Molecular Genetics of Nematode Development 
and Behavior 
Paul W. Sternberg, Ph.D. — Associate Investigator 
Dr. Sternberg is also Associate Professor of Biology at the California Institute of Technology and Adjunct 
Assistant Professor of Anatomy and Cell Biology at the University of Southern California School of 
Medicine, los Angeles. He received a B.A. degree in biology and mathematics from Hampshire College and 
a Ph.D. degree in biology from the Massachusetts Institute of Technology for work with Robert Horvitz. 
He did postdoctoral research in yeast molecular genetics with Ira Herskowitz at the University of 
California, San Francisco. Dr. Sternberg is also a Presidential Young Investigator. 
USING the nematode Caenorhabditis ele- 
gans, our laboratory takes a molecular genet- 
ics approach to basic questions in developmental 
biology and neurogenetics: What are the molecu- 
lar mechanisms by which cells interact to estab- 
lish a spatial pattern of cell types? What is the 
genetic and cellular basis for morphogenesis? 
What establishes the asymmetry of individual 
cells? How are the instructions for innate behav- 
ior encoded in the genome? Our major strategy is 
to identify mutations that make cells or animals 
misbehave and then to study the functions of the 
genes defined by these mutations, using a combi- 
nation of molecular cloning and genetic analysis. 
A second strategy is to clone nematode homo- 
logues of genes identified in mammals and then 
to elucidate the functions of those genes in 
nematodes. 
In this past year we focused on the develop- 
ment and function of the C. elegans male spic- 
ules — innervated structures crucial to successful 
mating. Each of the two spicules comprises nine 
cells: two sensory neurons, one motoneuron, and 
six supporting cells. By studying spicule develop- 
ment, we have identified a new example of in- 
duction during nematode development. In the de- 
veloping male, either of two cells signals spicule 
precursor cells to generate particular sets of spic- 
ule cells. This inductive signaling process re- 
quires the tin- 3 growth factor, the let- 2 3 tyrosine 
kinase, and the let-60 ras genes that we cloned 
over the last two years, tin- 3 encodes an induc- 
tive signal for the hermaphrodite vulva, and it is 
likely that it acts as an inductive signal for proper 
spicule development as well. Thus we have 
found that a cascade of proto-oncogenes specifies 
cell fates in several aspects of nematode develop- 
ment. In addition to this inductive signal, at least 
three other signals are also necessary for the 
correct specification of spicule precursor cells. 
Because C. elegans hermaphrodites are inter- 
nally self-fertilizing — each animal producing 
both sperm and ova — male mating and thus spic- 
ule function is dispensable. Thus mutant strains 
defective in male mating can be easily propa- 
gated and the mating process studied. We have 
used a simple behavioral test — the ability of 
males to sire progeny — to isolate mutants that are 
unable to mate. Some mutant males have obvious 
defects in the development of male-specific 
structures. Others, called Cod (for copulation 
defective) , are anatomically normal yet defective 
in mating behavior. 
By studying the Cod mutants, we hope to eluci- 
date how genes control each step in male mating 
behavior. We have isolated a set of mutants, have 
characterized the mating defect of each strain, 
and have begun placing these mutations on the 
genetic map. Most of the mutants analyzed are 
defective at only a single step in the mating pro- 
cess. These steps include 1) attraction to her- 
maphrodites, 2) maintaining contact with her- 
maphrodites, 3) location of the vulva, 4) 
insertion of spicules, and 5) transfer of sperm. 
For example, a mutant male defective in step 4 
will locate the hermaphrodite vulva but fail to 
insert his spicule. Having mutants blocked at de- 
fined steps will allow us to identify genes neces- 
sary to specify this innate behavior. 
To identify the cells responsible for each step 
in mating behavior, we kill individual cells with a 
laser microbeam and observe the consequences. 
For example, the spicule motoneuron and both 
spicule sensory neurons are necessary for spicule 
insertion. One of our G protein a-subunit genes is 
expressed in one of these neurons, suggesting 
that it might serve to regulate spicule insertion. 
Another G protein, homologous to human G,,, is 
expressed in male diagonal muscles, required for 
initial steps in mating. We have begun to test 
whether any of the Cod mutants are defective in 
these G protein genes. 
The establishment of cellular asymmetry is a 
fundamental aspect of cell regulation. We have 
begun to study this problem in the context of the 
2° vulval precursor cell. The lin-18 gene (cell 
lineage gene number 18) is necessary for the 
asymmetry of the 2° vulval precursor cell. We 
have mapped the lin-18 gene to a manageable 
region of the X chromosome. We have found that 
the 2° cells will orient posteriorly in the animal, 
unless they receive a signal from the developing 
gonad to orient anteriorly. This signal is distinct 
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