Gene Maps

In Neurospora the ascospores are ejected successively—the asci elongate, one at a time protrude through the opening in the perithecium and shoot ascospores in groups of eight and then retract. Thus, if the time of their collection is kept short, well-separated groups of tetrads (i.e., products of single meiotic events) can be collected (Figure 5.7).

Neurospora is the first organism for which techniques were developed for detecting and analyzing chromosomal rearrangements in which segments between chromosomes become exchanged, or deleted, or inverted (Figure 5.8), either spontaneously, by UV irradiation or by DNA-mediated transformation. These are types of mutations. The rearrangement of chromosome structure is manifested as visually defective ascospores in shot octads (Perkins, 1994). The defective ascospores are white while non-deficiency ascospores are black. To identify the nature of chromomosome rearrangements, the strain to be tested is crossed to a normal strain in a petri dish. After ten days, the perithecial surface is briefly inverted over a slab of water agar to collect spontaneously shot groups of eight ascospores (unordered tetrads) and examined under a binocular microscope to score defective octad (black:white) classes as 8:0, 6:2, 4:4, 2:6 and 0:8. The frequency of the 4:4 type is diagnostic of the type of translocation, whether insertional or reciprocal (Figure 5.9). Unordered octads are used in many aspects of meiotic genetics, including the measurement of cross-over frequencies based on the proportion of asci of parental ditypes, tetratypes and nonparental ditypes. From the data, the distance between a gene and centromere can be determined and gene maps constructed.

To facilitate the mapping of a newly discovered gene, a linkage tester strain, alcoy, having three reciprocal translocations was developed in which six linkage groups are represented as T(IR; IIR) al-1, T(IVR; VR) cot-1 and T(IIIR; VIL) ylo. The symbol T refers to translocation, the numbers refer to linkage groups and the symbols R and

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