3 On Equivalence Groups and the Notch/LIN-12 Communication System

Abstract

The original concept of equivalence groups arose from a series of cell ablation studies in the nematode Caenorhabditis elegans (Kimble et al. 1979; Sulston and White 1980; Kimble 1981). One of the organs studied was the vulva, the egg-laying structure of female and hermaphrodite nematodes, which derives from a group of six precursor cells (VPCs, vulva precursor cells) organized in a linear array in the ventral ectoderm (Sulston and Horvitz 1977). Although only three of the VPCs normally give rise to vulval tissue, laser ablation of these cells causes the remaining three, which would become hypodermal cells, instead to adopt a vulval fate and give rise to a normal vulva (Sulston and White 1980; Kimble 1981; Sternberg and Horvitz 1986). Similarly, by ablating each of the VPCs, individually or in combination, and analyzing the behavior of the remaining cells, these studies showed that all six VPCs are capable of becoming any of the three precursor cell types that give rise to the mature vulva. The six VPCs are therefore multipotential, and since they can replace each other, they are defined as an “equivalence group” (Kimble et al. 1979). These studies have also shown that there is a clear hierarchy of cell-fate decisions within an equivalence group, where a default or primary fate has precedence over the other alternative fates: If the cell that would acquire the primary fate is removed, one of the other cells in the equivalence group will replace it and become the primary cell. However, the converse...