10 LL GIENOLS, AU DUB ONG U La eee 
Genetic research may provide the solution to these problems. Indeed, 
one genetic-control mechanism has already been successfully used. This is 
the ‘‘sterile-male’’ technique used to control the screw worm fly, a serious 
livestock pest found in the southeastern United States. In this control meas- 
ure, male flies are sterilized by gamma radiation. These flies are then re- 
leased to compete for mates with normal males. The sterilized male flies 
that are successful in mating cause their female mates to produce sterile 
eggs. A sharp drop in the screw worm fly population occurs quickly, partly 
because the sterilized males competed successfully with normal males, and 
partly because female screw worm flies mate only once during their life- 
time. 
As good as this program has been however, it suffers from one 
significant defect. The sterilized males effectively reduce the fly population 
in the next generation, but they do not convey sterility to any members of 
that generation. Thus, screw worm flies retain the potential to increase their 
population level at any time that sterilized males are no longer produced 
and released. 
This difficulty may now be solved, however, thanks to biologist Karamjit 
Rai of Notre Dame. Rai and his colleagues have succeeded in producing a 
breed of sterile mosquitoes by manipulating the chromosomes that trans- 
mit hereditary characteristics from one generation to the next. As you may 
recall, chromosomes are rod-like bodies within the nuclei of cells. The 
actual carriers of genetic information, the genes, are arranged in sequences 
along the length of the chromosomes. Rai’s initial success could very well 
spell out the end of the mosquito as a pest and health hazard, and may even 
show the way to elimination of other insect carriers of disease. 
Rai’s work was done with the yellow-fever mosquito—Aedes aegypti. 
In brief, what he has done is quite simple. By using penetrating radiation, 
he was able to break certain chromosomes into pieces. When the pieces 
then rejoined, they sometimes linked up in abnormal arrangements. One 
of these rearrangements produced male mosquitoes that are 70 to 90 
per cent sterile. But significantly, these semi-sterile males are essentially 
unaffected in terms of other inherited characteristics. Thus, they look like 
and behave like normal male Aedes mosquitoes. Most important, the com- 
petitive ability of the semi-sterile males in seeking mates is not affected. 
Thus, when successful in mating, these males pass their sterility along to 
the next generation), about 80 per cent will inherit the sterility factor and 
will also pass it to succeeding generations in about the same ratio.’’ Based 
on these findings, Rai then programmed a computer to predict what would 
happen in a controlled area if semi-sterile males were released to mate. 
This computer simulation revealed that it would take only three to six 
months (five or six generations) to eradicate the mosquitoes. 
Let’s look more closely at Rai’s achievement. Each individual cell of 
an Aedes mosquito contains just three pairs of chromosomes—a total of 
six. Three of these originally came from the sperm cell of the “father;” the 
other three were contributed by the egg cell of the ‘““mother.’” Each chro- 
mosome from the male mosquito is matched by a chromosome from the 
female. These similar chromosomes, which both carry genes for the same 
set of hereditary characteristics, function in the same manner. The matched 
