CELL INTERACTIONS IN SLIME MOLD 



(ACRASINA) DEVELOPMENT 



A.J. Kahn 



Department of Zoology, Syracuse University 

 Syracuse, New York 



Ontogenetically meaningful exchanges occur 

 between cells and tissues during the develop- 

 ment of all multicellular organisms. In the 

 discussion to follow, evidence will be presented 

 that cellular slime molds are not exceptional 

 in this regard. 



The now rather familiar life cycle of cellu- 

 lar slime molds is shown in Fig. 1. While this 

 drawing was prepared to illustrate Dictyo- 

 stelium purpureum, in essential features, it is 

 representative of all members of the family 

 Dictyosteliaceae. Our attention today will be 

 focused upon the early stages of the cycle, 

 beginning with spores (Fig. lA) and terminating 

 with the formation of the pseudoplasmodium 

 (aggregate) (Fig. ID). 



A nutrient agar plate inoculated with bac- 

 teria and Dictyostelium (or Polysphondylium) 

 spores will generally show all stages of the 

 life cycle after two or three days of incubation 

 at 25°C. To obtain greater developmental uni- 

 formity, amoebae may be pregrown in liquid 

 culture, harvested at the end (or during) the 

 growth phase, washed and dispensed upon a 

 non-nutrient substrate. D. purpureum amoebae, 

 under these conditions, begin to aggregate 

 after a few hours and complete development in 

 less than twenty-four hours. Furthermore, 

 since most of the cells aggregate at about the 

 same time, substantial synchrony is achieved. 



In many respects, a spore is like a zygote — 

 each spore possessing the ability to germinate, 

 grow and develop into a complete multicellular 

 unit. Thus, if spores are isolated and cultivated, 

 genetically pure clones can be derived. 



Table I presents a summary of the various 

 types of cellular interaction that have been 

 detected during early slime mold development. 

 Some of these phenomena are much better known 

 than others but all, I believe, are worthy of 



inclusion in this survey. 



The first item in Table I indicates that 

 spores may interact to limit spore germina- 

 tion. Russell and Bonner (1) showed that a sig- 

 nificantly higher percentage of germination 

 occurs in sparse (dilute) groups of spores than 

 in dense groups. There are two possible ex- 

 planations for this observation. When a spore 

 germinates, it may release into the environ- 



Flg. 1. 



Life cycle of cellular slime molds. A) Spores; B) germi- 

 nated amoebae; C) feeding and cell division; D) aggrega- 

 tion; E) culmination (differentiation of fruiting body); 

 F) sorocarp (fruUng body) consisting of stalk and spore 

 mass (sorus). 



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