Hyphal Development in Neurospora Crassa - Essay Example

A fungus is a eukaryotic organizm which receives nutrients and energy from outside through absorption of these materials. These organisms do not have a formal digestive system, therefore, the process of food digestion to a level to be absorbed takes place outside the body. Fungi are a diverse group of organisms in characteristics; based on these features some fungi are benefical to man, som eare very dangerous to man and some are benefical to the unverse at a broader context. The beneficial effect of these organisms is that they are responsible for th fermentation process resulting inbread, beer an yougurt such as, yeast. Some of the fungi are a good source of food, like some mushrooms. At the same time out of these mushrooms are very poisonous and an individual who eats these mushrooms s/he may end up to death if untreated appropriately. At universal level, as fungi are found almost in all types of environment and so they are an imortant component of the ecosystem where they take part in the decomposition of the deaying material. Initally, fungi were placed in the plant kingdom because of their resemblance. Later on, it was found that they are closer to animals instead of plants. But they have been separately grouped in their own kingdom (Fungi, 2006). Fundi have been divided in five divisions based on their mode of reproduction. These divisions are (Fungi, 2006): The Chytridiomycota are also known as chytrids. These fungi produce zoospores that are capable of moving on their own through liquid by simple flagella. The Zygomycota are known as zygomycetes and reproduce sexually with meiospores called zygospores. Tthe arbuscular mycorrhizal fungi is the second name of the Glomeromycota. One species has been found to form zygospores but all other species reproduce asexually. The Ascomycota also known as sac fungi or ascomycetes because of their morphological resemblance. Mmbers of this division form meiotic spores called ascospores, which are enclosed in a special sac-like structure called an ascus. Sseveral ascomyctes have been used for elucidating principles of genetics and heredity, one member of the group is Neurospora crassa. The Basidiomycota, the members of this division are also called the club fungi or basidiomycetes. They produce meiospores called basidiospores on club-like stalks. In 1927, Shear and Dodge gave a very comprehensive account of fungus Neurospora. They called it with this name because of its resemblance to nerves; the striations which develop on the wall of ascospore are like nerves. They discovered the mating types A and a of this genus Neurospora and further described the life histories of three species, two eight-spored heterothallic species named N. crassa and N. sitohila and one four-spored homothallic species N. tetrasperma. (Perkins, 2002) Their explanation of the characteristics related to thallism was based on their work on nucleus of these species. In fact, there is programming of ascus development in N. tetrasperma in a way that each of the four ascospores encloses two nuclei of the opposite mating type. So there is self-fertilization of single-ascospores (Raju, 2003). While in eight-spored N. crassa, the situation is a bit different and which is expected, as it is eight-spored as compared to the four-spored species. In N. crassa, the ascospore pairs are aligned themselves in linear fashion and exhibit genetic events during the process of meiosis. The process of crossover, which occurs at the four-strand stage during the meiotic division, becomes visually obvious. In N. crassa, the segregation of alleles during meiotic division is not fixed but it varies depending upon the relation between the gene marker and the centromere. The closer the gene marker to the centromere the lesser the chances of crossover between the two and the segregation of alleles takes place at the first division of meiosis resulting in a 4:4 ascospore pattern for the alleles. On the other hand, when the gene marker and the centromere are at a distance then the chances of cross over between the gene marker and the centromere are increased. In this situation the segregation of alleles often occurs at the second division of meiosis; the pattern of the segregation of alleles could be 2:2:2:2 or 2:4:2. (Raju, 2003) Actually, this was Dodge who pioneered in working on Neurospora by starting the work in 1927 which was further built by others including Barbara McClintock who was able to demonstrate in forties the similarities of chromosome behaviour between Neurospora and higher plants and animals. (Perkins, 2002) In 1958, the Noble Prize was awarded to George and Edward for their historical work on genetic mutations and nutrition, which was later on, received fame as one-gene one-enzyme hypothesis. The basis for this glorious work was the published work of Dodge on Neurospora and following that he provided the strain of Neurospora to the team of the two future Noble Prize winners (Beadle, 1941). They could demonstrate through their work on mutation induced through X-rays on different strains of Neurospora. They found that mutant strains could not