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.

Two New ced Genes: ced-5 and ced-6

Ronald E. Ellis and Bob Horvitz

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 
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.