Name: ______________________
Date: ________________
Glowing Fish - First Genetically Modified Organism
Available as Pets
Genetic Engineering in Animals
The first genetically modified organism to be made available as a pet was the GloFish.
The GloFish was essentially a zebrafish that had been modified with bright red, green
and orange fluorescent colors. Like other genetically modified organisms, the GloFish
was merely an experiment, but it soon grew to prominence on the ornamental fish market
(a market that sells fish based on appearance).
GloFish Development
Zebrafish are native to rivers in India and Bangladesh. They measure approximately
three centimeters long and feature gold and dark blue stripes. However, natural
zebrafish have troubles breeding in the United States due to the fact that they are
tropical fish. This means they cannot survive in the wild. These facts played heavily into
the questions regarding the pros and cons of genetic engineering of ornamental fish.
While working at the National University of Singapore in the late 1990s, Dr. Zhiyuan
Gong and his associates extracted a protein from a jellyfish and inserted the gene into
the zebrafish genome. What does this mean, you ask? We can start by stating that
each organism has an instruction manual, or chemical blueprint, in each one of his or her
cells. That instruction manual is called DNA. Each “page” of the instruction manual
contains steps to build a different part of the organism. Each “page” or segment of
DNA, is called a “gene”. In simpler terms, Dr. Gong removed the gene, or chemical
instructions, that enable jellyfish to glow, and stuck it into the DNA of a zebrafish,
creating a transgenic organism.
The purpose of this experiment was to produce a fish that could be used to detect
pollutants and environmental toxins. The development of a fluorescent fish needed to be
completed first in order to continue the experiment. The goal was to create fish that
would “glow” in the presence of toxins. Gong and his team worked very hard to with
zebrafish, and created multiple versions of these glowing fish. Some contained jellyfish
DNA while others contained genes from sea coral giving them a red and yellow color.
The fluorescent genes are extracted from jellyfish or sea coral and then injected
into zebra fish eggs. The genes were then inserted into the DNA found within the
zebra fish eggs. With these newly inserted genes, the body of the zebra fish is
now capable of giving off a fluorescent glow. Scientists also added an “on-andoff” switch made of DNA, that cause the Glow-gene to be turned on or off in
certain conditions. The on/off switch is called a promoter.
So far, the researchers have succeeded in isolating two types of gene promoters
in the zebra fish an estrogen-inducible promoter and a stress-responsive
promoter. These promoters have been used to drive the fluorescent color genes in
transgenic zebrafish. Such fluorescent-colored transgenic fish will be able to
respond to the presence of chemicals like estrogen through the estrogenic
promoter and heavy metals and toxins through the stress-responsive promoter.
The fish will immediately display the color depending on the type of environment
the color has been specified for. For example, if a transgenic zebra fish is
swimming in waters that contain estrogen (a female hormone that promotes
female sex characteristics), the estrogen-inducible promotor will be activated and
the fish will begin to glow.
Techniques for generating Transgenic Organisms
The three principal methods used for the creation of transgenic animals are DNA
microinjection, embryonic stem cell-mediated gene transfer and retrovirusmediated gene transfer.
a) DNA microinjection.
This method involves injecting a chosen gene, or genes, from one organism into the
egg of another. The gene can be transferred from an organism of the same
species, or an entirely different species. It is one of the first methods that
proved to be effective in mammals and in many other types of species (Gordon and
Ruddle, 1981). When the new gene is inserted into the hosts DNA it will either
cause the hosts cells to produce more or less of the new protein or a protein that
already exists in the host cell. The insertion of DNA is, however, a random
process, and there is a high probability that the introduced gene will not insert
itself into a site on the host DNA that will allow it to be expressed. Once the egg
is made it will be transferred to female recipient that will act as a surrogate
mother. Sometimes the act of mating is vital for the female body to change and
nurture the growth of a developing embryo. To simulate this act often time’s
scientist will use male zebra fish that have been vasectomized.
b) Embryonic stem cell-mediated gene transfer. (Knockout Method)
Stem cells are undifferentiated cells that have the potential to develop, or
differentiate, into any type of cell (neuron, heart cell, eye cell, skin cell, etc.) In
the stem cell method, the transgene (gene from the foreign organism) is inserted
into a colony of stem cells in a Petri dish. These cells are then incorporated into
an embryo when the embryo is a small ball of cells. The result is a chimeric animal,
meaning some of the cells in the organism have the transgene in them, and some of
them are “normal” without the transgene. The challenge with this is that not
every cell has the gene of interest. In order to get every cell of an organism to
have this transgene you would have to breed them for a minimum of 10-20
generations before you made a fully transgenic organism.
c) Retrovirus-mediated gene transfer.
Viruses work by taking over the cell, and forcing the cell to replicate the virus.
Essentially, viruses hijack the cell and use them to make more viruses (and then
invade other cells). Retroviruses are a kind of virus that can be used to transfer
a transgene into the (hopefully) newly transgenic organism. Scientists insert the
transgene of interest into the virus’s genetic material, and the virus does the
rest: enters some of the cells of an embryo, inserts itself into the DNA of the
organism, and the cell begins to express the gene. Offspring derived from this
method are chimeric, i.e., not all cells carry the retrovirus. The challenge with
this is that not every cell has the gene of interest. In order to get every cell of
an organism to have this transgene you would have to breed them for a minimum
of 10-20 generations before you made a fully transgenic organism.
For any of the above techniques the success rate in terms of live births of
animals containing the transgene is extremely low. Providing that the genetic
manipulation does not lead to abortion, the result is a first generation of animals
that need to be tested for the expression of the transgene. Depending on the
technique used, the first generation may result in chimeras.
Questions: Use the article to answer the questions.
1. What is a zebrafish?
2. What is a glofish?
3. What is a gene?
4. What is a promoter?
5. What is a transgenic organism?
6. Explain how scientists created glofish from zebrafish?
7. Explain how scientists used glofish to study water pollution before they
became commercially available for use as pets?
8. What is estrogen? Why wouldn’t it be good to have it in water supplies?
9. In your opinion, are there ethical issues surrounding genetically modified
glofish? Explain.
11. How does this article relate to the movie “Avatar”?
12. How will this article help you answer the question, “How do you create an
Avatar?”
13. Why did scientists use the DNA microinjection method, rather than the other
methods? Explain.