Meiotic Events

B.O. Dodge and C.C. Lindegren perceived Neurospora as a marvelous organism for genetic analysis as the four products of a single meiosis are packaged as ascospores in a single row in the order of their formation in ascus (usually called a tetrad, although in Neurospora the meiotic division is followed by a mitotic division; hence, each of the four meiotically produced ascospores are duplicated as sister ascospores, thus an octad). The ascospores are large enough, measuring approximately 29 X 15 ^m, to be dissected manually under a dissecting microscope and their phenotypes and genotypes to be determined. This procedure, called tetrad analysis, allows tracing the order of the events in meiosis. For example, by crossing a mutant strain that affects ascospore color, it can be determined simply by visual examination of the asci in opened perithecia (Figure 5.4) whether the separation of parental alleles occurred in the first meiotic division without crossing over (first division segregation) or in the second division as a result of cross over between the gene pair and centromere. As an example, the 4:4 ratio of mating types (A or a) or of ascospore color marker (cys-3) in the octad demonstrates that alleles separate during meiosis, validating Mendel's First Law of Segregation. (In genetic nomenclature for Neurospora, the name of a gene is generally abbreviated to the first three letters. For example, the mutant allele cys refers to cysteine

Genetic Recombination Fungi

Figure 5.4 Opened perithecium of Neurospora crassa. Photomicrograph of a rosette of maturing asci from a cross of wild-type (+) X cys-3 mutant (m) allele that affects cysteine biosynthesis and results in unpigmented (white) ascospores. Because the final division is mitotic, contiguous sister ascospores have the same genetical constitution. One ascus at top center and two asci at upper left show four black : four white firstdivision segregation. The remaining mature asci show second division segregation patterns (2+, 2m, 2+, 2m) resulting from crossing over between cys-3 and centromere. Photo courtesy of Namboori B. Raju.

Figure 5.4 Opened perithecium of Neurospora crassa. Photomicrograph of a rosette of maturing asci from a cross of wild-type (+) X cys-3 mutant (m) allele that affects cysteine biosynthesis and results in unpigmented (white) ascospores. Because the final division is mitotic, contiguous sister ascospores have the same genetical constitution. One ascus at top center and two asci at upper left show four black : four white firstdivision segregation. The remaining mature asci show second division segregation patterns (2+, 2m, 2+, 2m) resulting from crossing over between cys-3 and centromere. Photo courtesy of Namboori B. Raju.

while the superscript "+" is used to distinguish the wild allele from the mutant allele). In a cross of a wild N. crassa with pink-orange carotenoid pigment, i.e., al+ and aerial mycelium (col+) to a double mutant albino (white, impaired in carotenoid biosynthesis) and colonial (restricted growth in the form of a colony), four types of progeny are produced in equal proportion: al+ col+ (wild type with orange, aerial mycelium), al col (albino colonial), al+col (pink-orange colonial) and al col+ (albino, spreading mycelium), in conformity with Mendel's Second Law of Independent Assortment. From a Neurospora cross, a significant number of sexual progeny can be grown in small tubes and analyzed in about four weeks, providing considerable savings both in space and time than would be possible with either a pea plant or a fruit fly. Neurospora has been the main source of information about the basic recombination mechanisms in eukaryotes. It is an ideal classroom material for teaching of microbiological and genetic methods.

Tetrad analysis demonstrated that meiotic recombination by crossing-over between linked loci on the same chromosome occurs after chromosome replication between chro-matids and not whole chromosomes. In a cross (AB X ab) where genes A and B are linked, with alleles a and b, the appearance of two parental and two recombinant products (a tetratype) led to the conclusion that crossing-over occurs after chromosome replication between non-sister homologous chromatids at the four-strand stage (Figure 5.5). Had it a B

Crossing-over before replication

Meiotic tetrad

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