Altruism is pervasive in nature. A worker honey bee spends its entire life searching for food and caring for its sister, queen, and young, but does not produce any more. In the widow spider, the male allows the fertilized female to eat him, and thus feed himself and his young. A meerkat, a mongoose found in Africa, assumes the role of sentinel, staying on mounds or rocks, guarding predators, instead of looking for food, while the rest of the clan is eating. If a predator is seen, it will alert others.
Many people have agreed that “there is no greater love than to lay down one’s life for one’s friends”.
How can the emergence of altruism in all its various forms be explained?
A genetic switch
Much of the progress in answering this question has come from studies of a simpler organism that is easier for researchers to study: the social amoeba. Dictyostelium discoideum. The take-home message from the study is that genes that make worker bees altruistic also help copies of genes in the queen and her offspring to be passed on to the next generation, even if the workers don’t reproduce. .
The so-called ‘green beard’ gene allows individuals who possess it to identify and be more cooperative.
Alternatively, the green beard gene may cause an individual to behave dangerously towards someone who has a different version of the gene.
Thus, scientists have postulated, the green beard gene encodes some kind of tag that helps the genome to understand identity (ie self-recognition).
Amoeba is altruistic
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 people a ‘lawn’ of bacteria grown in a Petri dish. When the bacteria are depleted, the amoebae stop growing and gather hundreds of thousands to form multicellular aggregates that are visible to the naked eye.
These aggregates then turn into fruiting bodies, each a few millimeters high. The fruiting body consists of a slender stalk made of dead cells, and holds a spore droplet at the top. About 20% of the amoebae in the aggregate altruistically sacrifice themselves to form a stalk. The remaining 80% are spores.
Small fauna, such as ants and earthworms, disperse spores to new food sources where they germinate to release amoebae. The newly released amoeba then repeats the cycle of growth, division, and dispersal.
Beware of cheaters
Not all amoebae in an aggregate necessarily show kinship. Some may be distantly related, so these aggregates can form genetic chimaeras – structures in which not all amoebas have the same genome. And when the genome is different, there is a risk that some strains can find a way to ‘cheat’ and avoid becoming stalk cells, and as a result become expressed in more than a fair share among the spores.
How not D. discoid ensure that cheaters do not prosper?
In 2017, researchers from the University of Manchester, in England, reported that two genes in D. discoid genome – called tgrB1 and tgrC1 – displayed all the traits you would expect in a green beard gene. On May 11 this year, researchers at Baylor College of Medicine in the US reported the evidence D. discoid amoebae use this gene to navigate the risk of chimerism.
The discovery shows how sophisticated genetic machinery can give these deceptively simple life forms the ability to express universal goodness.
Both studies were published in the journal Natural Communication.
Separate kith from relatives
At tgrB1 and tgrC1 genes are located next to each other in the D. discoid genome, and are written together (so if one isn’t written, the other isn’t either). They contain the information for cells to make two cell surface proteins called TgrB1 and TgrC1. The TgrB1 protein in one cell binds to the TgrC1 protein in another cell. If the bond is strong, the TgrB1 protein is activated, and gives altruistic behavior – manifested as the amoeba’s willingness to form a stalk.
The bond between TgrB1 and TgrC1 proteins from cells of the same strain is strong, and leads to self-recognition and cell cooperation. Pure cultures of cells that do not have tgrB1 and tgrC1 genes fail to evolve because they cannot recognize each other as the same.
At tgrB1 and tgrC1 genes are also highly polymorphic: in the same population D. discoid amoebae, they have many variants. In fact, they are among the most variable genes Dictyostelium genome. When researchers in a 2017 study examined 20 strains of amoeba isolated from common locations, they found 18 different variants of each gene.
Researchers can relate the difference in tgr gene sequence between two strains to the efficiency of cells separated from each other in mixed aggregates and formed. separate fruiting body. Specifically, when TgrB1 and TgrC1 proteins bind together in cells of a weakly diverged strain, TgrB1 fails to activate, and the cells separate from each other instead of cooperating.
When researchers remove tgrB1 gene but leave it on tgrC1 genes intact, amoeba does not lie ‘non-self’ amoebae that carry different tgrC1. However, it is deceived that with the same tgrC1 variants are themselves – that is, their relatives. Every family has a black sheep!
From Haldane to Voltaire
On the other hand, amoebae which the researchers activated tgrB1 The gene itself (by introducing mutations) is passed down to the stalk when it is mixed with its family.
The findings reveal the logic used by the greenbeard gene to produce altruism and the corresponding ability to keep altruistic amoeba from being exploited by greedy humans.
The British-Indian geneticist JBS Haldane (1892-1964) is said to have said that he would jump into a river to save eight cousins but not seven – but he would also jump to save two cousins but not one. The quote highlights the fact that we share half of our genes with siblings and one-eighth with first cousins.
As we know, amoebae do not use rational numbers. Instead, they used rapidly evolving genes to estimate their kinship with other amoebas. If the genes have not diverged at all, or have diverged very little, there is kinship and it is good for cooperation. If they have diverged significantly, there is less kinship and therefore cooperation is risky.
To paraphrase the philosopher Voltaire (1694-1778), if tgr genes do not exist, it will be necessary for amoebae to invent them.
DP Kasbekar is a retired scientist.