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Studies of a Genetic Disease of Trifolium repens Simulating a Virosis

S. S. Atwood and K. W. Kreitlow
American Journal of Botany
Vol. 33, No. 2 (Feb., 1946), pp. 91-100
Stable URL: http://www.jstor.org/stable/2437323
Page Count: 10
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Studies of a Genetic Disease of Trifolium repens Simulating a Virosis
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Abstract

A selected plant of white clover, which showed severe mottling of its leaf blades similar to the symptoms caused by virus, was crossed with an unrelated healthy plant, and the F1 segregated into 12 mottled and 33 healthy. A total population of 4,193 plants from various B (backcrosses), F2, and F3 families was classified in the greenhouse. All segregations were well defined, with no cases of intermediate injury. Ratios of 9:7, 3:1, 3:5, 1:3 and no segregation were obtained in different families, depending on the genotypes involved. Most of the obtained ratios fitted the expected closely, but in general there was a deficiency of mottled plants, particularly in the families segregating 9:7 or 3:5. These results are explained by assuming two independent dominant factors, both of which must be present for the development of the necrotic lesions. Evidence from B, F2, and F3 families suggested that the abnormal and healthy plants bearing the largest number of dominant factors tended to be lethal. No significant differences in stand were obtained, however, between the several segregating ratios. The number of leaves at the age of six weeks was used as a measure of vigor; the 1,673 healthy B and F2 individuals averaged 8.97 leaves per plant and the 605 abnormal plants 4.76; the difference between these averages was highly significant. The differences in average number of leaves were also highly significant between the different segregating ratios among both healthy and abnormal types. These averages were inversely correlated (r = -0.944) with the number of dominant factors borne by the different classes, indicating a highly significant dosage effect caused by the same genes that conditioned the mottling but with the dosage effect superimposed on the sharply bilateral segregation observed with presence or absence of mottling. All attempts to transmit the mottling by inoculating or grafting gave negative results, giving further evidence that the agent in this case was not a virus. This condition is of interest since it is another example of the fact that some heritable factors may induce types of morphological responses similar to those induced by certain disease agents.

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