Gene Action 



165 



sexually reproducing microorganisms have 

 yielded changed phenotypes whose under- 

 lying mechanisms proved to be transmissible 

 even though no changes in the genotype had 

 occurred. One of these phenomena is enzy- 

 matic adaptation as studied in yeasts and 

 other microorganisms. When a population 

 of these organisms is placed in contact with 

 some substrate to which it does not respond 

 typically on account of (absolute or relative) 

 lack of suitable enzymes, it may gradually 

 acquire the necessary enzymes. Once having 

 acquired them, and as long as the specific 

 substrates are present, the enzymes are re- 

 produced and descendents of the enzymati- 

 cally adapted cells continue to possess high 

 enzyme activity. However, after removal of 

 the specific substrates, the cells soon lose 

 their adaptive enzymes. The ability of cells 

 for enzymatic adaptation is controlled by 

 genes (Winge and Laustsen, '39; Linde- 

 gren, Spiegelman, and Lindegren, '44). These 

 genes, then, do not control in an absolute 

 sense the presence of specific enzymes in the 

 cell bvit the potentiality of enzyme forma- 

 tion. Monod ('42) and Spiegelman and Dunn 

 ('47) have shown that the formation of one 

 enzyme frequently occurs in competition 

 with formation of another enzyme so that 

 enzymatic adaptation may involve not only 

 the rise of one type of enzyme but the de- 

 cline in quantity of others. In analogy with 

 these findings, Spiegelman ('48) views dif- 

 ferentiation "as the controlled production of 

 unique enzymatic patterns." 



Parallel studies on Paramecium aurelia 

 are concerned with antigenic properties (Son- 

 neborn, '47). It has been found that various 

 strains all contain four different antigens, 

 designated as 1, 2, 3, and 4, but that in any 

 one strain only one antigen, the "primary," 

 either 1 or 2 or 3 or 4, produces a very high 

 titer of antibody in homologous antiserum 

 and that only the primary antigen is con- 

 cerned in the immobilization reaction when 

 the animals are subi'ected to antiservim. There 

 exist, then, four different types of strains as 

 characterized by their primary antigen. Each 

 strain is highly stable, breeding true during 

 asexual fission to its specific antigenic pat- 

 tern of a given primary and the correspond- 

 ing three secondary antigens. It is, how- 

 ever, relatively easy to induce changes from 

 one type to another: by exchange of cyto- 

 plasm between conjugants derived from dif- 

 ferent types, or, in type 3 animals, by the 

 mere fact of conjugation with type 1 or 2 

 animals, or by exposure to antisera, or after 

 x-raying. Sometimes the changes go spe- 



cifically from one type to another given type, 

 sometimes two or more diverse antigenic 

 strains may be isolated from the asexually 

 produced progeny of a single altered indi- 

 vidual which apparently exists in a tempo- 

 rarily unstable state. Genetic analyses in- 

 volving crosses or autogamy show clearly 

 that the changes in antigenic type are purely 

 cytoplasmatic, taking place within the frame- 

 work of an unchanged nuclear genotype. 

 Similarly as in enzymatic adaptation of mi- 

 croorganisms, a differentiation of antigenic 

 systems, perhaps depending on the establish- 

 ment of one or the other of several alternative 

 equilibria, can serve as a model for develop- 

 mental differentiation of multicellular or- 

 ganisms. 



The mechanisms by means of which the 

 nuclear genes control the potentiality of 

 enzyme formation or of antigenic types are 

 still obscure. Hypotheses have been proposed 

 which assume the production by the nuclear 

 genes of partial replicas of themselves which 

 reach the cytoplasm and are endowed with 

 the genie property of reproducing themselves 

 or the cytoplasmic complexes of which they 

 become part. Proofs for the existence of 

 "plasmagenes" of this type are not available 

 at present (Schultz, '50). Weiss (review in 

 '50) has outlined a concept which involves 

 interaction of self-perpetuating cytoplasmatic 

 elements with primordial gene products. "Ac- 

 cording to this concept, differentiated proto- 

 plasmic units would owe their origin and 

 their specific shapes to two entirely different 

 processes, occurring in different places. They 

 would be propagated in the nuclear center, 

 and be remodeled in the cytoplasm." 



An interpretation of gene action in de- 

 velopment when transcending the level of 

 intracellvilar states may be fitted into the 

 concept of embryonic fields. Theoretically, 

 specific alleles may either change the extent 

 or type of a field or they may change the 

 reactivity of cells to an existing unchanged 

 field. These alternatives may be illustrated 

 by the example of the sex-comb in Dro- 

 sophila melanogaster. It consists of a row of 

 heavy spines located on the first tarsal seg- 

 ment of the foreleg of males (Fig. 44^). The 

 sex-comb is absent in females. The different 

 developmental fate of the foreleg could be 

 due either to the fact that the male genotype 

 of an anterior leg disc leads to the existence 

 of a male type field which calls forth sex- 

 comb formation in the appropriate region, 

 in contrast to the female genotype which 

 does not lead to a sex-comb-inducing field; 

 or to the presence of an appropriate field 



