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?
Answer
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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