Altruism is widespread in nature. Worker honey bees devote their entire life to foraging and caring for their sister, the queen, and her offspring, but do not themselves reproduce. In widow spiders, a male allows a female fertilised by him to eat him, and thus nourish herself and her offspring. A meerkat, a mongoose found in Africa, assumes the role of a sentinel, perching itself on a mound or rock, keeping a lookout for predators, instead of foraging for food, while the rest of the clan is feeding. If a predator is sighted, it alerts the others.

Many humans themselves have agreed that “greater love has no man than he who lays down his life for his friends”.

How can the emergence of altruism in all these diverse forms be explained?

A genetic switch

Most of the progress in answering this question has come from studies of a simpler organism that has been easier for researchers to study: the social amoeba Dictyostelium discoideum. The take-home message of these studies is that if a gene makes a worker bee altruistic, it also helps the copy of the gene in the queen and her offspring to be passed on to the next generation, even if the worker herself does not reproduce.

Such so-called ‘green-beard’ genes allow the individuals bearing them to recognise and preferentially cooperate with each other.

Alternatively, a green-beard gene could provoke individuals to behave harmfully towards those carrying a different version of the gene.

Thus, scientists have postulated, green-beard genes encode some kind of tag that helps the genome to know their identity (i.e. self-recognition).

Altruistic amoebae

Dictyostelium discoideum is a free-living, fast-growing, unicellular amoeba. In the wild, it feeds on bacteria that grow on decaying vegetation. In the laboratory, researchers have been known to feed them a bacterial ‘lawn’ grown in a Petri dish. When the bacteria run out, the amoebae stop multiplying and gather in the hundreds of thousands to form multicellular aggregates visible to the naked eye.

The aggregates then transform into fruiting bodies, each a few millimetres tall. A fruiting body is composed of a slender stalk made of dead cells, and it holds aloft a droplet of spores. About 20% of the amoebae in an aggregate altruistically sacrifice themselves to form the stalk. The remaining 80% become the spores.

Small fauna, such as ants and earthworms, disperse the spores to new food sources where they germinate to release amoebae. The newly released amoebae then go on to repeat the cycle of growth, division, and dispersal.

Beware of cheaters

Not all the amoebae in an aggregate necessarily share kinship. Some could be only distantly related, so the aggregates can potentially be genetic chimaeras — structures in which not all amoebae have the exact same genome. And when the genome differs, there is a risk that some strains may have found a way to ‘cheat’ and avoid becoming stalk cells, and as a result become represented in more than their fair share among the spores. 

How does D. discoideum ensure that cheaters do not prosper?

In 2017, researchers from the University of Manchester, in the U.K., reported that two genes in the D. discoideum genome — called tgrB1 and tgrC1 — displayed all the properties one would expect in a green-beard gene. On May 11 this year, researchers at the Baylor College of Medicine in the U.S. reported evidence that D. discoideum amoebae use these genes to navigate the risk of chimerism.

Their findings demonstrate how sophisticated genetic machinery can confer these deceptively simple life-forms the ability to express a universal virtue.

Both these studies were published in the journal Nature Communications.  

Separating kith from kin

The tgrB1 and tgrC1 genes are located next to each other in the D. discoideum genome, and are expressed together (so if one isn’t expressed, the other isn’t either). They contain information for cells to make two cell surface proteins called TgrB1 and TgrC1.  The TgrB1 protein on one cell binds to the TgrC1 protein on another.  If the binding is strong, the TgrB1 protein is activated, and confers altruistic behaviour — manifesting as the amoeba’s willingness to form the stalk.

The binding between the TgrB1 and the TgrC1 proteins of cells of the same strain is strong, and leads to self-recognition and cell-cell cooperation. Pure cultures of cells that lack the tgrB1 and tgrC1 genes fail to develop because they are unable to recognise each other as being alike.

The tgrB1 and tgrC1 genes are also very polymorphic: within the same population of D. discoideum amoebae, they have multiple variants. In fact, they are among the most variable genes in the Dictyostelium genome. When the researchers in the 2017 study examined 20 strains of the amoeba isolated from a common location, they found 18 different variants of each gene.

The researchers were able to correlate differences in the tgr gene sequences between two strains to the efficiency with which their cells segregated from each other in mixed aggregates and formed separate fruiting bodies. Specifically, when the binding of TgrB1 and TgrC1 proteins across the cells of diverged strains was weak, TgrB1 failed to be activated, and the cells split away from each other instead of cooperating.

When the researchers deleted the tgrB1 gene but left the tgrC1 gene intact, the amoeba did not cheat on ‘non-self’ amoebae that carried a different tgrC1. Instead, it cheated those with the same tgrC1 variant as itself — i.e. its kin. Every family has its black sheep!

From Haldane to Voltaire

On the other hand, amoebae in which the researchers activated the tgrB1 gene alone (by introducing a mutation) were relegated to the stalk when mixed with their kin.

These findings demonstrate the logic that green-beard genes use to produce altruism plus the corresponding ability to keep altruistic amoebae from being exploited by greedy ones.

The British-Indian geneticist J.B.S. Haldane (1892-1964) is reputed to have said that he would jump into a river to save eight cousins but not seven — but also that he would jump in to save two brothers, yet not one. His quip highlights the fact that we share one-half of our genes with a sibling and one-eighth with a first-cousin.

As far as we know, amoebae do not use rational numbers. Instead, they use their rapidly evolving genes to estimate kinship with other amoebae. If the genes have not diverged at all, or have diverged very little, there is kinship and it is good to cooperate. If they have diverged significantly, there is less kinship and hence cooperation is risky.

To paraphrase the philosopher Voltaire (1694-1778), if tgr genes did not exist, it would be necessary for the amoebae to invent them.

D.P. Kasbekar is a retired scientist.