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''Pathosystem studies'' are conducted at the higher systems levels, and normally involve the interaction of a population of the parasite with a population of the host. In a wild plant pathosystem, both the host and the parasite populations exhibit genetic diversity and genetic flexibility. Conversely, in a crop pathosystem, the host population normally exhibits genetic uniformity and genetic inflexibility (i.e., clones, pure lines, hybrid varieties), and the parasite population assumes a comparable uniformity. This distinction means that a wild pathosystem can respond to selection pressures, but that a crop pathosystem does not. It also means that a system of locking (see below) can function in a wild plant pathosystem but not in a crop pathosystem. ''Pathosystem balance'' means that the parasite does not endanger the survival of the host; and that the resistance in the host does not endanger the survival of the parasite. This is self-evident from the evolutionary survival of wild plant pathosystems, as systems, during periods of geological time.〔Robinson, R.A.(2010) Self-Organizing Agro-Ecosystems; Sharebooks Publishing http://www.sharebookspublishing.com〕 The gene-for-gene relationship〔Flor, H.H. (1942); “Inheritance of pathogenicity in Melampsora lini.” Phytopath., 32; 653-669.〕 is an approximate botanical equivalent of antigens and antibodies in mammals. For each resistance gene in the host, there is a corresponding, or matching, gene in the parasite. When the genes of the parasite match those of the host, the resistance does not operate. There are two kinds of resistance to parasites in plants: * Vertical resistance〔Vanderplank, J.E. (1963); “Plant Diseases; Epidemics and Control.” Academic Press, New York & London, 349pp.〕 involves a gene-for-gene relationship. This kind of resistance is genetically controlled by single genes, although several such genes may be present in a single host or parasite individual. Vertical resistance is ephemeral resistance because it operates against some strains of the parasite but not others,〔Vanderplank, J.E. (1963); “Plant Diseases; Epidemics and Control.” Academic Press, New York & London, 349pp.〕 depending on whether or not a match occurs. In agriculture, vertical resistance requires pedigree breeding and back-crossing. It has been the resistance of choice during the twentieth century. * Horizontal resistance〔Vanderplank, J.E. (1963); “Plant Diseases; Epidemics and Control.” Academic Press, New York & London, 349pp.〕 does not involve a gene-for-gene relationship. It is the resistance that invariably remains after vertical resistance has been matched. It is genetically controlled by polygenes and it is durable resistance as many ancient clones testify. Its use in agriculture requires population breeding and recurrent mass selection. Infection is the contact made by one parasite individual with one host individual for the purposes of parasitism. There are two kinds of infection: * Allo-infection〔Robinson, R.A. (1976); “Plant Pathosystems.” Springer-Verlag, Berlin, Heidelberg, New York, 184pp.〕 means that the parasite originates away from its host and has to travel to that host. The first infection of any individual host must be an allo-infection. Vertical resistance can control allo-infection only. It normally does this with a system of locking (see below) which reduces the proportion of allo-infections that are matching infections. * * Auto-infection〔Robinson, R.A. (1976); “Plant Pathosystems.” Springer-Verlag, Berlin, Heidelberg, New York, 184pp.〕 means that the parasite originates on, or in, the host that it is infecting. Auto-infection and all the consequences of a matching allo-infection, can be controlled only by horizontal resistance. This is because the parasite individual reproduces asexually to produce a clone (or else reproduces sexually and quickly reaches homogeneity of matching individuals) and auto-infection is thus matching infection. An epidemic is the growth of a parasite population which is made at the expense of the host population. There are two kinds of epidemic: * Continuous epidemics〔Robinson, R.A. (1987) Host Management in Crop Pathosystems. Macmillan, New York, Collier-Macmillan, London, 263pp〕 have no break in the parasitism; they have no gene-for-gene relationships; they involve evergreen trees, and some perennial herbs. * Discontinuous epidemics〔Robinson, R.A. (1987) Host Management in Crop Pathosystems. Macmillan, New York, Collier-Macmillan, London, 263pp〕 have regular breaks in the parasitism, due to an absence of host tissue during an adverse season, such as a temperate winter or tropical dry season; they often have a gene-for-gene relationship against some of their parasites; they involve annual plants, some perennial herbs, and the leaves and fruits of deciduous trees and shrubs. == ''Gene-for-gene relationship - the n/2 model'' == The n/2 model (pronounced either ‘en over two’ or 'half en') suggests the mode of operation of the gene-for-gene relationship in a wild plant pathosystem.