Development of the Drosophila Peripheral 
Nervous System 
Hugo J. Bellen, D.V.M., Ph.D. — Assistant Investigator 
Dr. Bellen is also Assistant Professor in the Institute for Molecular Genetics, Division of Neuroscience, and 
Department of Cell Biology, Baylor College of Medicine. Educated in Belgium, he received a degree in 
commercial engineering and began research in sociometry, but decided to pursue a career in medical 
science. He obtained a D. V.M. degree from the University of Ghent and a Ph.D. degree in genetics from the 
University of California, Davis, where, in John Kiger's laboratory, he studied mutations that affect be- 
havior in the fruit fly. Later, in Basel, Switzerland, as a postdoctoral fellow with Walter Gehring, he helped 
to develop the enhancer detection system. 
OUR research is centered on the development 
of the peripheral nervous system (PNS) of 
the fruit fly Drosophila melanogaster. We be- 
lieve that the results will help us understand the 
development of the nervous system in many eu- 
karyotic species. We have focused on the PNS of 
Drosophila because its cells are relatively easy to 
study and because sophisticated genetics can be 
applied to this model organism. 
We are presently studying two genes that are 
expressed at the outset of embryonic nervous sys- 
tem development and later in embryogenesis in 
most cells of the PNS. The genes were identified 
in enhancer detector screens in which regulatory 
sequences are determined by means of a jS-galac- 
tosidase reporter gene. The newfound genes 
are essential, since mutations cause embryonic 
lethality. Here we describe their preliminary 
characterization . 
One of the two genes is named couch potato 
because some homozygous mutant flies are via- 
ble but hypoactive and have poor jump re- 
sponses, poor flight abilities, and slow recovery 
from ether anesthesia. Some of these attributes 
are probably due to thoracic muscle defects that 
we observe in the adult fly. More than 10 inser- 
tional mutations in couch potato were recovered 
from several enhancer detector screens, and a 
careful genetic analysis revealed that none 
caused a complete loss of function of the gene. 
Some insertional alleles cause recessive embry- 
onic lethality, but no defects were observed in 
the PNS of the mutant embryos. 
In order to define the null phenotype, we per- 
formed an excision mutagenesis of one of the in- 
sertions and recovered an embryonic lethal muta- 
tion in the gene that, by genetic criteria, can be 
defined as a complete loss-of-function mutation. 
Some embryos lacking the couch potato gene ex- 
hibit defects in the neuronal pathways of PNS and 
lack some peripheral nerve cells. Hence the 
couch potato gene seems to play a role in the 
development of the PNS as well as in the develop- 
ment of the adult thoracic muscles. 
A molecular analysis of couch potato was initi- 
ated with a genomic DNA fragment next to one of 
the enhancer detector insertions. This fragment 
allowed us to isolate three different couch potato 
cDNAs. These were labeled for a nonradioactive 
whole-mount in situ hybridization technique, 
which showed that the gene is expressed in the 
PNS, including the glial cells, and in some glial 
cells of the central nervous system. In all cases 
the expression patterns of |8-galactosidase in the 
embryos that carry an enhancer detector and the 
transcripts of couch potato are essentially the 
same. 
All the insertions that were recovered are clus- 
tered in a 450-bp genomic fragment. In addition, 
there is a very good correlation between the de- 
fects that we observe in embryos or adults and the 
position of these insertions; insertional alleles 
that cause a particular phenotype are clustered 
within 50 bp. These data, combined with the ge- 
netic data, indicate that the genomic sequences 
next to the insertions contain some of the gene's 
regulatory sequences that confer the tissue- 
specific expression pattern. That this is indeed 
the case has been shown by bringing these se- 
quences upstream of a i8-galactosidase reporter 
gene in transformed flies. The transformed em- 
bryos express the reporter in a subset of cells 
(PNS of head and glial cells) in which couch po- 
tato is expressed, showing that these sequences 
contain at least some couch potato regulatory 
sequences. 
The expression of the couch potato gene in the 
glial cells is interesting, as we observe defects in 
the pathways of the peripheral neurons in a sub- 
stantial fraction of the embryos lacking the gene. 
The glial cells may play a key role in helping the 
neuron find its normal target organ, tissue, or 
cell. It is therefore possible that the lack of ex- 
pression of the gene in the glial cells causes a 
neuronal pathway defect. 
To test the hypothesis that the glial cells play a 
role in neuronal pathway formation, we would 
like to ablate them. Hence we have devised a tech- 
nique that should allow specific ablation of these 
cells early in development. It makes use of a tem- 
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