Recently I was reading a paper by Kuzdzal-Fick et al. about my all time favourite social amoeba Dictyostelium discoideum, or as I like to call them, dictys, and it got me thinking: is altruism a real thing among non-sentient beings? Altruism, as Wikipedia defines it, is “the principle or practice of concern for the welfare of others”. In biological organisms, it can be defined as “an individual performing an action which is at a cost to
themselves”. Now, a little a bit of background for those of you who are not familiar with these social amoebas: dictys are a species of single-celled eukaryotes that primarily live in soil and can be found just about anywhere. When dictys favourite food source, bacteria, are readily available, they live happily as individual amoeba, but when this food source becomes scarce they send out signals to each other and begin to aggregate together. Once aggregated, they form a slug that is able to move around as one multicellular organism and then form a fruiting body. Here’s a link to a cool little video of this occurring: Dicty Lifecycle.
The fruiting body formed is composed of 80% fertile spores and 20% sterile stalk. Here is where my question about altruism comes into play. Do the 20% of dictys that become the sterile stalk sacrifice themselves to save the other 80%? Altruism is often driven by culture and religion, or by high relatedness. In this paper, the experimenters were trying to discover if high-relatedness (having a common origin) could stop so-called ‘cheaters’ from destroying multicellular populations. These cheaters work by forming disproportionately more spores, while forcing others to form more than their share of the sterile stalk, but are unable to form fruiting bodies on their own. To test this question, two experimental evolution experiments were undertaken to measure the ‘cheating ability’ of evolved lines in social competition with their ancestors. One line was constructed to have low relatedness and the other high relatedness. The results showed populations with low-relatedness had a higher proportion of cheaters than populations with high-relatedness. They also showed if a single-cell bottleneck occurred every 100 generations, cheaters that could eliminate cooperation were unable to spread.
Does this cheating occur because dictys work in an altruistic manner or could there be another explanation? After doing some digging around in the literature I came across this interesting paper: here. This paper explained if dicty cells were lacking the copine A (cpnA) gene they were unable to form fruiting bodies as this gene is necessary in pre-stalk cells (cells that will become the stalk). This explanation that ‘cheaters’ are missing necessary genes, which results in them being unable to become pre-stalk cells, and therefore unable to form fruiting bodies, makes a lot more sense.
It is a hard to imagine these tiny single-celled amoeba are working for the ‘greater good’, but a mutation that causes them to ‘cheat’ is something I can comprehend. What are your thoughts on altruism? Do you believe dictys could be ‘performing the ultimate sacrifice which is at a cost to themselves’?