Maggot's hair and bug's eye: Role of cell interactions and intrinsic factors in cell fate specification

The fate of a cell may be determined by cell lineage, by interaction between the cell and its environment, or by both. The importance of cell lineage in specifying cell fate varies considerably in the development of different cells. At one extreme, cell lineage may be highly determinate; the fate of a cell can be predicted from its ancestry. At the other extreme, cell lineage may play no role. On the surface, these two extremes appear to be very different developmental strategies. Appearances, however, can be deceiving. Are these two extremes really different in their underlying mechanisms? This issue has been addressed in the fly sensory nervous system, which includes some sensory neurons, such as the ones that innervate sensory bristles, that are generated by strict cell lineage, and others, such as photoreceptors, that are generated with no influence imparted by cell lineage. It is now possible to compare the molecular mechanisms used in constructing sensory bristles with those for the production of photoreceptors, because considerable progress has been made in understanding how a cell acquires the potential to become a neural precursor and how it realizes that potential and gives rise to sensory neurons in the fly. The emerging picture is that the development of fly sensory neurons is a progressive process. The developmental potential of sensory precursor cells becomes progressively restricted as development proceeds. This progressive restriction of developmental potential is the outcome of a number of discrete steps. Each step is the result of the action of a small number of genes, often interacting with one another (reviewed in Ghysen et al., 1993). Even though sensory bristles and photoreceptors appear to use very different developmental strategies, the former but not the latter involving strict cell lineage, many of the steps and the genes that control those steps are common to the development of both. The difference in the involvement of cell lineage can be attributed to the differential use of a few groups of genes.