Understanding How Eggs Work 
hind, then gene cloning could be replaced by a 
coffeemaker. 
Gene purification has two major benefits. First, 
the structure of the gene's product, a specific 
protein that actually becomes part of the egg, can 
now,be determined by DNA sequencing. Second, 
specific antibodies can be prepared that bind 
only to the product of that particular gene. When 
these antibodies are used, eggs that appeared nor- 
mal in the microscope can suddenly be seen to 
have specific defects that are responsible for their 
inability to function normally. Such molecular bi- 
ological studies allow us to understand in much 
greater detail where particular gene products are 
located in a egg, where they go as development 
begins, and what role they are likely to be playing 
in the process of development. 
Covert Signals 
One project in our research group concerns an 
unusual role played by the egg's nucleus during 
oogenesis. The nucleus of a cell contains its 
genes and is the site where gene activity is con- 
trolled. Normally, when a gene becomes active, it 
copies itself and releases the copies into the cyto- 
plasm, where they instruct the cell how to make a 
particular protein. However, the nucleus doesn't 
usually target these copies to any particular re- 
gion of the cell; they diffuse away from the nu- 
cleus in all directions through the cell cytoplasm. 
This year we have carried out several studies sug- 
gesting that Drosophila egg nuclei play a more 
sophisticated role during certain key times in 
oogenesis. 
The oocyte nucleus always moves from a cen- 
tral location in the egg to a spot near the upper 
surface of the egg. To learn if this migration 
might be important, we destroyed the nucleus 
with a laser microbeam just before it moved up- 
ward, and allowed the eggs to continue develop- 
ing. If the nucleus simply broadcasts products 
generally throughout the egg, its loss should 
cause only generalized defects, and possibly 
arrest further egg development. Instead, the up- 
per surface of each treated egg was specifically 
affected; it now continued to develop as though it 
were the lower surface. (The two surfaces can be 
distinguished by differences in their curvature 
and in the eggshell.) This led us to suspect that 
the egg nucleus was moving to the upper surface 
in order to send specific information to that re- 
gion. The most likely recipients of this informa- 
tion were some of the cells that surround the en- 
tire surface of the egg as it develops, called 
follicle cells. The egg nucleus appeared to be 
giving special instructions only to the follicle 
cells that lie just above the upper surface. With- 
out these instructions these cells were unable to 
carry out functions that were necessary to pro- 
duce a normal egg upper surface. 
Once we suspected that the oocyte nucleus and 
the follicle cells were communicating, we sought 
to identify specific genes that might be involved 
in transferring developmental information. Sev- 
eral genes were known previously whose absence 
causes defects in egg development very similar to 
those produced by our laser treatments. For exam- 
ple, the gene torpedo encodes a type of receptor 
protein found in all the follicle cells that may 
receive the signal from the oocyte nucleus. We 
have subsequently identified, cloned, and begun 
to study a new gene, zeppelin, that is also re- 
quired for upper surface development. This gene 
appears to be expressed preferentially in the up- 
per follicle cells. It now appears feasible to iden- 
tify most if not all the genes used by the oocyte to 
signal the upper follicle cells and to use them to 
learn exactly what the transmitted message con- 
sists of and how it is received and implemented. 
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