VIRUS‐ANTAGONISM TESTS AND THEIR LIMITATIONS FOR ESTABLISHING RELATIONSHIP BETWEEN MUTANTS, AND NON‐RELATIONSHIP BETWEEN DISTINCT VIRUSES

IN 1929, the writer reported that a yellow-mosaic virus that was derived from the common light-green mosaic (Nicotiana Virus 1)2 of tobacco, could not become established in tobacco plants in which the common-mosaic virus was already established. It was pointed out further that the same yellow-mosaic virus does become established in tobacco in which the virus of mild dark-green mosaic virus (N. Virus 6) is established. Kunkel (1934) working with Nicotiana sylvestris Spegaz. and Comes. found that a yellow-mosaic virus (aucuba mosaic), similar to the one used by McKinney (1929, 1935), induced local necrotic lesions when wiped on the leaves of the healthy plants, but when this virus was wiped on the leaves of plants carrying the systemic virus of tobacco-common mosaic, no necrotic lesions appeared. This reaction and also the inhibition of one systemic virus by another were considered to be expressions of acquired immunity. In a later paper Kunkel (1935) indicated that these inhibiting reactions serve to show the natural relationship or nonrelationship between viruses; and in 1936 he used the systemic method in his studies on the relationships and nonrelationships between certain viruses of the peach. Since Nicotiana Virus 1 mutates (McKinney, 1935, 1941) it is possible to study combinations of mutants, which are obviously related. In certain of these combinations, the writer has found very cleancut evidence of virus antagonism or incompatibility, but in other combinations domination by one virus is less evident. It was found also in certain other mosaic viruses, not generally regarded as related, that symptom domination was irregular when certain combinations were made, even though the viruses were compatible. Because so many virologists are attempting to determine natural relationships in the plant viruses on a basis of symptom domination alone, tests were carried out to obtain further information on the method as it applies to the necroticlesion reaction and as it applies to symptom reactions 1 Received for publication July 2, 1941. These studies, though strictly biological in scope, are a part of the program supported by funds obtained under Bankhead-Jones Project S. R. F. 2-17, U. S. Department of Agriculture, Bureaus of Plant Industrv, and Agricultural Chemistry and Engineering, cooperating. The paper was presented before the Third International Congress of Microbiology, September 6, 1939, New York, N. Y. Abstract published in the Proceedings of the Congress. Additional information obtained subsequent to the Congress is included. The author wishes to acknowledge the assistance of Matthew Koerner in conducting the tests. 2 For the present, the writer chooses to designate the viruses used in these studies in accordance with the terminology used by Smith (1937). when the second virus introduced is systemic in the test plant. Reasons for regarding these reactions as types of antagonism are presented. TESTING SYSTEMIC VIRUSES AGAINST SYSTEMIC VIRUSES.-Related viruses.-Young plants of Nicotiana tabacum L., variety Wisconsin Havana Seed, were inoculated with Nicotiana Virus 1 (James Johnson's source), the virus which causes the common or wild-type, light-green mosaic of tobacco. When mosaic symptoms appeared the plants were inoculated again with a strain which was known to have descended from N. Virus 1. ; Several methods of inoculation were used in preliminary studies to determine the one most suitable for use in this work. Introducing the paired viruses simultaneously did not prove altogether satisfactory as there was sometimes a difference in incubation time, and sometimes misses occurred with the cucumber and potato mosaic viruses. In view of these irregularities, one virus was established first, and, when mosaic symptoms were well established in one or two leaves, the second virus was introduced. Since the second virus could always be established in the apical zone of the stem and small leaves for at least a short period, the second inoculations were made in that region by means of wiping and needle punctures. Native virus extracts were used without diluting. The following mutants from Nicotiana Virus 11 were studied: Two severe yellow-mosaic viruses, designated for the purposes of this paper BSY and CSY,3 one mild-green mosaic virus, designated AMG, all derived directly from the common or wild type, and one mild-yellow-mosaic virus designated aMY which was derived directly from AMG. When Nicotiana Virus 1 had been introduced first, the yellow-mosaic mutants induced a few vellow spots in a few of the young leaves, but in all plants N. Virus 1 and its characteristic mosaic symptoms quickly dominated and all introduced mutants were completely suppressed in subsequent tissues as indicated in table 1. When the mutants were established first and the plants were then reinoculated with Nicotiana Virus 1, the initial progress of the latter was retarded, but eventually it suppressed the mutants and dominated completely in the new leaves. However, when tobacco plants with BSY mosaic were inoculated with N. Virus 1 diluted 1,OOOX in distilled water, the latter virus sometimes failed to become established during 3BSY was described previously (McKinney, 1935) as type A yellow-mosaic virus. Mutant CSY differs from BSY in its inability to induce typical local necrotic lesions on Nicotiana sylvestris. Like BSY it also induces severe vellow mosaic in tobacco.