synthesize important substances like vitamins, growth factors and amino acids for which mutation has occurred in them. But if that specific substance has been provided in the culture medium in which these strains were being grown then the deficiency of that specific substance could be managed very well (Beadle, 1941). Their work was on three strains of Neurospora; one of them was N. crassa. This strain could not produce para amino benzoic acid (PABA) but could grow smoothly when kept in minimum medium in which PABA has been added. Through this work the assumption of gene and enzyme specificities being the same order was supported (Beadle, 1941). In fact these characteristics are shown by N. crassa because of its haploid structure and it needs defined media and its genetic characteristics are well understood. Based on these features it was possible to isolate the first auxotrophic mutant (Beadle, 1941). Although the first eukaryotic genome studied, analysed and explored in detail was a fungal one, Saccharomyces cerevisiae. But the major limitation on its part is that it is strictly unicellular with a very specialised genome, which has got most genes, which lack introns (Hynes, 2003). Comparatively, the mycelial fungi have been more promising in advancement on molecular genetics in eurokaryotes. Among these filamentous fungi, N. crassa is being utilized more frequently because of its classical genetics. In molecular genetics discipline the major areas of the use of N. crassa are enzyme production and antibiotic synthesis (Hynes, 2003). The work of George and Edward not only bring them a Noble Prize but also disseminated the role of Neurospora in this regard. Afterwards, Neurospora received enormous attention and became the model filamentous fungus for a variety of discoveries like, vegetative incompatibility, recombination, gene conversion, itragenic complementation, biological blocks, repeat induced point mutations, signal transductions, chromosome rearrangements, meiotic drive, population biology and post-translational gene silencing. (Perkins, 1992). Galagan et al have recently discovered and published the latest development in Neurospora genome; this is related to its sequence, which states that there are over 10,000 protein-coding genes (Galagan, 2003). Signalling Pathways Responding to signalling pathways is an important characteristics possessed by N. crassa through which it performs multiple activities. Among filamentous fungi, Aspergillus nidulans provides better information as compared to N. crassa. (Aspergillus, 2006) A. nidulans has been extensively studied on the balance between growth, reproduction and secondary metabolism in mycelial fungi (Adams, 1998; Calvo, 2002) Mitogen activated protein (MAP) kinases and cyclic AMP-dependant protein kinases and are those which are involved in the sequence encoding in signal pathways and have been identified in bulk quantities in N. crassa Two component histidine kinases and calcium-signalling components are also found in sufficient number in N. crassa these kinases and calcium signalling components play key role in the signalling pathways and response of the fungi to the light. In contrast, the other species like S. cerevisiae has got very less number of these kinases and signalling components. Because of the availability of these kinases and signalling components which add to the ability of N.crassa to respond blue light which is important stimulus as far as the sexual and asexual development and circadian rhythm (Alex, 1996). Genome Defense: Post-transcriptional gene silencing The epigenetic phenomenon, which is well known for the genome defence against foreign invaders like, viruses. N. crassa has contributed a lot in understanding this phenomenon, which revealed that there are two important mechanisms, which are found in Neurospora. These mechanisms are posttranscriptional gene silencing, named as: quelling and meiotic silencing (Hynes, 2003). Although quelling is found in plants, nematodes and some mammals and this is related to RNA interference (RNAi). While meiotic silencing is exclusively possessed by N. crassa. It involves the recognition of unpaired DNA and silencing of all genes coded by that specific DNA. These both pathways are unique and need RNA-dependant RNA polymerase (Hynes, 2003). Repeat-Induced Point Mutation and Genome Evolution Studies on N. crassa have been important in understanding this phenomenon. The exploration in this area revealed the methylation of DNA and histone proteins. The premeiotic dikaryotic stage is important when mutation occurs. At this stage the duplicated sequences, which are approximately 400 base-pair long and have more than 80% identity are recognized. A high number of mutations occur with a preference at CpA dinucleotide. Skewness of dinucleotid frequencies is recognized at this stage, which are helpful in detecting the sequence where mutation has occurred. Eventually, gene is inactivated due to the methylation of the mutated sequence (Hynes, 2003). After analysing the whole genome, now the repeat-induced point mutation becomes clearer and its impact can be seen for gene evolution. There are very few gene