Neurospora, a genus of ascomycete fungi, is commonly known as red bread mould. The name “Neurospora” came from nerve-like striations on the sexual spores, also known as ascospores. They produce orange-coloured asexual spores called conidia. N. crassa is the well-known species of this genus and has been extensively used in many research studies such as epigenetics, circadian rhythm, photobiology, etc. Neurospora, in the asexual phase, produces haploid spores (conidia) that germinate to form multiple branched hyphae, which combine together to form mycelium. In sexual reproduction, different mating-type strains fuse together to form fruiting bodies. These fruiting bodies (perithecia) shoot the ascospores towards the light.
Neurospora in a natural habitat, the picture depicts growth of N. crassa on a burned tree in Portugal
Characteristics of Neurospora Helpful in Research Studies
- A short life span of 10-15 days helps researchers get results within a short span of time.
- They are haploid, which means, unlike humans, they have only one set of chromosomes. This helps in the expression of both dominant and recessive genes in progeny.
- The whole genome of Neurospora has been mapped. This helps researchers in identifying the role of every gene by creating knockouts through genetic manipulation.
Genome sequencing in Neurospora
- Neurospora is useful in tetrad analysis. Tetrad, a set of four spores, is formed as a result of meiosis. This helps researchers study phenomena like meiosis, crossing over, and recombination.
- Neurospora is preferred over other model organisms like E. coli and S. cerevisiae because they share similar features with higher eukaryotes, such as DNA methylation, circadian rhythm, etc.
- Tools for genetic manipulations are available for Neurospora. Wild type and mutated (knock-out) strains can be easily obtained that help in analyzing the role of a particular gene.
Uses
In Biotechnology Studies
N. crassa was the first species to be used as an experimental model by Dodge in the late 20s. Since then, it has been used extensively in many research studies such as photobiology, epigenetic processes, population biology, circadian rhythm, etc. Neurospora was a part of a Nobel Prize-winning study of ‘one gene–one enzyme hypothesis’ by Beadle and Tatum. According to this theory, every single gene encodes an enzyme which in turn is responsible for a single step in a metabolic pathway. They made Neurospora mutants by exposing the spores to radiation and grew them in separate test tubes containing minimal media. They observed that mutant spores died in minimal media because they were not able to make a particular crucial molecule required for its growth.
In Studying Circadian Rhythm
Circadian rhythm is a biological clock of 24 hours that synchronizes inner physiology with the external world. It is also known as the sleep-wake cycle. These are physical and behavioural changes that respond mainly to light and darkness. Circadian rhythm is an endogenous clock found in almost all living beings. Neurospora has been served as a crucial model for studying the process and physiology of circadian rhythm in humans. Like humans, the circadian rhythm in Neurospora has three parts – input, a central oscillator, and output. The function of the central oscillator is to generate a rhythm of about 24 hours. The core complex has two sets of protein pairs, the negative arm, and the positive arm. The negative arm contains FRQ/FRH complex (FFC) and CK1, whereas the positive arm has white-collar complex (WWC). WWC consists of WC1 and WC2, which stimulates the expression of frq. In Neurospora circadian cycle begins in the subjective night when WCC binds to the frq promoter and starts the translation of frq mRNA. FRQ protein binds with FRH forming FRQ-FRH complex (FFC) and then enters the nucleus. In the nucleus, it forms a complex with CK1 that inhibits WCC by phosphorylating it. WCC ultimately stops frq translation and exits the nucleus. Lack of WCC decreases the synthesis of FRQ, by late afternoon. This leads to the degradation of FRQ by SCF- ubiquitin ligase complex. Unbound WCC then binds the frq promoter and starts its translation into FRQ protein, and the cycle starts again.
Nucleosome dynamics regulate Neurospora circadian clock
As a Model in Photobiology Research Studies
Light affects biological and physiological processes almost in all living beings. Neurospora has been used as a tool to study biological responses such as the role of light in controlling the circadian rhythm, development of sexual (ascospores) and asexual (conidiospores) structures, the direction of ascospores dispersal, etc. Studies have found that these biological responses to light involve light-induced gene regulation in Neurospora. It was found that nearly 5.6% of genes show rapid transcription when stimulated by light. Studies have found that it was mainly UV/blue light that is responsible for light responses in Neurospora. Genetic analysis on Mutants has shown that photoreceptor WC-1 and WC2 forms a complex WCC that binds to specific DNA sequence and starts the expression of many light-responsive genes.
DNA Methylation Studies on Neurospora
DNA methylation is an epigenetic process of regulating gene expression by transferring methyl groups on cytosine in DNA. Methylation of cytosine inhibits the binding of transcription factors to the DNA sequence. In eukaryotic organisms, DNA methylation plays an important role in processes such as genome imprinting, gene silencing, and inactivation of X chromosome. Neurospora has proved to be an excellent system to study the cause and role of DNA methylation. DNA methylation is not mandatory in Neurospora, making it an important model to study methylation. Studies in Neurospora, Arabidopsis, and mice have shown that there are similarities in the function and process of DNA methylation in different eukaryotic and prokaryotic organisms. It was found that blocking of deacetylation of histones in Neurospora reduces DNA methylation. It was also discovered that mutation of dim5 and dim2 genes in Neurospora hampers the process of DNA methylation. Research studies have shown that in Neurospora DNA methylation depends on DIM-2(DNA methyltransferase), directed by DIM-5 (histone H3 methyltransferase). Scientists have isolated methylated DNA from Neurospora and found that methylated sequences consist of transposons that were mutated due to repeated sequences. These research studies concluded that Neurospora uses DNA methylation as a defence mechanism against repeated or duplicated sequences (transposable elements).
DNA methylation and the formation of heterochromatin in Neurospora crassa
Programmed Cell Death (PCD) in Neurospora
Programmed cell death, also known as cellular suicide, is a process to get rid of unwanted cells. In this process, unwanted cells are eliminated from the body through either apoptosis or autophagy. Various triggers like exposure to stress, viral invasion can cause programmed cell death in Neurospora. PCD is also involved in other life processes like sexual and asexual reproduction, ageing, fungal non-self recognition. Cell death in Neurospora occurs when hyphal fusion occurs between genetically dissimilar strains. This triggers cell death in the fusion compartment and neighbouring cells, causing rejection of heterokaryon formation, this is known as heterokaryon incompatibility. Programmed Cell death in these incompatible heterokaryon leads to plasma membrane shrinkage DNA condensation followed by cell death. It was observed that certain chemicals, such as Phytosphingosine (PHS) can also induce cell death in Neurospora. PHS is a shingolipid that has antifungal properties. Researchers have shown that treating Neurospora with PHS can cause ROS (reactive oxygen species) production, condensation, and fragmentation of DNA leading to cell death.
Programmed Cell Death in Neurospora