538 
PACIFIC SCIENCE, Vol. XXI, October 1967 
tial determinant for subsequent developmental 
events. 
The effectiveness of ethionine up to the 
point of actual stalk formation indicates that 
at least some of the proteins essential for nor- 
mal development are formed right up to the 
point of culmination. Wright and Anderson 
(I960) have shown that certain ethanol-insolu- 
ble, methionine-containing proteins are pro- 
duced at an increased rate during the stage of 
pre-culmination. It will be of interest to de- 
termine whether these proteins are enzymes 
involved in cellulose synthesis or if they are 
structural proteins responsible for cell polariza- 
tion and differentiation. Evidence from experi- 
ments with colchicine in conjunction with elec- 
tron microscopy indicates the occurrence of such 
a proteinaceous cytoskeleton in Dictyostelium 
(Hohl and George, 1966). 
SUMMARY 
Ethionine progressively inhibits the develop- 
ment of Dictyostelium discoideum, from a con- 
centration of 7.5 X 10 -4 M (which induces 
somewhat smaller but normal fruiting bodies) 
to a concentration of 1.5 X 1° -2 M (which 
results in complete inhibition). Intermediate 
concentrations produce a variety of distorted 
forms. With increasing concentrations the in- 
hibitory effect is first noticed in spore differen- 
tiation, then in cellulose sheath production, 
followed by stalk cell differentiation, and finally 
in morphogenetic movement. 
Simultaneous addition of methionine re- 
verses the effect of ethionine, the final result 
depending on the ratio of ethionine to methio- 
nine rather than on the absolute amounts of 
either substance administered. Ethionine exerts 
its effect at any time in the life cycle up to the 
actual formation of the stalk, the final ap- 
pearance of the fruiting bodies being a function 
both of the stage at which the ethionine was 
applied and of the period of time the cultures 
were in contact with it. The results indicate 
that, first, ethionine acts as a competitive in- 
hibitor of methionine, and, second, the pro- 
tein or proteins incorporating ethionine and 
thereby rendered biologically inactive are being 
produced continuously up to the time of actual 
stalk formation. 
REFERENCES 
Filosa, M. I960. The effects of ethionine on 
the morphogenesis of cellular slime molds. 
Anat. Rec. 138:348. 
Gerisch, G. 1961. Zellfunktionen und Zell- 
funktionswechsel in der Entwicklung von 
Dictyostelium discoideum. III. Getrennte 
Beeinflussung von Zelldifferenzierung und 
Morphogenese. Roux’ Archiv Entw. mech. 
153:158-167. 
Hohl, H. R., and R. P. George. 1966. Col- 
chicine inhibition of cell polarization and 
cellulose synthesis in Dictyostelium. J. Cell 
Biol. 31(2) :47A-48A. 
and K. B. Raper. 1963. Nutrition of 
cellular slime molds. I. Growth on living and 
dead bacteria. J. Bacteriol. 85:191-198. 
1964. Control of sorocarp size 
in the cellular slime mold Dictyostelium dis- 
coideum. Develop. Biol. 9:137-153. 
Sussman, M., and N. Lovgren. 1965. Prefer- 
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polysaccharide transferase during cellular dif- 
ferentiation in the slime mold, Dictyostelium 
discoideum. Exptl. Cell Res. 38:97-105. 
Wright, B. E., and M. L. Anderson. I960. 
Protein and amino acid turnover during dif- 
ferentiation in the slime mold. II. Incor- 
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