DEVELOPMENTAL STABILITY AND SIGNALING AMONG CELLS

The production of stable phenotypes depends from the earliest stages o f development upon high levels of somatic cellular selection amongst c ells or cell lineages. The signals exchanged amongst cells can reveal important aspects of a cell's phenotype, and might thereby be used in darwinian processes of developmental selection. Based upon an optimali ty model, we suggest that stable phenotypes require a substantial inve stment in two mechanisms of inter-cellular selection: ''quality-select ion'' mechanisms regulate the average phenotype of a group of cells; ' 'stability-selection'' mechanisms regulate the variance in cell phenot ypes. Variance in cell phenotypes may arise from developmental-error o r other stochastic processes, or be generated as is true of the immune system, as part of a developmental strategy. The model shows that sta bility-selection mechanisms may exert the stronger effect on overall o rgan or body performance. Selection based upon reliable inter-cellular signalling of phenotypic properties may be the key way that bodies an ticipate and then constrain variance in cell phenotypes around the opt imal cellular attributes, and suggests an advantage of developmentally -selected systems over instructional ones. High levels of investment i n stability mechanisms also ensure homogeneous collections of cells th at can translate ''upwards'' into developmentally stable organ systems and phenotypes. Environmental and genetic factors, as well as the pre valent mode of selection, may all affect developmental stability and t hereby give rise to varying degrees of somatic selection. (C) 1998 Aca demic Press.