Detection Of Genetic Variation In Populations

Burnett (2003) described a population as a pattern of distribution of individuals with distinctive but comparable morphology and genotypes. A basic requirement for the study of populations is that the fungus is recognized in nature and enough samples are collected over a wide range of geographical distances for inferences. A fungus suited admirably for population studies is Neurospora (Ascomycotina) since it produces distinctive orange conidia that allow its practically unambiguous recognition in nature. Practical techniques have been devised for sampling, transportation and reliable identification of its species in the laboratory (Perkins and Turner, 1988; 2001). Over 4000 samples have been collected globally and collections preserved, providing the most valuable resource material for population studies (Turner et al., 2001). For fungi whose mycelium is in soil, e.g., Fusarium oxysporum (Fungi Anamorphici), the fungus is isolated from soil by dilution plating on a selective medium (Gordon et al., 1992). For fungi immersed in substrate such as wood, blocks can be taken and plated to isolate the fungus. Using this method, Rayner (1991) and colleagues detected different basidiomycete individuals, demarcated by interactive zones of lines in a single branch of an oak tree.

13.6.1 Isozymes

Variation in strains can be due to variants of proteins that catalyze the same reaction but differ in net charge due to differences in their amino acid composition and can therefore be resolved by electrophoresis. The protein extracts of samples are subjected to polyacry-lamide gel electrophoresis and the difference in mobilities of a specific enzyme are compared after visualization following a specific staining reaction. Such variants, called isozymes, imply diversity in the amino acid sequences of the proteins analyzed and of the genes that encode them. Spieth (1975) surveyed N. intermedia for general proteins (acid phosphatases and esterases) and detected a high degree of genetic variability among natural populations. This technique is widely used to detect variation in all forms of life. The technique ultimately detects variation among DNA sequences that code for isozymes.

The late blight fungus Phytophthora infestans (Straminipila) collected world-wide was long considered a single asexual clone. However, populations from the highlands of central Mexico were more variable than collected elsewhere and the isozyme analysis was used to demonstrate genetic diversity in populations. The finding of protein polymorphism in P. infestans isolates was consistent with the discovery of heterothallism and sexual reproduction, suggesting that this region is the site of origin of this fungus In the 1970s and 1980s, the analysis of isozymes of glucose-phosphate isomerase in populations of the P. infestans from Europe, the United States and Canada suggested an intercontinental spread of the fungus from Mexico to Europe through the transport of diseased potato tubers. As a result of man-mediated migration of strains on diseased crops, sexual reproduction elsewhere has become potentially possible, alerting the plant pathologists to protect potato crops by a continuous program of resistance breeding.

Section 13.4.1 referred to the control of wheat rust in the United States by the eradication of barberry, the alternative host of this heterothallic fungus on which sexual reproduction occurs (see Appendix). Protein markers and pathogenecity tests were used to investigate the role of sexual recombination in maintaining variability within the population of Puccinia graminis f.sp. tritici. By 1975, there was a notable difference in the number of physiological races as the sexual stage had ceased to function. In regions where barberry is absent, like Australia, the pathogen population has more limited isozyme and virulence phenotypes than in regions where opportunities for sexual reproduction exist (Burdon and Roelfs, 1985).

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