Discovery Of Intraspecies Variability

13.4.1 Physiological Races

Figure 13.1 Infection types produced by wheat stem rust, Puccinia graminis var. tritici, on differential varieties of wheat. 0, Entirely immune. 0;, Practically immune. 1, Extremely resistant. 2, Moderately resistant. 3, Moderately susceptible. 4, Completely susceptible. Tracing from Stakman and Harrar (1957).

Author:Two labels for "0", please verify. Is 0; correct? Would 0a be better?

Figure 13.1 Infection types produced by wheat stem rust, Puccinia graminis var. tritici, on differential varieties of wheat. 0, Entirely immune. 0;, Practically immune. 1, Extremely resistant. 2, Moderately resistant. 3, Moderately susceptible. 4, Completely susceptible. Tracing from Stakman and Harrar (1957).

that is, it has a parts of its life cycle on wheat and the other on an alternate host, barberry. When this was recognized, a barberry destruction program was undertaken in the United States to control rust epidemics with some limited success as new races of wheat rust continue to arise through mutation in the absence of sexual reproduction.

The application of pathogenecity tests show that intraspecific groups of individuals restricted to a characteristic host (forma speciales) is a common feature of several plant pathogenic fungi. For example, forma speciales have been discovered in Phytophthora infestans, in the powdery mildew (Erysiphe spp.), in the downy mildew (Peronospora spp.) and in Fusarium oxysporum.

13.4.2 Vegetative Compatibility

Since the majority of species are non-pathogenic, the pathogenecity test for detecting intraspecies variation used for the rust fungi is of limited application. In the pre-molecular era, the formation of heterokaryon, involving hyphal fusion between strains followed by mixing of their protoplasm, was a common method to determine their genetic relatedness. When two test strains are paired opposite to each other on agar media, frequent anastomoses occur between the hyphae of two closely related fungal strains. If not, the mycelia of the test strains confronted on nutrient media show an antagonistic reaction consisting of vacuolated, dying and often pigmented hyphal cells between them called the barrage reaction. Fusion of hyphae is controlled by genes called heterokaryon incompatibility (het) loci or vegetative incompatibility (vic) loci. For example, 11 and 8 het loci in N. crassa and Aspergillus nidulans, respectively, and 10 vic loci in Fusarium oxysporum have been identified. The hyphae of two fungal strains fuse to form a homogeneous mycelium (heterokaryon) only if the alleles at each of the corresponding het or vic loci are identical. This method is more reliable if the test strains have genetic markers whose defect can be remedied by complementation. In one of the procedures, the test strains are grown in media containing chlorate. The spontaneous nitrate non-utilizing (nitrate reductase) mutants as chlorate-resistant mutants are selected. These strains are paired against each other on minimal media plates with nitrate as the nitrogen source. Strains of the same vegetative compatibility group (VCG) form prototrophic heterokaryons whereas the strains of different VCG are incapable of forming prototrophic heterokaryons. Based on this test, Jacobson and Gordon (1990) resolved a morphologically identical population of the muskmelon wilt fungus Fusarium oxysporum f.sp. melonis into several incompatible groups and demonstrated intraspecies variability in this facultative parasite. This method has also been used to detect variation in the population of honey fungus, Armillaria mellea (Basidiomycotina), in Australia. The vegetative compatibility test reflects closeness but not the genotypic identity of the individuals.

13.5 GENERATION OF VARIATION 13.5.1 Mutation and Heterokaryosis

Ultimately all variations arise from mutations. Although a rare event, of the order of 10-6 per nucleus, mutation assumes greater importance in fungi than in other organisms because the coenocytic fungal mycelium contains thousands or perhaps millions of nuclei. The effect of mutation on phenotype may not be detectable because of masking of the mutant nuclei by the wild-type nuclei in the heterokaryon. It is only when the mutant nuclei segregate as uninucleate spores and germinate to produce a growth variant containing descendants of the mutant nuclei that the mutation is revealed (Chapter 2). Such a variant may differ morphologically from the original strain (Adhvaryu and Maheshwari, 2000). Chapter 3 refers to the finding that a single multinucleate spore of mycorrhizal fungi contains genetically different nuclei. More than one ITS sequence has been obtained from a single multinucleate spore. Since sexual reproduction is unknown in these fungi, heterokaryosis is believed to play an important role in the variability of Glomales (Sanders, 1999).

13.5.2 Transposable Elements

Transposable elements are insertion sequences in DNA that have an intrinsic capability of transposing within the host genome. They contribute to genetic variation by both modification of gene sequence and modification of chromosome structure through translocation, deletion and duplication and have been identified in fungi (Daboussi, 1996). Transposons are present in multiple copies and consequently can be discovered through DNA sequencing. The plant pathogenic fungi Fusarium oxysporum and Mycosphaerella graminicola are typical examples where various types of transposons have been discovered.

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