the determination and temporal control of optic 
lobe development, respectively. The so gene en- 
codes a homeobox-containing protein whose loss of 
function results in the inability of the optic lobe 
primordium on the surface of the embryonic ecto- 
derm to invaginate. The transient and highly spe- 
cific pattern of expression of so in the optic lobe 
placode prior to invagination is consistent with an 
early determinative role in this process. The so gene 
also plays a critical and very early role in the devel- 
oping retina. 
The ana gene, a heterochronic mutation, plays a 
novel role in regulating brain development. Loss of 
ana function results in an accelerated development 
of the optic lobes; neuronal differentiation occurs 
precociously. This loss of coordination between de- 
velopmental events in the eye and optic lobes re- 
sults in a severe defect in the projection patterns of 
the R cells. The ana gene encodes a 70-kDa glyco- 
protein that is secreted by glial-like cells surround- 
ing the primordium. A model that is currently being 
tested is the hypothesis that ana controls timing in 
the developing visual system by regulating the rate 
of cellular proliferation in the optic lobes. (The 
work on the ana gene was supported by a grant from 
the National Institutes of Health.) 
Development of Specific Patterns 
of Neuronal Connectivity 
in the Drosophila Visual System 
The R cells form remarkably precise patterns of 
synaptic connections in the optic lobes. The eight R 
cells from each ommatidium send axons, arranged 
in a precise fashion within a fascicle, into the devel- 
oping lobes. The R1-R6 cells terminate in the first 
optic ganglion, the lamina; the R7 and R8 cells pro- 
ject through the lamina and into the medulla, where 
they make synapses. These axons all project into the 
brain in a retinotopic manner. Dr. Zipursky and his 
colleagues are using a variety of genetic screens to 
identify mutations that affect the development of 
these projections, as a first step to understanding the 
underlying molecular mechanisms regulating the 
formation of specific synaptic connections in the fly 
visual system. 
Dr. Zipursky is also Associate Professor of Bio- 
logical Chemistry at the University of California 
School of Medicine, Los Angeles, and Member of 
the Molecular Biology Institute at UCLA. 
Books and Chapters of Books 
Cagan, R.L., and Zipursky, S.L. 1992. Cell choice 
and patterning in the Drosophila retina. In Deter- 
minants of Neuronal Identity (Shankland, M., 
and Macagno, E.R., Eds.). San Diego, CA: Aca- 
demic, pp 189-224. 
Articles 
Biggs, W.H., III, and Zipursky, S.L. 1992. Primary 
structure, expression, and signal-dependent tyro- 
sine phosphorylation of a Drosophila homolog of 
extracellular signal-regulated kinase. Proc Natl 
Acad Sci USA 89:6295-6299. 
Cagan, R.L., Kramer, H., Hart, A.C., and Zipursky, 
S.L. 1992. The bride of sevenless and sevenless 
interaction: internalization of a transmembrane li- 
gand. Ce// 69:393-399. 
Kramer, H., Cagan, R.L., and Zipursky, S.L. 1991. 
Interaction of bride of sevenless membrane- 
bound ligand and the sevenless tyrosine-kinase 
receptor. Nature 352:207-212. 
Krantz, D.D., Zidovetzki, R., Kagan, B.L., and Zi- 
pursky, S.L. 1991. Amphipathic /? structure of a 
leucine-rich repeat peptide. / Biol Chem 
266:16801-16807. 
Van Vactor, D.L., Jr., Cagan, R.L., Kramer, H., and 
Zipursky, S.L. 1991. Induction in the develop- 
ing compound eye of Drosophila: multiple mech- 
anisms restrict R7 induction to a single retinal 
precursor cell. Ce// 67: 1 145-1 1 55. 
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