536 
PACIFIC SCIENCE, VoL XXI, October 1967 
ment. Of developmental stages, spore formation 
is also the most sensitive to treatment with 2- 
mercaptoethanol, as shown by Gerisch (1961). 
Spore formation and normal morphogenesis are 
closely linked, as spores have been found only 
in normal sori, and normal-appearing sori al- 
ways contained spores. 
Whereas no morphogenesis takes place in 
the presence of 1.5 X 10 -2 M ethionine, the 
simultaneous addition of methionine in increas- 
ing amounts leads to the formation of pro- 
gressively more normal appearing fruiting 
bodies (Fig. 3). The structure of the resulting 
sorocarps follows the same pattern as described 
above for ethionine-treated populations. When 
the concentration of methionine approaches 
twice that of ethionine the fruiting bodies look 
normal but are somewhat smaller in size (equal 
to stage 2) compared with controls growing 
on methionine alone or on the buffer alone. If 
the experiment is repeated with 1/5 the con- 
centration of ethionine and a correspondingly 
lowered amount of methionine the results re- 
main unchanged. This demonstrates that the 
ratio of ethionine to methionine is the critical 
factor, rather than the absolute amounts of 
either compound. Ethionine clearly behaves 
like a competitive inhibitor of methionine. 
Next, ethionine sensitivity at the various de- 
velopmental stages (after cessation of vegetative 
growth) was determined. For this, populations 
were exposed to 1.5 X 10 -2 M ethionine at 
selected points in their development for various 
periods of time. Some of the results are sum- 
ETHIONINE 
METHIONINE 
TYPICAL FORMS 
TYPE 
CONC. (M) 
CONC. (M) 
NO. 
0 
0 
Hit* 
1 
0 
1.5 XlO' 2 
HI 
1 
1.5 X I0' 2 
0 
^ 
6 
1.5 x I0" 2 
0.75xl0' 2 
4 
I.5xl0- 2 
I.5xl0' 2 
JLASlh 
3,2 
I.5xl0" 2 
3.0x10" 2 
ML 
2,1 
Fig. 3. Influence of ethionine and methionine, 
alone and in combination, on the development of 
Dictyostelium discoideum. The results represent the 
final stages of development regularly reached after 
24 to 30 hours. 
Fig. 4. Sensitivity of various stages of slime mole 
development to the inhibitory action of ethionine. 
The time scale indicates hours after deposition of 
the washed myxamoebae populations together with 
the onset of aggregation (ag), migration (mig), 
and culmination (cul) of the control population. 
Solid lines denote the presence of ethionine, dotted 
lines of methionine. The resulting types of sorocarps 
are numbered according to Figure 1. They represent 
the final levels of development reached under the re- 
spective conditions. The delay in time to reach these 
final stages with respect to the controls is approxi- 
mately equal to the time the populations were exposed 
to ethionine. 
marized in Figure 4. Before and after the 
treatment with ethionine the cells were kept 
on equimolar concentrations of methionine. This 
was done in order to assure a quick removal of 
any possible free ethionine in the internal 
amino acid pool (Wright and Anderson, I960) 
that might obscure the results. In fact, it was 
found that when the cells were transferred 
(after treatment with ethionine) to buffer alone, 
instead of to buffer plus methionine, morpho- 
genesis was permanently inhibited. This 
strongly indicates the presence of a rather large 
internal pool where the ethionine can persist 
for hours, unless it is exchanged for exogenous 
methionine. The main conclusions to be drawn 
from Figure 4 are: (1) the entire morpho- 
genetic part of the life cycle up to the actual 
formation of the stalk is sensitive to ethionine, 
and (2) the inhibitory effect of ethionine is a 
gradual one, the damage becoming more severe 
the longer the cells are in contact with the 
substance. An important point is that even if 
ethionine is administered just prior to com- 
mencement of stalk production the inhibitory 
