Generation of Variability in Pathogens
Pathogens have a wide range of flexible mechanism for producing variability. The following discussion is based on fungi, but many of the features and the conclusions drawn are applicable to other pathogens as well. New variability in fungal pathogens may be created in the following ways: 1) Mutation 2) Sexual reproduction, 3) Heterokaryosis, and 4) Para sexual reproduction.
Mutation:
Spontaneous mutations are the ultimate source of all genetic variation present in the biological world. Spontaneous mutations occur at a low rate 10-5 to 10-6. but this rate is large enough when it considered along with astronomical number of asexual and sexual spores produced by fungal pathogens. With mutation frequency of 10-6 , the number of mutant at a locus of Puccinia recondite produced per day per hectare of wheat field has been estimated at 100,000. Studies have also been focussed on estimating the frequency of mutant virulent to specific resistance genes. In case of Puccinia graminis tritici, the rates of mutation fir virulence to Sr5, Sr15, Sr21, and Sr9e were high, but mutation for virulence to Sr26 was not detected. Gene Sr26 is derived from Agropyron elongatum, for some unknown reason, mutations for virulence to other genes derived from related species is also rare. One way for a bio tropic pathogen to become virulent to a resistance gene is to delete the corresponding avirulence allele, this, however, would have detrimental effects on the competitive ability of the mutant.
Sexual Reproduction:
Sexual reproduction is a common feature of most of the pathogenic fungi. But in the pathogens belonging to fungi Imperfect, sexual reproduction has not been detected. Sexual reproduction involves the fusion of two haploid cells, which may or may not be sexually differentiated, to produce a cell containing two nuclei, a dicaryon. The duration of dicaryon stage varies from a very brief period to the entire life cycle, except for brief diploid ad haploid generations. The two nuclei of a dicaryon ultimately fuse to produce a diploid and haploid generations. The two nuclei of a dicaryon ultimately fuse to produce a diploid nucleus, which undergoes meiosis to produce four haploid nuclei. The haploid nuclei may or may not divide mitotically before they produce haploid sexual spores, which on germination give rise to the haploid phase of life cycle.
Heterokaryosis:
In many fungi, e.g. Fungi Imperfecti, the hyphae are multinulecleate during their active growth. These nuclei may be genetically identical or of dissimilar genotypes. Heterokaryosis is produced by the fusion of vegetative hyphae, which are genetically dissimilar, this fusion is not affected by the mating type. Heterokaryosis occurs in nature, and is of adaptive value. Heterokaryosis are generally more various than homocaryons. During mitosis, the different nuclei of a heterocaryon may get included in different cells producing homocaryons. Thus, Heterokaryosis is capable of storing a limited amount of genetic variability, which is released through ‘segregation’ of the whole nuclei during mitosis; this appears to have an adaptive significance. There is evidence that Heterokaryosis can change the pathogenicity and may even give rise to new races of a pathogen.
Para Sexual Reproduction:
In para sexual reproduction, diploid nuclei are produced in vegetative cells from which haploid nuclei are produced through mitotic irregularities, but not through meiosis. Diploid cells occur in the heterocaryon at frequency of 10-7 and result from the fusion of the two nuclei present in the heterocaryon. These diploid cells give rise to haploid cells at a frequency of 10-3. Crossing over does not take place during haploidization of diploid nuclei; hence a chromosome is inherited as a single unit. However, somatic recombination does take place in low frequency, i.e. one chiasma per 10 nuclei. But somatic recombination is not associated with somatic reduction (production of haploid nuclei through mitosis); the two events are independent of each other and may take place in the same or in different nuclei.