ced-1,
ced-2,
ced-5,
ced-6,
ced-7, and
ced-10 are needed for the efficient engulfment and removal of programmed cell death (PCD) corpses (Ellis et al., 1991, Genetics 129: 79-94).
mec-4(
u231) is a dominant mutation that causes the degenerative death of six neurons that mediate body touch sensitivity. This
mec-4(d)-induced cell death does not operate through PCD: double mutants
ced-3(
n717);
mec-4(
u231),
ced-4(
n1162);
mec-4(
u231), and
ced-9(
n1950);
mec-4(
u231) experience degenerative death with kinetics indistinguishable from wild type. By contrast, we have found that the six engulfment ced genes are needed for the removal of
mec-4(d)-induced degenerative corpses (Chung and Driscoll, WBG, Vol. 14 No. 1). This suggests that the removal of cellular debris does not depend on apoptotic-specific signaling. The engulfment ced group can respond to any corpse. We wondered if any unidentified genes might be involved in the removal of degenerative corpses. To this end, we screened for mutations that would result in persistent
mec-4(d)-induced corpses. We mutagenized
lon-2(
e678)
mec-4(
u231) hermaphrodites with EMS. The F1 progeny were collected; ten animals were placed on a plate. In the F2 generation, L4 animals were examined using a high magnification dissection microscope. In a
mec-4(d) background, only 2% of animals still display degenerative corpses in the L4 stage. We screened for L4 animals with large persistent
mec-4(d)-induced corpses. From a screen of 30,400 genomes, we found three interesting mutations. Two of the new mutations cause 30% to 40% of the animals at the L4 larval stage to still display
mec-4(d)-induced corpses. One of these mutations is an allele of
ced-7. The second has proven to be an allele of
ced-12.
ced-12 is a novel engulfment gene isolated by the Hengartner Laboratory (Gumienny et al., 11th Int'l C. elegans Meeting, 1997). This demonstrates that like the other engulfment ced genes,
ced-12 mediates removal of both PCD and degenerative death corpses. The third mutant from this screen causes vacuolated corpses similar to those induced by
mec-4(d). However, these corpses are not localized to the area of the six touch receptors; they are seen through out the body, especially the anterior half of the animal. The screen that we have designed allows us to examine many mutagenized genomes in a short period of time. However, this screen probably misses mutations that allow persistence of small corpses, or only modestly increases persistence in L4 (this is true for most single mutant ced genes); we could only select for animals with persistent corpses large enough to be seen under a dissection microscope. Genes with maternal effects should also have escaped detection. The fact that our screen, which is not designed to select for PCD genes, identifies alleles of two engulfment ced genes further confirms that PCD corpses and necrotic corpses are removed from the worm by the same undertaker genes.