A Method for Selection of Biochemical Mutants of Neurospora.

13 Lederberg, J., Genetics, 32, 505-525 (1947). 14 It should be noted that these data were secured by the plating of undiluted hcultures. When such cultures are diluted and plated the hbacteria form microcolonies which are not visible to the naked eye. Mutations to the h+ condition occur in these microcolonies and in some cases this results in the formation of visible h+ colonies. These plate mutations obscure the characterization of the number of h+ organisms originally present in the hculture. They can be eliminated by omitting asparagine from the minim.al medium. Asparagine was present in the medium used for the experiments reported in this paper. Although some plate mutations occur under the crowded conditions formed by plating undiluted cultures, their number is insignificantly small. Even after 5 days less than 5 per cent of all h+ colonies are formed by.plate mutation, a source of error well within that attributable to random sampling (Ryan, F. J., and Schneider, L. K., Genetics, in press). It is interesting to note that a variance analysis of the number of h+ colonies on plates of 10-6 dilutions of different hcultures indicates that they form a homogeneous population (P = 0.54; Pfor the variance in the number of h+ colonies on plates of different samples of the same culture = 0.51). This is independent evidence that 10-6 dilutions of hcultures contain no h+ organisms but that h+ colonies formed on plates of such dilutions are derived from events occurring on the plates. 16 The spontaneous mutations may conceivably be to a condition of susceptibility to respond to the testing conditions. This notion is operationally the same as mutation to the ability the testing conditions demonstrate. 16 Ryan, F. J., Cold Spring Harbor Symposia Quant. Biol., 11, 215-227 (1946). 17 Lederberg, J., and Tatum, E. L., J. Biol. Chem., 165, 381-382 (1946); see also reference 12. Under the conditions of this experiment microcolony mutation adds an insignificantly small number of h+ colonies. 18 Lindegren, C. C., and Lindegren, G., Science, 102, 33-34 (1945). 19 Carpenter, C. M., Bahn, J. M., Ackerman, H., and Stokinger, H. E., Proc. Soc. Exp. Biol. Med., 60, 168-171 (1945); Sullivan, M., Stahly, G. L., Burkeland, J. M., and Myers, W. G., Science, 104, 397-398 (1946). 20 Monod, J., Ann. Inst. Past., 72, 879-890 (1946). 21 van Loghem, J. J., Zbl. Bakt. I. Abt. Orig., 57, 385-387 (1911). 22 Fildes, P., Gladstone, G. P., and Knight, B. C. J. G., Brit. J. Exp. Path., 14, 189196 (1933)., 23 Doermann, A. H., Arch. Biochem., 5, 373-384 (1944).