Lineage-Specific Gene Expression 
in Caenorhabditis elegans 
James D. McGhee, Ph.D. — International Research Scholar 
Dr. McGhee is Professor in the Department of Medical Biochemistry at the University of Calgary, Alberta. 
He received his B.Sc. degree in physiology and biochemistry from the University of Toronto and his Ph.D. 
in molecular biology from the University of Oregon. His postdoctoral research was done in the laboratory 
of Gary Felsenfeld at the National Institutes of Health. He is a Medical Scientist of the Alberta Heritage 
Foundation for Medical Research. 
ONE of the most important problems in devel- 
opmental biology is to understand the mech- 
anism of lineage-specific gene expression. That 
is, how does a developing embryo manage to ex- 
press a particular gene in one cell or cell lineage 
and not in the many other cells of its body? Fur- 
thermore, how does it tell time, in order to ex- 
press this gene only at the correct point in 
development? 
We are approaching these problems in the sim- 
ple nematode, or roundworm, Caenorhabditis 
elegans. The reasons for choosing such a simple 
animal are that it comprises only about a thou- 
sand cells and, more importantly, that the divi- 
sion pattern and cell lineage of every one of these 
is known. We are studying the expression of 
genes in the C. elegans intestine, since this is the 
simplest lineage available. 
When the developing embryo has only eight 
cells, one of these (called the E cell) is the pro- 
genitor of the animal's entire intestine. As a 
marker for biochemical differentiation of the gut, 
we have characterized a simple hydrolytic en- 
zyme, a nonspecific carboxylesterase. As shown 
in the figure, the gene coding for this esterase 
(called ges-1, standing for gut esterase) is only 
expressed in the developing intestine. Presum- 
ably the natural function of the enzyme has some- 
thing to do with the worm's digestion, but mu- 
tants that do not express esterase appear to be 
perfectly viable. 
The ges-1 gene is transcribed from the genome 
of the embryo at a point in development when 
the embryo has only 100-200 cells and the devel- 
oping gut lineage consists of only four cells. Mi- 
cromanipulation experiments have shown that 
ges-1 is expressed normally in embryos in which 
non-gut cells have been removed or destroyed. In 
other words, we have no evidence for any interac- 
tion between gut and non-gut cells. An intriguing 
feature of ges-1 expression is that it is completely 
dependent on DNA synthesis during the cell cycle 
in which the embryo has a total of eight cells, i.e., 
the cell cycle just after the gut has been clonally 
established. 
The problem of understanding intestinal- 
specific expression of the ges-1 gene is twofold. 
One must define regions in the DNA to which 
transcription factors bind, and then identify and 
characterize the transcription factors that bind to 
these regions. To address the first part of the 
problem, we have injected the cloned DNA from 
the ges-1 gene back into a ges-i-nonexpressing 
mutant and have been able to reconstitute accu- 
rate gut-specific expression. We have used this 
transformation assay to identify sequences that 
appear to be necessary for correct gut expression. 
We have also found regions, however, whose 
deletion causes the ges-1 gene to be expressed not 
in the gut but in specific sets of cells in the phar- 
ynx, in the body wall musculature, and in the 
hypodermis. These cells belong to either the sis- 
ter or cousin lineage of the gut. 
The simplest model to explain these ectopic 
staining patterns invokes lineage-specific repres- 
sors that would normally keep the ges-1 gene si- 
lent. When the repressor-binding site is deleted, 
the normally silent gene can be expressed. We are 
testing this model by attempting to identify short 
discrete sequences that cause lineage-specific re- 
pression of a gene that is otherwise ubiquitously 
expressed. We are also attempting to provoke ec- 
topic expression of the ges-1 gene by injecting 
large amounts of putative repressor-binding sites 
as a competitor. 
There is a curious and unexpected feature of 
these ectopic ges-1 staining patterns. With con- 
structs that produce staining in pharynx, mus- 
cles, and hypodermis, the three staining patterns 
are by and large exclusive: individual embryos 
show only one of the patterns, and combinations 
of patterns are rare. Whatever the molecular ex- 
planation turns out to be, the phenomenon sug- 
gests that the embryo is making a decision very 
early in development and stably propagating this 
decision thereafter. Indeed, this decision must ac- 
tually be made at or before the two-cell stage of 
the embryo, since this is the last point when the 
different lineages are common. 
We are also working on the second part of the 
problem of lineage-specific gene expression, 
namely to identify the protein factors that bind to 
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