Host-induced modifications of viruses.

Viruses exhibit extensive adaptability to growth in various hosts or tissues. It was widely held in the past that virus adaptability reflected a peculiar plasticity of virus heredity, which allowed it to be directly influenced by its host cells. The alternative interpretation of virus adaptation to new host cells as due to spontaneous mutations, which provide a range of genotypes for the new hosts to select from, always had authoritative proponents (see Findlay, 1939). This viewpoint finally gained wide recognition (see Burnet, 1946)) partly as a consequence of the development of phage genetics and of the interpenetration of various branches of virology. It is now recognized that most variation in virus properties is caused by viral mutations, and that virus plasticity results from the variety of genotypes present in the large viral populations. It was, therefore, an unexpected development when a new type of virus variation was discovered in bacteriophages. This has been called host-induced or host-controlled variation (Luria and Human: 1952; Ralston and Krueger, 1952; Anderson and Felix, 1952; Bertani and Weigle, 1953). Its outstanding characteristics are that it is strictly phenotypic, non-hereditary, and that it is determined by the host cell in which a virus has been produced. In this paper, I shall summarize the features of this phenomenon; I shall compare it with other instances of nonhereditary phage variation; and I shall attempt to assess its possible bearing on certain problems in other areas of virology.