Evolutionary questions raised by cellular slime mould development

Abstract

The cellular slime moulds (CSMs) are amoeboid organisms whose life cycle can be viewed in two ways. Firstly, because free-living amoebae come together to build bodies, they are ideal models for studying multicellular development in terms of the properties of single cells. Secondly, coming together and participating in an integrated unit implies social behaviour. Consequently differentiation (especially in the advanced CSMs) can be seen as a form of division of labour in which only some amoebae get to transmit their genes to the next generation. Viewed thus, their life cycle is ideally suited for studying the evolutionary basis of cooperation with some members of the cooperating group exhibiting altruistic behviour. The present review takes the second approach. We examine alternative explanations for social behaviour (i.e., multicellular development) based on individual-level and group (including kin-group) selection. Non-clonal fruiting bodies are likely to be common in nature; we show a case with at least nine genotypes. The CSMs display both individual and group-level adaptations and both levels of selection operate in their appropriate contexts. The review ends with questions for the future and indicates how studies of CSM development might help to explain the evolution of altruism in this group.