A high degree of genetic relatedness among the nuclei of cells in a given organism may sustain multicellular cooperation, finds a paper published in Nature Communications this week. The study, which looked at the fungus Neurospora crassa, suggests that a high degree of genetic relatedness selects for cooperation among the cells of multicellular organisms, but that, conversely, when relatedness is low cells compete in order to reproduce rather than investing in the individual as a whole.
High genetic relatedness has long been hypothesized to be fundamental for the evolution of multicellularity, as supported by findings in slime moulds and bacteria. However, direct tests have been lacking because levels of relatedness are difficult to manipulate in the lab.
Duur Aanen and colleagues investigated the evolution of cooperation in N. crassa by allowing two strains of the fungus to evolve in parallel, one with and one without the ability to form chimeras (single organisms with cells from different zygotes) with unrelated cells. This experimental set up allowed the authors to study different degrees of relatedness: the fungi that form chimeras are characterized by low-relatedness, whereas those that do not - instead developing clonally from single cells - are characterized by high-relatedness. They find that multicellular cooperation, measured in terms of spore production, remained unchanged in strains with high relatedness, but decreased in those with low relatedness, resulting in an average three-fold decrease in spore yield. The authors show that the low-relatedness strains evolved to incorporate cheating mutants - cells that increase their own fitness at the expense of the fitness of other cells in the chimera.
Therefore, this experimental test suggests that high genetic relatedness selects against cheating and maintains cooperation within multicellular organisms.