Genetically Modified Birds

 
3.2 Limitations to the Ideal Process for Genetic Modification 
One of the obstacles in the production of transgenic aves was the low-efficiency of introducing foreign 
DNA into the chicken genome. Procedures that have worked for other animals are difficult, if not impossible, 
due in part to the unique reproductive physiology of the chicken [9]. 
Researchers using birds in genetic engineering experiments are trying to overcome the inaccessibility 
of the birds‟ fertilized ova. In mammals, such as mice, the fertilized ova are relatively easy to obtain in large 
numbers, and they have visible pronuclei for DNA microinjection. However, a hen produces only one fertilized 
ovum per day. This ovum, the yolk, is large and fragile. Its cytoplasm makes the pronucleus impossible to 
visualize for DNA microinjection. By the time the egg is laid the embryo has already begun to develop on the 
yolk and has about 60,000 cells. Another problem is that the viruses that are used to carry the foreign genes into 
the birds can dangerously replicate: “The major problem associated with the use of retroviral vectors is the 
generation of infectious virus that can be indefinitely transmitted”. 
Also, because of the unique features of avian reproduction. During fertilization in birds, several sperms 
can penetrate the ovum instead of one sperm. So, it is impossible to identify the male pronucleus that will fuse 
with the female pronucleus. 
3.3 THE PRACTICAL PROCESSES USED FOR GENETIC MODIFICATION OF BIRDS 
FIG. 6: The Practical Processes Used For Genetic Modification Of Birds 
Source: [4]. 
In the mid-1980s, a researcher at the Institute of Animal Physiology and Genetics Research in 
Edinburgh injected foreign DNA into single-cell chicken embryos and then cultured them through hatching in 
vitro. The embryos was removed from the hen‟s oviduct, placed in various vessels containing solutions similar 
to those in an egg, then placed in artificial eggshells sealed with glue made from albumen and antibiotics, and 

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Genetically Modified Birds 
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mechanically incubated through hatching. New Scientist called them “the world‟s first test-tube chickens” 
allowing researchers to create “„super-chicken‟ by inserting foreign DNA into chick embryos” [10]. 
A National Institutes of Health (NIH)-supported study of the avian liver was described to express 
(manifest a trait linked to a particular gene) recombinant proteins in vivo. The research was designed to estimate 
use of the avian liver “to influence growth rates, metabolism, body fat composition, and the effectiveness of 
several drugs” and be a model for treatment of human genetic diseases. In the experiment, avian leukosis 
retroviral vectors were used to introduce a recombinant rat neomycin-resistance gene into chicken embryos 
before and during incubation. Upon hatching, the surviving chicks were killed by cervical dislocation (neck-
breaking) and their tissues were frozen in liquid nitrogen for further analysis [11]. 
A method of producing transgenic chickens based on microinjection of foreign DNA from two 
different types of bacteria into the birds was described by some researchers [12]. First, the researchers 
artificially inseminated hens with semen pooled from young roosters. Then they killed the hens by intravenous 
injection of an anaesthetic overdose of Expiral. Next they opened the dead hens‟ abdomens and removed the 
oviduct section containing the shell-less fertile eggs. They then placed the eggs in surrogate shells and injected 
the bacterial DNA into the cytoplasm of the germinal discs of the zygotes (the one-cell embryos). Next they 
filled the shells with a culture medium and sealed them shut. Following this, they analyzed the fate of the 
plasmid DNA microinjected into the germinal discs of embryos who survived for at least 12 days in culture. Of 
the 128 original ova, seven chicks survived to sexual maturity. Of these, one rooster transmitted the bacterial 
DNA to 3.4 of his offspring. Of these offspring, those who survived to sexual maturity were bred to produce 
transgenic offspring, “demonstrating that stable transmission of foreign DNA can be obtained by our method” 
The aim of a study published in 1995 in Transgenic Research was “to develop a safe retroviral system 
to obtain transgenic chickens” [13]. The problem addressed was the fact that replicating avian retroviruses used 
in vivo can be pathogenic even after a long time, increasing the risk of disease states associated with chronic 
viral infection. As the avian spleen necrosis virus (SNV) is closely related to mammalian retroviruses, and it has 
been found that SNV can infect human cells, the researchers sought a way to produce transgenic chickens using 
replication defective vectors based on a system (an “ecotropic” system) which is able to infect avian cells only. 
Using vectors derived from ecotropic avian leukosis viruses, the researchers microinjected foreign bacterial 
genes into the sub germinal cavity of unincubated chicken embryo blastoderms (tiny reproductive cells). DNA 
was then extracted from these embryos and injected into another group of unincubated chicken embryos who 
were then incubated. Of the group of 1550 chicken embryos infected with the ecotropic vectors, only 36 
hatched. According to the researchers, most died early after injection due mainly to the opening of their 
eggshells. One surviving rooster managed to transmit vector DNA to his progeny, at a rate of 2.2 percent. The 
researchers concluded that their data showed “the efficacy of ecotropic avian retroviral vectors to produce 
transgenic chickens”.