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Permanent transformations in populations of microorganisms are most frequently brought about by environmental selection of randomly occurring mutants. Changes that can be environmentally induced in all or most cells of a population and retained in environments that are non-inducing have also been reported. One instance is the alteration in serotype of protozoa such as paramecium, occasioned by a variety of treatments (1, 11, 12). Examples of bacterial resistance to drugs and other chemicals have recently been discussed in a symposium on adaptation in microorganisms (5). The precise biochemical mechanisms for these transformations are obscure, but there is general agreement that they involve shifts to new, heritable, steady state levels of cytoplasmic metabolic pathways within the genetic framework of the cell. In terms of known biochemical events, the clearest example of induction and maintenance of a new steady state is that of the adaptive ,8-galactoside enzyme system in populations of Escherichia coli. Cells induced to synthesize the ,8-galactoside permease and enzyme, when transferred to a new situation under which the parent phenotype remains nonindluced, will maintain their adaptation indefinitely (2, 3, 4, 9). In these experiments, however, traces of the inducing agent were necessary in order for the maintenance effect to continue. Earlier work with Chlorella vuilgaris (15) indicated that populations of this organism were able to adapt to the presence of the antimetabolite selenomethionine, and that the change, despite two subcultures (approx 20 generations), was maintained in medium free of the analogue. This adaptation has now been studied in greater detail. The present paper will describe the morphological events that occur during the adaptation as well as evidence that a transformation of all cells takes place rather than a selection of mutants. The following paper (17) will give data to show that the transformation can be

Mass adaptation to selenomethionine in populations of Chlorella vulgaris