Molecular Genetics of Nematode Development 
and Behavior 
Paul W. Sternberg, Ph.D. — Assistant Investigator 
Dr. Sternberg is also Assistant 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 Medi- 
cine, 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 Searle Scholar. 
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 tail. 
The concerted morphogenesis of male-specific 
neural cells, along with specialized muscle and 
epidermal cells, forms the copulatory male tail, 
which allows males to mate with hermaphro- 
dites. Because C. elegans hermaphrodites are in- 
ternally self-fertilizing — each animal producing 
both sperm and ova — male mating is a dispens- 
able behavior. Thus mutant strains defective in 
male mating can be easily propagated and the 
mating process studied. We have used a simple 
behavioral test — the ability of males to sire prog- 
eny — to isolate more than 100 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. 
Of the males with obvious abnormal tails, we 
have focused on those with abnormal spicules — 
innervated structures crucial to successful mat- 
ing. Each of the two spicules comprises nine 
cells: two sensory neurons, one motoneuron, and 
six supporting cells. So far we have identified sev- 
eral genes that act at various stages in develop- 
ment of the spicules. For example, two genes are 
necessary for production of the hardened cuticle 
that surrounds each spicule. 
We have identified several genes that are re- 
quired for the decision between alternative neu- 
roblast fates. Two cells, Ba and B/3, each normally 
generate distinct sets of spicule cells. Two 
genes promote differentiation as B/?, while four 
others promote differentiation as Ba. Of the 
former class, the let-60 gene (lethal gene 60) is 
of special interest because we have discovered 
that the level of its activity determines whether 
the neuroblasts in question become Ba or B^: low 
let-60 activity makes a cell BjS, high let-60 activ- 
ity makes a cell Ba. This set of genes also controls 
the fates of two other sets of cells whose fates 
during development depend on signals from 
neighboring cells, suggesting that these genes en- 
code a general mechanism that controls the fates 
of cells during development. The products of let- 
23 and let-60 are homologous to mammalian 
proto-oncogenes, epidermal growth factor (EGF) 
receptor and ras, respectively, suggesting that 
common mechanisms might control the fates of 
cells in mammals and nematodes. 
Male mating is the most complex nematode be- 
havior. By studying the Cod mutants, we hope to 
elucidate how genes control each step in this be- 
havior. 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 24 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 3 
will endlessly circle the hermaphrodite search- 
ing for the vulva. A minority of the mutants are 
ineffective at more than one step. Having mutants 
blocked at defined steps will allow us to identify 
genes and cells necessary to specify this innate 
behavior. 
We are also identifying the cells responsible for 
each step in mating behavior by killing individual 
cells or structures with laser microbeam irradia- 
tion and observing the consequences for behav- 
ior. For example, the spicule motoneuron and at 
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