On the same drier side of humour; if you give a dog a hammer, nails and the wood, can it build its own kennel and even if it could, would it be a work of art? Obviously, both the title and the question that follows are tongue-in-cheek! However, there is a serious side to this article. Though it may sound a little metaphysical, there is reason to consider all living things to be in some sense, sentient or perhaps even, ‘intelligent’. Star Trek’s engineer Scotty, might have responded, “but not as we know it, Jim”! Most biologists would agree that the words ‘thought’ and ‘mind’ should only be applied to animals with a nervous system. However, they would probably also agree that evolution has crafted the genetics of all living things such that their responses to the environment are logical (Mr Spock would agree?) – plants grow towards the light; when threatened with desiccation a snail retracts into its shell and secretes an operculum to retain moisture. These are behaviours crafted by evolution and built into an organism’s genetic make-up. They are of course, entirely logical and absolutely necessary to their survival. However, there are behaviours demonstrated by even the simplest of organisms that require scientists to adopt language that is usually reserved for animals with a nervous systems. In a previous article written for a local nature magazine (https://petermobbs.com/can-an-amoeba-think/) I described how the single-cell cilliate, Stentor sp., habituates to repeated stimuli. Habituation is considered to be a very basic form of learning and memory. It was Victorian microscopists who first ventured the suggestion that the behaviour of protists could not be explained in same the terms that one could explain the mechanisms of a clock. At the time I wrote the article several labs were studying Stentor with a view to better understanding the molecular mechanisms underlying its habituation to repeated stimuli. Back then, I would probably have denied that a protist could demonstrate any more complex form of learning. However, I think I was wrong.
Having laid the ground for what follows, let me return to the more humorous side of things. Clearly, a dog can’t build it’s own kennel and its architectural skills are somewhat limited. On the other hand, the testate amoeba Acella sp., builds the most exquisite ‘kennel’ using only the tools built into its genetics. This ‘teste’ is made from the polysaccharide, chitin. I took the picture of it shown above using a technique in which multiple micrographs are combined such that everything is in focus at once. Despite and perhaps also because of this, some of the details of it have been lost. It is a truly beautiful and complex structure and though my interests in the behaviour and morphology of insects have often been a cause of awe, I have to say that this amoeba, takes the cake! I have often encountered these testes in pond water samples but they have either been embedded in debris or otherwise contaminated and this particular one was the first in which I could clearly see that the amoeba was ‘at home’ (see the picture below of a single plane of focus at a level about half way through he depth of the teste). One thought that occurred to me when I first saw this structure was ‘how could a unicellular organism possibly build such a complex structure? It seems that that question is not easy to answer for I have yet to find any description of the process by which it is formed. Further, the function of the teste has been the subject of debate. The obvious suggestion is that it is for defence against predators and doubtless, having a rigid chitin or other form of shell, will protect the amoeba. However, there is strong evidence that it acts as an offensive mechanism. Actin filaments within the amoeba’s cytoplasm (you can think of them as tiny contractile ropes) are linked to the chitinous shell. The processes of the amoeba can ‘grab’ their prey through the hole in the base of its teste and pull it in. The force that the amoeba can exert is such that the teste is deformed but the cells of its prey are broken, the prey is pulled within the teste, and then engulfed and digested. One cannot help but be amazed at the complexity of both the structures and the behavour that allows an amoeba to engulf diatoms much bigger than itself, up to 100 microns long (3 times its diameter). Bear in mind that the cells walls of diatoms are made of silica – as strong as, and closely related to, glass.
Having dealt with the structure and function of the amoeba’s ‘kennel’ which surely beats anything that a dog can manage, I want to move on to the ability of amoebae to learn. In this case, I do not mean to habituate to a repeated stimulus like Stentor can, though doubtless amoeba can do this too. Rather, I mean to behave like a dog! More-or-less everyone is familiar with Ivan Pavlov’s experiments on dogs and conditioned memory. He famously showed that after a few presentations of food accompanied by the ringing of a bell, the bell alone was sufficient to make the dogs salivate – the dog had learned to associate the bell’s ringing with food. My feeling would be that most neuroscientists would not expect such behaviour could be demonstrated by an aneural animal. However, it appears that an amoeba can learn and remember the association between a sign of food and another stimulus.
In 2019 a group of scientists led by De la Fuente published a paper in the journal Nature1 that would seem to show that amoebae are capable of associating a stimulus with the presence of food. The work is built on the responses of amoebae to an electric field – if you connect a battery across the fluid in a Petri dish the amoebae within it will migrate towards the negative pole of the battery. The so-called ‘galvanotactic response’ of amoebae has been known for a long time, though its purpose, if it has any, is obscure. Perhaps a less surprising behaviour of amoebae is their movement toward a food source. Many amoebae feed on bacteria, the presence of which they can sense due to the presence of a peptide (a fragment of a protein) in the water around them. The diffusion of the peptide (nFMLP) in the water creates a gradient along which the amoeba moves and homes in on its bacterial prey. Incidentally, that behaviour in itself requires some form of memory; in order to know you are heading in the right direction you have to know (remember) the concentration of the stimulus at one point and be able to compare it to that at another. So, if nFMLP is introduced into the water on one side of a Petri dish, the amoebae all migrate towards it. The appeal of the food is high! De la Fuente and colleagues combined presentations of nFMLP with the galvanotactic stimulus such that the presence of food was associated with the positive pole of the battery – the pole which under normal circumstance the amoebae were disinclined to move towards. Subsequently, amoeba ‘trained’ in this way migrated toward the positive pole when the galvanotactic stimulus was presented in the absence of nFMLP. The ‘Pavlov dog amoebae’ had learned to associate food with the positive pole of the battery – a reversal of their normal galvanotactic behaviour.
I am very impressed with the amoeba both in terms of its dog-kennel building abilities, its Pavlonian and other complex behaviours. I am inclined to think that the reason that until quite recently its behaviour and physiology had been overlooked by scientists resulted from two things; the unwillingness in modern times of grant awarding bodies to fund research on such ‘lowly’ creatures and the ever decreasing number of scientists interested in studying ‘lower’ animals. Of course, one is not entirely unrelated to the other. I suspect there is also a sort of ‘snobbery’ around these things – I have not forgotten fellow scientists who asked of me and other people interested in invertebrates, why we weren’t studying vertebrates – God only knows what they would have said/do say about studies of protozoa. These creatures have so much to offer in terms of understanding cellular physiology and behaviour and I am pleased to see that there remain some labs around the world that have recognised this.
Dogs rule but amoebae are not far behind!
1 https://www.nature.com/articles/s41467-019-11677-w Evidence of conditioned behavior in amoebae
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