TABLE I 



The Types of Cellular Interaction Which Take Place During the Early Development of 



Slime Molds. 



CELLULAR: 



SPORE 



BACTERIUM 



BACTERIUM 



VEGETATIVE AMOEBA 



AGGREGATIVE AMOEBA 



=^ SPORE (INHIBITION) 



-> SPORE (STIMULATION) 



-^ VEGETATIVE AMOEBA (ATTRACTION) 



VEGETATIVE AMOEBA ,( REPULSION) 

 AGGREGATIVE AMOEBA (ATTRACTION) 



Implemented by 



A) Chemotaxis; relay amplification 



B) Contact following; adhesion 



MULTICELLULAR: 



CENTER 



-^ 



CENTER (INHIBITION) 



ment a spore germination inhibitor. Or, spores 

 may compete for some essential factor during 

 germination. Thus, the first spores to become 

 active would remove this factor from the en- 

 vironment and limit the germination of the re- 

 maining spores. No evidence is available to dis- 

 tinguish between these two possibilities. 



The next item in Table I suggests that bac- 

 teria may stimulate spore germination. The 

 evidence for this phenomenon is limited to 

 some observations that I made several years 

 ago. I found that six to ten times more spores 

 would germinate in the presence of bacteria 

 than in their absence. How bacteria influence 

 germination is, unfortunately, not known. 



As is indicated by the next item in the table, 

 bacteria may also influence the movement of 

 amoebae. Samuel (2) demonstrated that amoebae 

 migrate toward bacteria probably in response 

 to a chemical released by the bacteria. The 

 possible relationship of bacterial-amoebal 

 chemotaxis to aggregation is of interest. It is 

 well established that aggregation in cellular 

 slime molds is largely the result of chemo- 

 taxis. Therefore, during the evolution of these 

 organisms, chemoreceptors must have evolved 

 for the receipt and translation of chemical sig- 

 nals. The first receptors were probably used 



to detect and capture bacteria. If this is so, then 

 perhaps the receptor(s) that operates in aggre- 

 gation might be nothing more than a modified 

 version of that used to detect bacteria and, as 

 such, still is somewhat sensitive to bacterial 

 attractant. This last assumption could account 

 for the observed absence of aggregation in the 

 presence of bacteria. Since the attractant re- 

 leased by the bacteria would compete for or 

 occupy receptor sites, no clear aggregation sig- 

 nal could be received until the bacteria were 

 removed. 



The next item in Table I indicates that 

 vegetative amoebae repulse one another. Samuel 

 (2) found that if amoebae are dispensed in small, 

 dense groups on an agar surface, they will 

 migrate from the group along rather direct 

 paths. This migratory activity is probably the 

 result of a "repellent" that accumulates when 

 vegetative amoebae are present at high density. 



Aggregation is the most complex series of 

 interactions that takes place in early slime 

 mold development. It is characterized by the 

 formation of migrating streams of cells (Fig. 

 ID). Stream formation is the result of two 

 mechanisms; chemotaxis (and related "relay 

 amplification") and "contact following." Relay 

 amplification describes Shaffer' s model of slime 



124 