pairs n N. crassa with high identity, while in S. pombe and fungal plant pathogen Magnaporthe grisea has comparatively more number of gene pairs with high identity. This may predict in a direction that restriction has been posed on N. crassa as far as the evolution of the families of genes with similar functions is concerned. In contrast, sugar transporters have bee seen to be similar in N. crassa and S. cerevisiae. But the diversity is seen more frequently in N. crassa. In N. crassa the repeat-induced point mutation process suggest that a few genes survive the mutation and this also favours the assumption that evolution of new functions would have taken place before the actual process of mutation took place (Hynes, 2003). Whatever would be the cost of the process, in this regard, N. crassa seems to be successful which has got a genome that has got the ability to encode various functions and a gene number out of which 20-30% is from man. Heat Shock Response One of the unique features of any living object is its response to environmental changes especially when these changes are of extreme level the response is immediate and brisk. On molecular level the living object react by the synthesis of heat shock proteins (HSPs) or stress response proteins. At the eukaryotic level, specifically, a large molecular size proteins (70 to 110kDa) is synthesized which is crucial for the maintenance of the structural and enzymatic proteins in such extreme situations (Kapoor, 1994). Genetically well-characterized model eukaryote, N. crassa response, to the stress which may be in the shape of physiological stress or thermal extremes, is immediate synthesis of various type of proteins, according to the needs, is started in huge quantities to combat the situation. Among various proteins are: HSP70, HSP80 and HSP90 (Kapoor, 1990). Circadian Rhythm Response of an organism to different timings of 24 hours in the shape of day (light) or nigth (dark). The response of an organism to these changes in the ligth is genetically determined. These circadian rhythms/clocks have common features across different organisms and the key component is feedback system. This feedback system is composed of two genetic interactions; negative and positive (Francois, 2005; Bell-Pedersen, 1996; Bell-Pedersen & Dunlap, 1996). Circadian clock in N. crassa works through an autoregulatory negative-feedback system with the involvement of three proteins. These are: FREQUENCY protein, FRQ, the representing protein; and the two white-collar proteins WC1 and WC2, the activating proteins. (Francois, 2005). During early days' work on N. crassa, frq gene was among the first genes to be studied. The relationship between frq gene and FRQ proteins is somewhat coordinated in a manner that during persistent darkness the frq gene level fluctuates; initially frq gene is at peak and after an interval of about 4-6 hours there appears a comparatively larger peak of FRQ proteins (Francois, 2005). In actuality, Neurospora circadian cycle has been divided into two phases: repression and de-repression. In first phase (repression) the FRQ proteins suppress their own synthesis when these are in abundance through negative feedback system. This phase lasts for about 14-18 hours. A level reaches when the concentration of FRQ proteins becomes too low, then frq increases in concentration and eventually stimulates the transcription of FRQ proteins, duration of this phase remains around 4 hours (Francois, 2005). (Source: Francois, 2005) The role of white-collar proteins is very important. They act as transcription factors at the level of transcription. The two WC1 and WC2 bind to form white collar complex (WCC). This complex binds to the promoter of frq gene and facilitates the process of transcription. This favourable action continues in the de-repression phase when high quantity of FRQ proteins is produced. In repression phase, the FRQ proteins bind itself to the WCC and inhibit its binding to frq gene thus holding the further synthesis of FRQ proteins for the time being. These both features have been illustrated in the figure 1(Francois, 2005). In fact, N. crassa has been very widely, very extensively and over a long period studied. The diversified feature possessed by this organism has been encouraging the scientists for a long to utilize this organism for exploring various issues of molecular level. The molecular genetics has depended on this organism to a greater extent in answering variety of questions and further improving the existing knowledge. Bibliography Adams T, Wieser JK & Yu J-H, 1998, 'Asexual sporulation in Aspergillus nidulans,' Microbiol Mol Biol Rev, vol. 68,pp. 35-54. Alex LA, Borkovich KA & Simon MI 1996, 'Hyphal development in Neurospora crassa: Involvement of a two-component histidine kinase,' Microbiology, vol. 93, pp. 3416-3421 Aspergillus nidulans database, 2006, 'Genomes,' Broad Institute, [Online] Available at: http://www.broad.mit.edu/annotation/fungi/aspergillus/index.html Beadle GW & Tatum EL, 1941, 'Genetic control of biochemical reactions in Neurospora,' Biological Department, Stanford University. 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