Tetrad Analysis in Fungi PDF

# Tetrad Analysis in Fungi ## Genetic Recombination: Study Using _Neurospora crassa_ - **The fungal mat or mycelium** (mycelia, plural) is composed of interwoven filaments called hyphae. - **The tips of hyphae** (hypha, singular) may pinch off asexual spores called conidia (conidium, singular), wh...

# Tetrad Analysis in Fungi ## Genetic Recombination: Study Using _Neurospora crassa_ - **The fungal mat or mycelium** (mycelia, plural) is composed of interwoven filaments called hyphae. - **The tips of hyphae** (hypha, singular) may pinch off asexual spores called conidia (conidium, singular), which germinate into more hyphae. - **The vegetative hyphae** are segmented, with several haploid nuclei in each segment. - **Hyphae from one mycelium** may anastomose with hyphae of another mycelium to form a mixture of nuclei in a common cytoplasm called a heterokaryon. - **A pair of alleles**, A and a, governs the two mating types. - Sexual reproduction occurs only when cells of opposite mating type unite. - **Specialized regions of the mycelium** produce immature female fruiting bodies (protoperithecia) from which extrude receptive filaments called trichogynes. - **A conidium or hypha** from the opposite mating type fuses with the trichogyne, undergoes several karyokineses, and fertilizes many female nuclei. - **Each of the resulting diploid zygotes** lies within an elongated sac called the ascus. - **The zygote divides by meiosis** to form four nuclei, followed by a mitotic division that yields four pairs of nuclei, maturing into eight ascospores. - **A mature fruiting body** (perithecium) may contain over 100 asci, each containing eight ascospores. - **The confines of the ascus** force the polar organization of division to orient lengthwise in the ascus and also prevent the meiotic or mitotic products from slipping past each other. - **Each of the four chromatids** of first meiotic prophase are now represented by a pair of ascospores in tandem order within the ascus. - **In the case of yeast** the ascospores representing the four chromatids of meiosis are in no special order, but in the bread mold Neurospora the ascospores are linearly ordered in the ascus in the same sequence as the chromatids were on the meiotic metaphase plate. - **The recovery and investigation of all of the products** from a single meiotic event is called tetrad analysis. - **Each ascus of Neurospora**, when analyzed for a segregating pair of alleles, reveals one of two linear ratios: - (1) 4: 4 ratio, attributed to first-division segregation - (2) 2:2:2:2 ratio, resulting from second-division segregation. ## First-Division Segregation - A cross between a culture with a wild-type (c+) spreading form of mycelial growth and one with a restricted form of growth called “colonial” (c) is diagrammed in Fig. 6-6(a). - If the ascospores are removed one by one from the ascus in linear order and each is grown as a separate culture, a linear ratio of 4 colonial: 4 wild type indicates that a first-division segregation has occurred. - **That is, during first meiotic anaphase both of the c⁺ chromatids moved to one pole and both of the c chromatids moved to the other pole.** - **The 4: 4 ratio indicates that no crossing over has occurred between the gene and its centromere.** - **The further the gene locus is from the centromere**, the greater is the opportunity for crossing over to occur in this region. - **Therefore, if the meiotic products of a number of asci are analyzed and most of them are found to exhibit a 4 : 4 pattern**, then the locus of c must be close to the centromere. ## Second-Division Segregation - Let us now investigate the results of a crossover between the centromere and the c locus [Fig. 6-6(b)]. - **Note that crossing over in meiotic propase results in a c⁺ chromatid and ac chromatid being attached to the same centromere.** - **Hence, c+ and c fail to separate from each other during first anaphase.** - **During second anaphase, sister chromatids move to opposite poles**, thus affecting segregation of c⁺ from c. - **The 2:2:2:2 linear pattern is indicative of a second-division segregation ascus produced by crossing over between the gene and its centromere.** ## Importance of Neurospora in Molecular Genetics - _N. crassa_ is used as a model organism in several areas of biological research including recombination, DNA repair, differentiation, morphogenesis and cell biology, and notably, DNA methylation and silencing because: - it is easy to grow culture with simple nutritional requirements - 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. - susceptibility to mutagenesis - entire genome of seven chromosomes has been sequenced - It is also important in the elucidation of molecular events involved in circadian rhythms, epigenetics and gene silencing, cell polarity, cell fusion and development. ## Genetic Recombination - Genetic recombination refers to the rearrangement of DNA sequences by the breakage and rejoining of chromosomes on chromosome segments. - **Or** - Genetic recombination is the exchange of genetic material between different organisms which leads to the production of offspring with combinations of traits.