The aim of the present article is to derive and illustrate in a simple form some of the important concepts in developmental biology. The development of the cellular slime mouldDictyostelium discoideum is an ideal model system for this purpose. I will outline the development of this organism at its multicellular stages and review some relevant studies focusing on the control of cell differentiation and pattern formation while deriving some key concepts in the current thinking about the control of development.

Aggregative multicellularity requires the ability of cells to recognise conspecifics. Social amoebae are among the beststudied of such organisms, but the mechanism and evolutionary background of species recognition remained to beinvestigated. Here we show that heterologous expression of a single Dictyostelium purpureum gene is sufficient for D.discoideum cells to efficiently make chimaeric fruiting bodies with D. purpureum cells. This gene forms a bidirectional pairwith another gene on the D. purpureum genome, and they are both highly polymorphic among independent wild isolates ofthe same mating group that do not form chimaeric fruiting bodies with each other. These paired genes are both structurallysimilar to D. discoideum tgrB1/C1 pair, which is responsible for clonal discrimination within that species, suggesting thatthese tgr genes constitute the species recognition system that has attained a level of precision capable of discriminatingbetween clones within a species. Analysis of the available genome sequences of social amoebae revealed that such genepairs exist only within the clade composed of species that produce precursors of sterile stalk cells (prestalk cells),suggesting concurrent evolution of a precise allorecognition system and a new ‘worker’ cell-type dedicated to transportingand supporting the reproductive cells.