Worm Breeder's Gazette 10(3): 99
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
After a cell has died by programmed cell death, its corpse is quickly engulfed and broken down by one of its neighbors. Mutations in the genes ced-1 and ced-2 slow this process of phagocytosis, so that the corpses of dead cells accumulate during development and are easy to see. Although ced-1 is a strictly zygotic gene, Hilary Ellis discovered several years ago that ced-2(e1752) mutants can be rescued if their mother provides wild-type ced-2 product. ced-2/+ animals born of a ced-2 mother are also wild-type, so ced-2 function can be provided by either the mother or the zygote. Hilary's observation explains why many ced-1 alleles, but only one ced-2 allele, have been found in screens of mutant F2 animals, and suggested that more genes like ced-2 might exist. Recently, new mutations that cause cell corpses to persist have been isolated in our laboratory. Chand Desai found n1812 as a spontaneous mutation in one of his stocks, and Michael Stern isolated n1813 as a gamma-ray induced mutation. We tested these recessive alleles for complementation with ced-1 and ced-2 and with each other, and found that each mutation defines a new complementation group. Genetic mapping confirms that these are new ced genes: n1812 is located on chromosome IV very near mec-3, and n1813 is on chromosome III between lon-1 and the left end of nDf16. We have chosen the names ced-5(n1812) and ced-6(n1813) for these genes. Both ced-5 and ced-6 mutants show maternal rescue, just like ced-2 mutants. However, careful examination of ced-6 animals from a ced-6/+ mother shows that though embryonic cell deaths are degraded normally, cell corpses formed later, during larval development, are not phagocytosed efficiently. This suggests that wild-type ced-6 product put in the egg by the mother is used up or diluted out during larval development. We are now testing ced-2 and ced-5 for this phenotype. The gene ced-5 is the only one of this group uncovered by a deficiency sDf2 fails to complement ced-5(n1812) for the phagocytosis of dead cells, and sDf2/ced-5 animals appear otherwise healthy, just as ced-5 homozygotes do. When we compare newly hatched ced-5(n1812) 282) 91)/sDf2 animals with ced-5( n1812) 282) worms, we see twice as many cell corpses still present in the pharynx in the ced-5/sDf2 animals. This suggests that n1812 lowers but does not eliminate ced-5 gene function. To identify more genes involved in the process of phagocytosing dead cells, we have developed a mutant screen that allows us to find genes of both the ced-1 and ced-2 varieties. We mutagenize sem-4(n1378) animals, which form bags-of-worms because they lack sex muscles ( Michael Stern, personal communication), and then screen F2 bags-of- worms using Nomarski microscopy to look for persistent cell corpses in the F3 embryos and larvae. Mutations that show maternal rescue like ced-2 should appear as bags full of Ced worms. We are now analyzing several new ced mutations isolated from our first test of this procedure. Why is ced-1 a zygotic gene while all of the other loci involved in the phagocytosis of dead cells show maternal rescue? One possibility is that the process of breaking down a dead cell involves two sets of genes: some are involved in specifically marking the cell corpse as an object that should be phagocytosed, and others encode products that are part of the cellular machinery necessary for phagocytosis. In this model it seems possible that genes in the first class could act only in dying cells, whereas genes in the second class could make products found in all cells, products that the mother also provides to the egg.