Course: SAT 170 Introductory Biotechnology

Unit 10 Learning Module

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- Unit 10
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Transgenic Animals and other Organisms
The mechanisms for generating genetically modified animals are similar but not exactly the same as those used for plants. For example, the use of retroviruses as a vector to introduce the gene of interest into an animal cell is one that is commonly used for the production of transgenic animals. This method does not work for plants as the retrovirus does not recognize plant cells as proper hosts (the same is also true for the TI plasmid that is effective at transforming plants but not animal cells because animal cells do not represent the appropriate host for that plasmid). See unit 7, for a complete description of the use of retroviruses as vectors in generating GMO’s. We also described microinjection as a way of introducing foreign DNA into cells in unit 7. For the production of transgenic animals, microinjection is performed on a zygote. Zygotes are the cells which result once an egg has been fertilized by a sperm cell. If you can alter the DNA of a zygote, you can change the genetic content of every cell in the animal that develops from that zygote. This form of microinjection is referred to as pronuclear microinjection. Another method, known as the embryonic stem cell method is also used for the production of transgenic animals. The embryonic stem cell method combines what is known about stem cells with that of GMO’s. For this method, stem cells are collected from the inner cell mass of a blastocyst (see unit 8 for a review of embryonic stem cells).  These stem cells are then mixed with the transforming DNA. Since some of the embryonic stem cells pick up that DNA, when they are placed back into a host blastocyst, they can develop into an animal which carries that DNA. Sperm-mediated transfer is another method used to generate transgenic animals. For this method, segments of DNA are attached to sperm cells. When the sperm cell enters the egg during fertilization, it carries that DNA with it. Transgenic animals can also be produced using gene guns and electroporation which were described previously with transgenic plants. Many different types of transgenic animals have been created using these methods including livestock such as sheep, cows, poultry, fish and shellfish.

Much like in plants, the main focus of transgenic animals in agriculture is to increasing yield from these animals. This may be accomplished through enhancing the growth rates to reduce the time needed to bring animals to market. One such example that is currently available is transgenic salmon developed by Aqua Bounty Technologies to carry a promoter that will activate a growth hormone year round, thus increasing the rate of growth of these modified animals compared to their wild-type counterparts.

Figure 1 - Transgenic salmon such as the one created by Aqua Bounty Technologies may soon be the norm among fish grown in fish farms such as the one shown here. The advantage of these transgenic salmon is an increase in overall size because of consistent stimulation of growth of these animals.

Figure 1 | Figure 2

Other modifications focus on improving the nutritional content of the animals and animal products. For example, chickens are being transformed to carry genes that alter cholesterol production which would results in eggs containing less cholesterol. Dairy cows have also been modified to carry genes that increase milk production and some have even received genes to alter the content of their milk. One such example comes from a transgenic bull named Herman which carried a human gene known as lactoferrin which is important for human development. The idea is that any female offspring that he sires can then produce milk that carries this protein.

Animal loss during the husbandry process is a real and expensive problem facing agriculturists. Thus, enhancing resistance of animals to certain diseases is another goal of agricultural biotechnology. By transforming animals to carry disease resistance genes, we can prevent costly losses of animals and their products. For example, taura syndrome is a viral disease which has proven to be devastating for shrimp. A company known as ViaGen has produced a transgenic shrimp that is resistant this disease.

The Enviropig is another popular story of a transgenic animal that addresses reducing feed cost as well as pollution from raising pigs. The Enviropig has been modified to create an enzyme that is released in the salivary glands of the pig. This enzyme allows pigs to more thoroughly digest their feed leading to a decrease in the amount of feed needed for these pigs. It also reduces the output of harmful phosphorous pollutants in the feces of such pigs since they are further digesting the phosphorous containing compounds in their feed.

Figure 2 - Pig farms such as that shown here are often criticized for the amount of phosphorous containing pollutants that are introduced into the soil and water from the pig feces. The introduction of the Enviropig may be able to help with this pollution problem and at the same time, decrease the amount of feed required to raise pigs.

The use of transgenic animals as a source of nutritional supplements and pharmaceuticals as well as disease models will be discussed in unit 11 where we examine medical biotechnology.

Figure 3

Transgenic Insects
Engineered insects have been developed for a couple of agricultural reasons. One such reason is to create biosabateurs which can disrupt the natural population of a pest insect. This is another method that has been employed for controlling insect populations in agriculture. Such biosaboteurs are insects which have been genetically modified to contain a lethal gene. The activation of this lethal gene is prevented in the laboratory. However, once released into the environment, the gene is no longer repressed. The hope is that by releasing these modified insects into the environment, they can mate with the wild-type insects and thus spread this lethal gene. The offspring which represent the next generation would then die off before being able to reproduce leading to a decrease in the size of the pest population.

Figure 3 - The pink bollworm (Pectinophora gossypiella ) shown here is known for being a pest to various agricultural crops particularly cotton. It has been the target of genetic modification both through the introduction of Bt cotton crops as well as through the introduction of pink bollworm saboteurs.

Transgenic Bacteria
Food processing aids have been isolated from genetically modified bacteria for agricultural purposes. One example of this is the modification of bacteria, and sometimes fungal organisms, to carry the genes necessary for producing the enzymes needed for the production of cheese from milk. These enzymes naturally exist in mammals. The genes for these enzymes have recently been isolated from mammals and inserted into bacteria to create commercially available rennet. The term rennet is used to describe this complex of enzymes needed to coagulate milk for the production of cheese.

The production of animal drugs from genetically modified bacteria, such as bovine growth hormone (BGH) is another focus of agricultural biotechnology. This BGH, also known as bovine somatotropin, can increase the milk production of dairy cows. The use of this hormone was limited as the primary source was cow cadavers. With the introduction of bacteria that had been genetically modified with the gene for bovine somatotropin the use of BGH has expanded.

Biofuels
Another focus of agricultural biotechnology is the use of plants and algae as a source of biofuels. Biofuels are fuels which are produced from organic matter, such as that generated from the biomass of a living thing. Plant wastes contain large amounts of sugars that, if made accessible can be converted to ethanol, one of the major forms of biofuels. Solazyme, a company based out of San Francisco has been working on creating bioreactors capable of producing large amounts of algae from which biofuels can be harnessed. We will discuss the role of biotechnology in the production of biofuels further in unit 14.