The uniformity in the exact cells undergoing apoptosis makes C. elegans an excellent model organism for uncovering genes involved in apoptosis. In a classical genetic screen to uncover such genes, which phenotypes may be informative?

 In a classical genetic screen designed to uncover genes involved in apoptosis using C. elegans as a model organism, the most informative phenotypes are those that disrupt normal apoptotic processes. Here are several phenotypes that may be informative:

1. Survival of cells that normally die - In C. elegans, the pattern of apoptosis during development is highly predictable, with exactly 131 cells programmed to die in each hermaphrodite. If genes required for apoptosis are inactivated or mutated, cells that are normally destined to die may survive, leading to a higher cell count than expected. Researchers can detect these surviving cells using various techniques, such as Nomarski differential interference contrast (DIC) microscopy.

2. Abnormal timing or sequence of apoptosis - Sometimes a mutation can cause cells to undergo apoptosis earlier or later than they normally would, or to alter the sequence in which cells die. This can provide insight into the genes that control the timing and order of apoptotic events.

3. Enhanced cell death - Mutations may also increase sensitization to apoptotic triggers, leading to more cells undergoing apoptosis than usual, which could indicate genes that normally suppress or regulate apoptosis.

4. Morphological defects in apoptotic cells - Cells undergoing apoptosis exhibit characteristic morphological changes, such as cell shrinkage and chromatin condensation. Mutants that affect these morphological features can be informative for identifying genes that execute the apoptotic process itself.

5. Defects in corpse engulfment and degradation - After a cell undergoes apoptosis, its corpse is normally engulfed by neighboring cells and degraded. Mutations in genes required for corpse clearance may result in persistent cell corpses that are easily detectable.

By observing these phenotypes and using genetic approaches like mutagenesis and subsequent screening, researchers can identify and characterize genes essential for proper apoptotic function in C. elegans.