al., 1983; Ellis and Horvitz, 1986; Desai et al., 1988; Ellis et al., 1991; Ellis and
Horvitz, 1991; Hengartner et al., 1992). Indeed, one of the future challenges in the
C. elegans cell-death field will be the identification of essential genes that also affect
programmed cell death.
Apoptosis can be separated into four distinct steps, each of which is defined by the
analysis of various mutants (Figures 2 and 3). (i) Initially, specific cell types are
committed to the apoptotic fate. (ii) Subsequently, the general apoptotic machinery,
which is used in all dying cells, is activated. (iii) Later, the recognition and engulfment
of dying cells by a neighboring cell proceeds. (iv) Finally, the remnants of engulfed
cells are degraded. Genes of the first type generally affect very few cells or cell types,
whereas the genes falling into the subsequent classes affect all cell deaths. These genes
will be described below (Figures 2 and 3).
Figure 3. The genetic pathway of programmed cell death in C. elegans.
Various developmental and environmental clues that can activate programmed cell death are
indicated in the left panels. Arrows indicate activation; T-bars indicate repression.
166 GENETICS OF APOPTOSIS