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I've read that the amoeba is capable of learning. Since these protists have no nervous system, it's safe to assume that even highly simplified learning mechanisms of the Aplysia are miles off in the evolutionary distance.

How does this species learn? I would presume that there is a complicated chain of receptor mediated transcription regulation, but what are the specifics? What types of receptors does an amoeba have to sense its environs?

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Amoebas are not animals, this term means wide range of similar, but not related unicellular organisms. Do you mean something like the learning behaviour of Physarum slime molde link? –  Marta Cz-C Dec 18 '11 at 13:50
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Just a note: qualifying a species of "lower organism" could make some purist cringe ... I dont have a straight answer, but you might be interested in this paper which describes a model of event memory in amoeba –  agrimaldi Dec 18 '11 at 17:00
    
@MartaCz-C Yes, I had written "animal" in and taken it out for a better substitute and ended up leaving it in. You are absolutely correct about its misuse. I have not encountered the study in your link, but I will take a look at it. –  jonsca Dec 19 '11 at 1:23

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up vote 14 down vote accepted

I'd like to know what is the reference for amoebic learning. I cannot comment directly on this, but there is some evidence for "adaptive anticipation" in both prokaryotes and single-celled Eukaryotes which do not have a nervous system.

In the case of E. coli, it has been shown that the bacteria can anticipate the environment it is about to enter. E. coli in the digestive tracts of mammals will typically be exposed to initially a lactose, and then later to a maltose environment as the bacteria pass down through the animal tract. This suggests that upon encountering a lactose environment, maltose operons are induced. I.e., upon encountering lactose, maltose is anticipated. This suggests a "genetic memory" of the sequence of sugar types where lactose is always encountered before maltose.

Further cultures (500 generations) of E. coli in the absence of maltose but in the presence of lactose reduced the maltose operon activity to negligible levels, suggesting that this is an adaptive prediction of environmental changes.

Mitchell, A et al., Adaptive Prediction of environmental changes by microorganisms, 2009, 460, 1038

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In addition to the excellent response up top (by Poshpaws), one can also imagine how these systems work by looking at recent synthetic examples of single-celled organism memory.

It is possible to design various bistable switches using protein pathways, RNAi, or other means that will latch a particular state. In that way, an organism could effectively "remember" one bit of data by querying the state of the toggle.

For a specific example, see the Gardner 2000 paper [1]. It's a transcription-level circuit, that in response to a certain stimulus can latch either high or low. While the synthetic version itself is not terribly robust, one could see how in nature a highly evolved / refined circuit could maintain state and effectively serve as "memory" for a single-celled organism.

[1] Gardner, et. al 2000 "Construction of a genetic toggle switch in Escherichia coli."

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This is an interesting point. A big issue with the toggle switch is that the network topology from Gardner's paper is rarely if ever seen in biology. However, there remains plenty of examples where hysteresis may occur in naturally relevant systems. –  bobthejoe Feb 24 '12 at 22:42

Someone is almost sure to prove me wrong about 30 seconds after I post this, but I don't think that the mechanistic aspects of learning are really all that well known in these study systems. The idea that it is occurring at all is recent enough (I've enjoyed Tanya Latty's Ph.D. work on this, for instance: http://www.tanyalatty.com/Home/research) that I think that much of the work on the mechanisms is still to come. I have an interest in learning myself, and I know that I haven't read anything specifically addressing your question - which, again, means that someone will provide a reference immediately following this, so just look for the next answer. :-)

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