Altruism in unexpected places: The science behind self-sacrifice
How can seemingly selfless acts evolve?
This article explores the concept of altruism in nature, where organisms act in ways that benefit others at a cost to themselves. The social amoeba Dictyostelium discoideum serves as a model organism for studying this phenomenon.
The green-beard theory and self-recognition
The article proposes the green-beard theory to explain altruism. This theory suggests that genes can encode a "tag" that allows organisms to recognize and cooperate with those who share the same tag, leading to altruistic behavior towards kin.
Dawkins called gene-based discrimination 'the Green-Beard Altruism Effect'. He envisaged a gene that encoded both a phenotypic label, the green beard, and the tendency to be nice to green-bearded individuals. Kinship is a major cause of identity by kind but the label is independent of genealogy.
Dictyostelium discoideum: A case study
Dictyostelium discoideumis 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.
Researchers identified two genes, tgrB1 and tgrC1, that influence this behavior.
Kin recognition through protein binding
The tgrB1 and tgrC1 genes produce proteins that bind to each other on cell surfaces. Strong binding between these proteins from the same strain promotes self-recognition and altruistic cooperation.
Genetic variation and preventing cheating
The tgr genes are highly variable, leading to different protein variants. Weak binding between TgrB1 and TgrC1 from different strains discourages cooperation, preventing "cheating" by non-kin amoebae.
Conclusion: The importance of kin selection
D. discoideum's altruistic behavior highlights the role of kin selection in evolution. Genes that promote cooperation among relatives ensure the survival and spread of those genes. The article ends by highlighting the ingenuity of natural selection in achieving seemingly complex behaviors through simple mechanisms.
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