other chemicals. The scientists could add or delete nutrients from the mixture of the medium. It is able to synthesize all amino acids and other chemicals needed for its growth, thus mutants in synthetic pathways would easily show up.
X-rays induced mutations in Neurospora, and the mutated spores were placed on growth media enriched with all essential amino acids. Crossing the mutated fungi with non-mutated forms produced spores which were then grown on media supplied with only one of the 20 essential amino acids. If a spore lacked the ability to synthesize a particular amino acid, such as Pro (proline), it would only grow if the Proline was in the growth medium. Biosynthesis of amino acids is a complex process with many chemical reactions mediated by enzymes, which if mutated would shut down the pathway, resulting in no growth.
Neurospora crassa has proven to be an excellent organism for studying various aspects of biology of mitochondria by biochemical and genetic approaches. As N. crassa is an obligate aerobe and its mitochondria are more similar to mammalian mitochondria than those of yeast. The recent sequencing of the genome of N. crassa and a gene knockout project that is under way make the organism even more attractive.
CONTRIBUTION TO SCIENCE
George Beadle and Edward Tatum during the late 1930s and early 1940s established the connection Garrod suspected between genes and metabolism. They used X rays to cause mutations in Neurospora, which affected a single gene and single enzyme in specific metabolic pathways. Beadle and Tatum proposed the “one gene one enzyme hypothesis” for which they won the Nobel Prize in 1958.