Examples
The concept remained a merely theoretical possibility under Dawkins' selfish gene model until 1998, when a green-beard gene was first found in nature, in the red imported fire ant (Solenopsis invicta). Polygyne colony queens are heterozygous (Bb) at the Gp-9 gene locus. Their worker offspring can have both heterozygous (Bb) and homozygous (BB) genotypes. The investigators discovered that homozygous dominant (BB) queens, which in the wild form form monogyne rather than polygyne colonies, are specifically killed when introduced into polygyne colonies, most often by heterozygous (Bb) and not homozygous (BB) workers. They concluded that the allele Gp-9b is linked to a greenbeard allele which induces workers bearing this allele to kill all queens that do not have it. A final conclusion notes that the workers are able to distinguish BB queens from Bb queens based on an odor cue.
The gene csA in the slime mould Dictyostelium discoideum, discovered in 2003, codes for a cell adhesion protein which binds to gp80 proteins on other cells, allowing multicellular fruiting body formation on soil. Mixtures of csA knockout cells with wild-type cells yield spores, “born” from the fruiting bodies, which are 82% wild-type (WT). This is because the wild-type cells are better at adhering and more effectively combine into aggregates; knockout (KO) cells are left behind. On more adhesive but less natural substances, KO cells can adhere; WT cells, still better at adhering, sort preferentially into the stalk.
In 2006, green beard-like recognition was seen in the cooperative behavior among color morphs in side-blotched lizards, although the traits appear to be encoded by multiple loci across the genome.
A more recent example, found in 2008, is a gene that makes brewer's yeast clump together in response to a toxin such as alcohol. By investigating flocculation, a type of self-adherence generally present in asexual aggregations, Smukalla et al. showed that S. cerevisiae is a model for cooperative behavior evolution. When this yeast expresses FLO1 in the laboratory, flocculation is restored. Flocculation is apparently protective for the FLO1+ cells, which are shielded from certain stresses (ethanol, for example). In addition FLO1+ cells preferentially adhere to each other. The authors therefore conclude that flocculation is driven by this greenbeard gene.
A mammalian example appears to be the reproductive strategy, in particular cooperation among spermatazoa, of the wood mouse.
Read more about this topic: Green-beard Effect
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