H. H. PLOUGH 



Amhersf College 



Chapter 17 



Genetic Implications of 

 Mutations in S. Typhimurium* 



The contribution that an account of studies in bacterial genetics can make 

 to the problem of heterosis must be indirect, since actual sexual or other 

 fusion in bacteria has not been observed and the weight of evidence is against 

 the view. Even the very interesting genetic evidence of recombination dis- 

 covered by Tatum and Lederberg (1947) in the K12 strain of the colon 

 bacillus, and now being developed by the capable studies of Lederberg (1947, 

 1949) and others, is still susceptible of other interpretations. Diploid strains, 

 if they occur at all, are certainly so rare as to be unimportant in the produc- 

 tion of hybrid vigor in bacterial populations. 



The apphcations of bacterial genetics to the problem of heterosis must be 

 rather in the information they make available concerning the kinds and fre- 

 quencies of gene mutations, and the ways in which they interact with each 

 other within populations. It has been generally recognized by geneticists 

 only recently that the bacteria are excellent material for studies of these 

 problems, though bacterial mutation was first mentioned by Massini in 1907, 

 and distinctive and precise food requirements for bacterial strains have been 

 known since 1913 (Hinselwood, 1946). Studies in the genetics of bacteria 

 have, of course, been greatly stimulated by the pioneer work on mutations 

 in fungi by Thom and Steinberg (1939), and particularly on Neurospora by 

 Dodge, by Lindegren, and by Beadle (1949) and his associates, as well as by 

 the important work on yeast as presented in Dr. Lindegren's chapter. 

 Long before the currently enlarging wave of interest in bacteria as objects 

 of genetic study, Gowen had shown that mutations of the same order of 

 frequency as in higher plants or animals were induced by radiation in Phy to- 



* This research was supported bva grant from the Atomic Energy Commission, Division 

 of Biology and Medicine #AT (30-1) -930. 



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