Understanding How Eggs Work 
Allan C. Sprcutling, Ph.D. — Investigator 
Dr. Spradling is also a staff member of the Department of Embryology at the Carnegie Institution of 
Washington in Baltimore and Professor of Biology and Microbiology at the Johns Hopkins University. He 
earned his B.A. degree in physics from the University of Chicago and his Ph.D. in cell biology from the 
Massachusetts Institute of Technology. His postdoctoral study was done at Indiana University with 
Anthony Mahowald. Dr. Spradling is a member of the National Academy of Sciences and has received 
many honors for his work. 
EGGS have always fascinated biologists. The 
union of egg and sperm initiates the complex 
developmental processes that ultimately produce 
a new multicellular animal. However, the roles 
played by egg and sperm in embryonic develop- 
ment are by no means equivalent. Even nonspe- 
cific treatments such as pricking with a needle 
will stimulate many types of eggs to develop in 
the absence of sperm. Without an egg, however, 
neither sperm nor any other cell can even begin 
the complex processes that lead to an embryo and 
ultimately to an adult. In fact, eggs have under- 
gone extensive and intricate preparations that al- 
low them to support and direct embryogenesis. If 
the mystery of a new life could be condensed into 
a single cell, then that cell would surely be 
an egg. 
Not surprisingly, egg cells are very different in 
structure as well as in biological capacity from 
other cells. Most chicken cells weigh less than 1 
millionth of an ounce, whereas a single chicken 
egg makes a nice breakfast. However, in addition 
to size, eggs diverge in many important ways from 
other cells. The genes within the egg's chromo- 
somes function differently. These alterations are 
so extensive that egg chromosomes appear in the 
microscope quite unlike those in other tissues. As 
a result of these differences, in eggs the products 
of many genes accumulate and are stored in spe- 
cial forms that can be utilized at precisely appro- 
priate times during embryonic development. In- 
deed the presence of so much stored material is 
one of the reasons for the enormous size of many 
eggs. 
A great deal remains to be learned about the 
structure of eggs. How many gene products are 
specially stored there, and what are their specific 
functions later in development? Is each product 
located in a particular place? What are the special 
mechanisms that allow gene products to be pro- 
duced and stockpiled in the appropriate manner? 
The sheer complexity of an egg, with its tens of 
thousands of specific, highly organized compo- 
nents, has until recently prevented all attempts to 
describe egg structure in molecular detail, much 
less to understand the logic that allows this struc- 
ture to begin development into an even more in- 
tricate adult organism. However, during the last 
1 0 years or so, the advent of powerful new meth- 
ods in molecular biology, such as gene cloning 
and gene transfer, in combination with Drosoph- 
ila (fruit fly) genetics, has begun to unravel the 
fascinating secrets stored inside an eggshell. 
Using Genetics to Study Eggs 
It is now possible to study structures as com- 
plex as eggs, because both genetics and molecu- 
lar biology allow us to take eggs apart gene by 
gene. Each component of an egg is specified by a 
gene carried on the chromosomes. Much research 
on eggs utilizes the fruit fly, since over 70 years 
of genetic studies have revealed far more about 
its genes than those of any other complex organ- 
ism, including the human. A genetic mutation in- 
activates one specific gene among the 20,000 to 
50,000 in the fruit fly. If that gene specifies an 
egg component, then the mutant fly will produce 
defective eggs. Frequently, however, the eggs 
can still partially function. For example, in a par- 
ticular mutant, development might begin nor- 
mally, but then stop due to a lack of stored food. 
By studying what goes wrong with the mutant 
eggs, biologists can learn much about what the 
product of that particular gene does normally. 
For example, in this case it would be concluded 
that the gene was involved in producing the food 
stored in the egg. To understand eggs it will be 
necessary, at a minimum, to create mutations and 
carry out such studies on all 20,000 fruit fly 
genes, one by one. 
If researchers were limited to looking at egg 
defects in a microscope, progress would still be 
slow. Many of the defective eggs would not con- 
tain any detectable problems; they just wouldn't 
work. It is at this point that the ability to clone 
molecularly the gene under study becomes es- 
sential to make further progress. Cloning is sim- 
ply a way of purifying an individual gene so that 
its DNA structure can be determined in the labora- 
tory. If someone could invent a piece of filter 
paper that would allow only one specific gene to 
pass through while the other 20,000 stayed be- 
417 
