Abstract
Reproductive altruism is an extreme form of altruism best typified by sterile castes in social insects and somatic cells in multicellular organisms. Although reproductive altruism is central to the evolution of multicellularity and eusociality, the mechanistic basis for the evolution of this behaviour is yet to be deciphered. Here, we report that the gene responsible for the permanent suppression of reproduction in the somatic cells of the multicellular green alga, Volvox carteri, evolved from a gene that in its unicellular relative, Chlamydomonas reinhardtii, is part of the general acclimation response to various environmental stress factors, which includes the temporary suppression of reproduction. Furthermore, we propose a model for the evolution of soma, in which by simulating the acclimation signal (i.e. a change in cellular redox status) in a developmental rather than environmental context, responses beneficial to a unicellular individual can be co-opted into an altruistic behaviour at the group level. The co-option of environmentally induced responses for reproductive altruism can contribute to the stability of this behaviour, as the loss of such responses would be costly for the individual. This hypothesis also predicts that temporally varying environments, which will select for more efficient acclimation responses, are likely to be more conducive to the evolution of reproductive altruism.