An individual-level selection model for the apparent altruism exhibited by cellular slime moulds

In Dictyostelium discoideum, cells that become part of the stalk or basal disc display behaviour that can be interpreted as altruistic. Atzmony et al. (Curr Sci 72:142-145, 1997) had hypothesised that this behaviour could be the outcome of an adaptive strategy based on differing intrinsic quality as reflected by phenotypes that indicate differences in potential for survival and reproduction, followed by intercellular competition among amoebae of differing qualities. Low-quality amoebae would have a poor chance of succeeding in the competition to form spores; they could enhance their chances of survival by adopting a presumptive stalk strategy. Here we extend the hypothesis by making use of recent findings. Our approach is based on the view that an evolutionary explanation for the apparent altruism of stalk cells in D. discoideum must apply broadly to other cellular slime moulds (CSMs) that exhibit stalk cell death. Further, it must be capable of being modified to cover social behaviour in CSMs with an extracellular stalk, as well as in sorocarpic amoebae whose stalk cells are viable. With regard to D. discoideum, we suggest that (a) differentiation-inducing factor, thought of as a signal that inhibits amoebae from forming spores and induces them to differentiate into basal disc cells, is better viewed as a mediator of competition among post-aggregation amoebae and (b) the products of the 'recognition genes', tgrB and tgrC, allow an amoeba to assess its quality relative to that of its neighbours and move to a position within the aggregate that optimises its reproductive fitness. From this perspective, all cells behave in a manner that is 'selfish' rather than 'altruistic', albeit with different expectations of success.