DICTYOSTELIUM DISCOIDEUM 1)4)5)
A species of amoeba which can switch from independent individuals to structured colonies, and reciprocally, according to environmental conditions.
This amoeba is a member of the Acrasiales, which consist of eight species. It offers interesting insights in the way colonies and societies may become organized, as well as the deeper significance of crowding effects.
J.T. BONNER's description of this amoebas' behavior can be thus resumed: The cycle will start when spores are sown in a sufficiently moist and nutritious environment for the bacteria on which the amoebae feed. After a time these start to multiply by division, thus producing numerous independent individuals. However, when the food supply nears exhaustion and the amoebae population is dense, they stop to multiply (probably a mass effect caused by very close interactions). Some become centers of attraction for others and they start to aggregate. After two to four hours of a wavelike process, all the amoebae will be collected in cell masses of different sizes and acquire communal and integrated properties and behavior. The agent of aggregation is acrasin (cyclical adenosine mono-phosphate – cAMP), i.e. a biochemical product which through successive pulses, in a growing situation of crowding, becomes locally more concentrated and attracts the amoebae (Chemotaxis). At the end of the process, most of the amoebae are part of a unique group. The final specialized role of individuals within the colony depends on the timing of their integration. The aggregated masses of amoebae show differentiated and variable collective behavior, according to positions within the whole, but start to migrate as a whole toward light and heat, even if gradients are very small. Finally, if adverse environmental conditions persist, the aggregated mass will stop moving and produce a stalk tipped by a small sphere, where spores become formed. These spores will disperse and restart the cycle when favorable conditions reappear. Individual and collective behavior are thus much closely related than commonly believed. It is noteworthy that the production of cAMP is started by one sigle individual, possibly one more strongly affected by starvation than any other. The process then propagates to more and more individuals, in a way reminiscent of crystallization in a solution starting from a small nucleus.
According to BONNER, a quite complex division of labor becomes apparent, depending of the position of the individuals within the colony, i.e. related to their positional value (1955, p.104-110).
I. PRIGOGINE and A. BABLOYANTZ did study this phenomenon in terms of dissipative structuration (1972).
For details about the biochemical aspects and models, see A. WINFREE (1980).
Similar propagation processes seem to be present under different guises in:
- periodic dissipative structures in unstable chemical solutions
- propagation of fires, avalanches, pandemics, etc… (percolation)
- optical-electrical reactive collective behavior in fish shoals
- acquisition of resistance to toxic agents in bacteria and insects panics and mass behavior in human societies
In all these cases this social-type propagation is a cyclical phenomenon progressing in a wave-like way from one or more foci.
It would be very interesting to discover in each case the self-generated and reproducing stimulating agent, corresponding to cAMP.
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Bertalanffy Center for the Study of Systems Science (2020). Title of the entry. In Charles François (Ed.), International Encyclopedia of Systems and Cybernetics (2). Retrieved from www.systemspedia.org/[full/url]
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