Scientists have gained valuable new insights into natural altruism (a selfless act of helping others) by studying the amoeba Dictyostelium discoideum.

What is Altruism in Animals?

Altruism in animals describes behaviors that may that may be a disadvantage to the individual performing them but benefit others, often in terms of reproductive fitness. It is commonly found in nature and can be observed in the following:

  • Worker Honey Bees: Devote their lives to foraging and caring for the queen and her offspring, without reproducing themselves.
  • Widow Spiders: Males allow themselves to be eaten by fertilized females, nourishing the female and offspring.
  • Meerkats: Serve as sentinels, watching for predators while others forage, and alerting the group if danger is near.

About Dictyostelium discoideum

  • Description: It is a free-living, unicellular amoeba feeding on bacteria.
  • Lifecycle: It transitions from unicellular amoebae to a multicellular slug and then to a fruiting body. 
  • Challenge: When food runs low, amoebae clump together to form fruiting bodies. Some amoebae become stalk cells, sacrificing themselves to lift spores (future generations) towards light.

About 20% of the amoebae altruistically sacrifice themselves to form the stalk. The remaining 80% become the spores. 

Kin Selection and the Mystery of Altruism:

  • Studies on the social amoeba Dictyostelium discoideum suggest that if a gene promotes altruism in one individual, it also aids in passing the gene to the next generation through surviving relatives.
  • Evolution typically favors traits that enhance an individual’s reproductive success. Kin selection theory offers a possible explanation of how seemingly selfless acts, like becoming a stalk cell in Dictyostelium discoideum, persist. 
  • This theory proposes that individuals can indirectly ensure the survival of their genes by helping close relatives (who share similar genes). By supporting their relatives, they increase the chances of their own genes being passed on to future generations. with two key functions:

Greenbeard genes are a proposed type of gene with two key functions:

  • Promote altruistic behavior (e.g., becoming a stalk cell in amoebae).
  • Act as a “tag” for self-recognition: These genes enable self-recognition and cooperation among genetically similar individuals, while preventing exploitation by cheaters.

Evidence for Greenbeard Genes in D. discoideum

  • tgrB1 and tgrC1 genes: These genes might be the greenbeard genes.

Protein Binding:TgrB1 protein on one amoeba binds to TgrC1 protein on another. This binding influences altruistic behavior (stalk cell formation).

  • Strong Binding: Strong binding between Tgr proteins of the same strain promotes cooperation among kin.
  • Weak Binding: Weak binding between Tgr proteins of different strains discourages cooperation.

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