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'' 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. The first published account of this fungus was from an infestation of French bakeries in 1843. ''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 show up in the offspring. Analysis of genetic recombination is facilitated by the ordered arrangement of the products of meiosis in ''Neurospora'' ascospores. Its entire genome of seven chromosomes has been sequenced.〔(Trans-NIH Neurospora Initiative )〕 ''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. Beadle and Tatum exposed ''N. crassa'' to x-rays, causing mutations. They then observed failures in metabolic pathways caused by errors in specific enzymes. This led them to propose the "one gene, one enzyme" hypothesis that specific genes code for specific proteins. Their hypothesis was later elaborated to enzyme pathways by Norman Horowitz, also working on ''Neurospora''. As Norman Horowitz reminisced in 2004 “These experiments founded the science of what Beadle and Tatum called ‘biochemical genetics’. In actuality, they proved to be the opening gun in what became molecular genetics and all developments that have followed from that.” In the 24 April 2003 issue of ''Nature'', the genome of ''N. crassa'' was reported as completely sequenced. The genome is about 43 megabases long and includes approximately 10,000 genes. There is a project underway to produce strains containing knockout mutants of every ''N. crassa'' gene. In its natural environment, ''N. crassa'' lives mainly in tropical and sub-tropical regions. It can be found growing on dead plant matter after fires. Neurospora is actively used in research around the world. It is important in the elucidation of molecular events involved in circadian rhythms, epigenetics and gene silencing, cell polarity, cell fusion, development, as well as many aspects of cell biology and biochemistry. Strains and other materials for working with Neurospora are available from the (Fungal Genetics Stock Center ) ==The sexual cycle== Sexual fruiting bodies (perithecia) can only be formed when two mycelia of different mating type come together (see Figure). Like other Ascomycetes, ''N. crassa'' has two mating types that, in this case, are symbolized by ''A'' and ''a''. There is no evident morphological difference between the ''A'' and ''a'' mating type strains. Both can form abundant protoperithecia, the female reproductive structure (see Figure). Protoperithecia are formed most readily in the laboratory when growth occurs on solid (agar) synthetic medium with a relatively low source of nitrogen. Nitrogen starvation appears to be necessary for expression of genes involved in sexual development. The protoperithecium consists of an ascogonium, a coiled multicellular hypha that is enclosed in a knot-like aggregation of hyphae. A branched system of slender hyphae, called the trichogyne, extends from the tip of the ascogonium projecting beyond the sheathing hyphae into the air. The sexual cycle is initiated (i.e. fertilization occurs) when a cell (usually a conidium) of opposite mating type contacts a part of the trichogyne (see Figure). Such contact can be followed by cell fusion leading to one or more nuclei from the fertilizing cell migrating down the trichogyne into the ascogonium. Since both ''A'' and ''a'' strains have the same sexual structures, neither strain can be regarded as exclusively male or female. However, as a recipient, the protoperithecium of both the ''A'' and ''a'' strains can be thought of as the female structure, and the fertilizing conidium can be thought of as the male participant. The subsequent steps following fusion of ''A'' and ''a'' haploid cells, have been outlined by Fincham and Day〔Fincham J RS, Day PR (1963). Fungal Genetics. Blackwell Scientific Publications, Oxford, UK. ASIN: B000W851KO〕 and Wagner and Mitchell.〔Wagner RP, Mitchell HK. (1964). Genetics and Metabolism. John Wiley and Sons, Inc., New York ASIN: B00BXTC5BO〕 After fusion of the cells, the further fusion of their nuclei is delayed. Instead, a nucleus from the fertilizing cell and a nucleus from the ascogonium become associated and begin to divide synchronously. The products of these nuclear divisions (still in pairs of unlike mating type, i.e. ''A/a'') migrate into numerous ascogenous hyphae, which then begin to grow out of the ascogonium. Each of these ascogenous hypha bends to form a hook (or crozier) at its tip and the ''A'' and ''a'' pair of haploid nuclei within the crozier divide synchronously. Next, septa form to divide the crozier into three cells. The central cell in the curve of the hook contains one ''A'' and one ''a'' nucleus (see Figure). This binuclear cell initiates ascus formation and is called an “ascus-initial” cell. Next the two uninucleate cells on either side of the first ascus-forming cell fuse with each other to form a binucleate cell that can grow to form a further crozier that can then form its own ascus-initial cell. This process can then be repeated multiple times. After formation of the ascus-initial cell, the A and a nuclei fuse with each other to form a diploid nucleus (see Figure). This nucleus is the only diploid nucleus in the entire life cycle of ''N. crassa''. The diploid nucleus has 14 chromosomes formed from the two fused haploid nuclei that had 7 chromosomes each. Formation of the diploid nucleus is immediately followed by meiosis. The two sequential divisions of meiosis lead to four haploid nuclei, two of the ''A'' mating type and two of the ''a'' mating type. One further mitotic division leads to four ''A'' and four ''a'' nuclei in each ascus. Meiosis is an essential part of the life cycle of all sexually reproducing organisms, and in its main features, meiosis in ''N. crassa'' seems typical of meiosis generally. As the above events are occurring, the mycelial sheath that had enveloped the ascogonium develops as the wall of the perithecium, becomes impregnated with melanin, and blackens. The mature perithecium has a flask-shaped structure. A mature perithecium may contain as many as 300 asci, each derived from identical fusion diploid nuclei. Ordinarily, in nature, when the perithecia mature the ascospores are ejected rather violently into the air. These ascospores are heat resistant and, in the lab, require heating at 60 °C for 30 minutes to induce germination. For normal strains, the entire sexual cycle takes 10 to 15 days. In a mature ascus containing eight ascospores, pairs of adjacent spores are identical in genetic constitution, since the last division is mitotic, and since the ascospores are contained in the ascus sac that holds them in a definite order determined by the direction of nuclear segregations during meiosis. Since the four primary products are also arranged in sequence, a first division segregation pattern of genetic markers can be distinguished from a second division segregation pattern. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Neurospora crassa」の詳細全文を読む スポンサード リンク
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