Tetrad Analysis in Fungi Help
Tetrad Analysis in Fungi
Fungi that produce sexual spores (ascospores) housed in a common sac (ascus; asci, plural) are called ascomycetes. One of the simplest ascomycetes is the unicellular baker's yeast Saccharomyces cerevisiae (Fig. 6-4). Asexual reproduction is by budding, a mitotic process usually with unequal cytokinesis. The sexual cycle involves the union of entire cells of opposite mating type, forming a diploid zygote. The diploid cell may reproduce diploid progeny asexually by budding or haploid progeny by meiosis. The four haploid nuclei form ascospores enclosed by the ascus. Rupture of the ascus releases the haploid spores, which then germinate into new yeast cells.
Another ascomycete of interest to geneticists is the bread mold Neurospora crassa (Fig. 6-5). 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, or fungal mat, 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 or (2) 2 : 2 : 2 : 2 ratio, resulting from second-division segregation.
First-Division and Second-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.
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 prophase results in a c+ chromatid and a c 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.
Practice problems for these concepts can be found at:
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