Cells within developing metazoa are directed to particular pathways of differentiation which lead to the highly ordered tissue constitution of the adult. Two mechanisms, different in principle, that direct developmental decisions are commonly proposed. In lineage mechanisms cells inherit developmental directives and are not influenced by their environment. In position-dependent mechanisms cells assess environmental cues to choose one of many potential developmental pathways. Both mechanisms operate in development and are not mutually exclusive, the final fate of a cell may be determined by a combination of both mechanisms.
In the developing eye of Drosophila, as cells choose their terminal cell type, lineage mechanisms do not operate and analysis of this system focuses upon the nature of developmental decisions directed by positional signals. The specific issues that can be addressed are the nature and transmission of the signals, the mechanisms the cells use to receive them, and the molecular events...
