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Title: Inheritance of resistance to Phytophthora infestans (Mont.) de Bary in hybrid derivatives of Solanum demissum Lindl
Author: Black, William
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1952
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1. The common strain and nine specialised strains of Phytophthora infestens were employed in testing seedlings and seedling progenies, obtained from four different breeding systems, for resistance to the disease. 2. The resistance exhibited by S. demissum (CPC 2127) and seedlings bred from it is due primarily to the hypersensitive condition of the protoplasm. This condition is manifested in the presence of one or more major resistance genes, of which four have been identified, viz. R₁, R₂, R₃ and R₄. 3. Each major gene confers resistance to the common strain and to a particular group of specialised strains of the parasite. The genes are inherited independently in simple Mendelian fashion. 4. A series of minor genes, associated with morphological and physiological characters of the plant, modify the phenotypic expression of the major gene system, and so determine the degree of susceptibility in susceptible phenotypes and the extent of necrosis in resistant ones. 5. In the early generations of S. demissum-S. tuberosum hybrids, the irregularity of chromosome behaviour and the presence of unpaired chromosomes caused the ratios of resistants resistants to susceptibles to vary widely from standard Mendelian ratios. 6. In certain progenies, particularly those obtained by crossing S. tuberosum plants with resistant hybrid derivatives of S. demissum, deviations from standard Mendelian ratios were consistent in their trend, and appeared to be due to some relationship between genes affecting disease resistance and incompatibility genes. 7. In certain parent seedlings with duplicate genes, derived from self- fertilised plants or from recombination crosses, partial auto-syndesis resulted in an excess of resistant segregates in the progenies. B. The interrelationships of ten strains of blight (Phytophthora infestans) and four major genes R₁, R₂, R₃ and R₄ controlling resistance to the disease in the potato are examined. 9. Since dominance of the genes is complete the maximum number of phenotypic reactions which can be distinguished is sixteen. This series represents the complete range of differential hosts for the material in question and is capable of distinguishing sixteen different strains of the parasite. 10. The reactions exhibited by the series form a concise statement statement of the various relationships and provide the necessary groundwork for the systematic classification of strains. The classification will afford a basis for the calculation of segregation- ratios to be expected from the mating of any pair of genotypes when infected with any strain or group of strains of the parasite. 11. Each specialised strain, being adapted to a particular Solanum genotype, is more prolific on it than on any other. This genotype is regarded as the natural host of the strain. 12. Specialised strains differ in their effect upon varieties of S. tuberosum: Those with the widest host range being least virulent. Also, they are less prolific and cause less damage than the common strain. 13. The exact mode of origin of strains is not clear but mutation appears to play a significant part. 14. Specialisation appears to progress in stages in certain directions determined by the genetic constitution of the various hosts. Examples of three progressive stages of specialisation are apparent in the material reported on here. If the fourth stage should be reached, the resulting strain will be capable of attacking all sixteen genotypes. 15. It is suggested that strains should be classified according to the genetical constitution of their natural hosts and that each should be known by the numeral pertaining to the genes in question.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Sc.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available