〔Robinson, R.A. (1996) Return to Resistance; Breeding Plants to Reduce Pesticide Dependence”. agAccess, Davis, California, 480pp.〕 It apparently functions as a system of locking in which every host and parasite individual has half of the genes in the gene-for-gene relationship (i.e., n/2 genes, where n is the total number of pairs of genes in that relationship). Each gene in the host is the equivalent of a tumbler in a mechanical lock, and each gene in the parasite is the equivalent of a notch on a mechanical key. Provided that each n/2 combination of genes occurs with an equal frequency, and with a random distribution, in both the host and parasite populations, the frequency of matching allo-infections will be reduced to the minimum. For example, with six pairs of genes, each host and parasite individual would have three genes, and there would be twenty different locks and keys; with a twelve-gene system, there would be 924 six-gene locks and keys. Given an equal frequency and a random distribution of every lock and key, the frequency of matching allo-infection would be 1/20 and 1/924, respectively. These figures are obtained from the binomial expansion illustrated by Pascal's triangle.〔Person, C.O. (1959); “Gene-for-gene relationships in host-parasite systems.” Can. J. Bot. 37; 1101-1130.〕〔Robinson, R.A. (1996) Return to Resistance; Breeding Plants to Reduce Pesticide Dependence”. agAccess, Davis, California, 480pp.〕 This system of locking cannot function in a crop pathosystem in which the host population has genetic uniformity. A crop pathosystem is usually the equivalent of every door in the town having the same lock, and every householder having the same key which fits every lock. A system of locking is ruined by uniformity, and this is exactly what we have achieved when protecting our genetically uniform crops with vertical resistance. It also explains why vertical resistance is temporary resistance in agriculture. This type of error is called sub-optimization and it results from working at too low a systems level. The system of locking is an emergent property that is observable only at the systems level of the pathosystem. Comparable biological emergents are the schooling of fish, and the flocking of birds, which cannot be observed at any systems level below that of the population. The n/2 model is also the most important hypothesis to emanate from the concept of the pathosystem.〔Robinson, R.A.(2010) Self-Organizing Agro-Ecosystems; Sharebooks Publishing, http://www.sharebookspublishing.com〕 It can also be argued that the gene-for-gene relationship must function on a basis of heterogeneity in the wild pathosystem because the gross instability of the 'boom and bust'〔Vanderplank, J.E. (1963); “Plant Diseases; Epidemics and Control.” Academic Press, New York & London, 349pp.〕 of modern plant breeding would have no evolutionary survival value.〔Robinson, R.A.(2010) Self-Organizing Agro-Ecosystems; Sharebooks Publishing, http://www.sharebookspublishing.com〕 A gene-for-gene relationship can evolve only in a discontinuous pathosystem.〔Robinson, R.A. (1987) Host Management in Crop Pathosystems. Macmillan, New York, Collier-Macmillan, London, 263pp〕 This is because it functions as a system of locking. A matching allo-infection is the equivalent of a lock being unlocked. With the end of the season, all matched (i.e., unlocked) host tissues disappear. With the onset of a new growing season, all discontinuous host tissue (e.g., new leaves of a deciduous tree, newly germinated annual seedlings, or newly emerged tissue of a perennial herb) is unmatched and each host individual has a vertical resistance that is functioning. This is the equivalent of re-locking. This alternation of matching and non-matching (or unlocking and re-locking) is an essential feature of any system of locking, and it is possible only in a discontinuous pathosystem. Conversely, in a continuous pathosystem just one matching allo-infection on each host individual is required for that individual to be parasitised for the rest of its life which, in the case of some evergreen trees, may endure for centuries. A gene-for-gene relationship is useless in such a pathosystem and, consequently, it will not evolve. Crops that are derived from a continuous wild pathosystem (e.g., aroids, banana, cassava, citrus, cocoa, coconut, date palm, ginger, mango, oil palm, olive, papaya, pineapple, pyrethrum, sisal, sugarcane, sweet potato, tea, turmeric, vanilla, yams) have no gene-for-gene relationships, not withstanding a few erroneous reports to the contrary. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Plant pathosystem」の詳細全文を読む スポンサード リンク
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