The establishment of Centre for Functional Genetics is based on the vision that domestic animals provide unique opportunities for studying the relationships between hereditary variation and phenotypic variation. We study all traits, from the occurrence of various diseases to animal behaviour.
We also make use of model organisms, for example mice, in order to study in greater detail genes which we have associated with a certain trait of the animal. The combination of genetic research on domestic animals and model organisms is a powerful method for better understanding gene function.
Just now we are in the golden age of genetics. Over the past 30 years increasingly sophisticated methods have become available for studying the genome and its function. This scientific breakthrough is comparable to access to the telescope for the first time for astronomical observations, or to the microscope to study cells. The genome of higher organisms is unbelievably complex and consists of billions of base pairs and tens of thousands of genes.
With the tools of modern genetics, it has become possible to study hereditary variation directly at the DNA level and to study its significance for the properties of the individual. We now have tools to reconstruct the evolutionary history of domestic animals and to understand at the molecular level what happened when humans bred domestic animals from their wild original forms (wolves, wild pigs, aurochs, etc). We also use domestic animals as a model for human medicine since most human genes have a counterpart in our animals. In most cases genetic function is well conserved between different species.
A centre of the type we have established is necessary so that we may fully achieve our ambition to couple genetic variation with the variation in individual traits. This research demands detailed knowledge of the phenotypes of animals as well as access to a broad arsenal of the tools of modern genetics.
Dogs and chickens
We conduct our research mainly on dogs and chickens, but some work is also done on horses, cattle and pigs. We use dogs mainly to study the hereditary cause of various diseases since it has a number of advantages as animal model for diseases which affect humans (see the article by Mia Olsson and Kerstin Lindblad-Toh in Miljöforskning 1/2008).
We use different crosses of chicken lines to study genetic factors that affect behaviour, appetite, growth, certain autoimmune diseases and simple morphological properties such as colouring and comb development. In this short summary I intend to give an example which well illustrates the type of new knowledge we generate and why this research is important.
How chickens acquired yellow legs
Most domestic chickens have yellow legs. The reason for the yellow colour is that the chickens store carotenoids in their skin. The carotenoids come from the food, and the more carotenoids the chicken has eaten, the yellower its legs will be. This property is caused by a recessive gene, and the wild type variant of the same gene gives rise to white legs. We have now shown that this trait is determined by a regulatory change in the gene that codes for the enzyme betacarotene deoxygenase 2 (BCDO2). This enzyme splits coloured carotenoids into a colourless molecule. The metabolism of carotenoids is essential since they are precursors for Vitamin A.
We could demonstrate that the difference between chickens with yellow and white legs is that this gene is not expressed in the skin of chickens with yellow legs but active in chickens with white legs. This means that carotenoids that are taken up in the skin are not split in chickens with yellow skin. The yellow colour therefore reflects how much carotenoids the chicken has eaten. Think of this the next time you are eating a corn-fed chicken! Corn is rich in carotenoids (lutein and zeaxanthin), and if chickens are fed on corn their skin will be especially yellow.
One may naturally wonder what difference it makes to know why some chickens have yellow legs while others have white legs. Do not researchers have more important problems to look into? Carotenoid-based pigmentation s is in actual fact extremely important in nature, and we daily come into contact with this. Examples that can be mentioned are the yellow breast of the great tit, the pink flesh of salmonid fish, the pink colouring of flamingoes – and who has not been fascinated by the bright red legs of the oystercatcher. These are all due to carotenoids that the animal takes in through its food.
Our study is an important breakthrough since this is the first time it has been possible to couple a specific gene with variation in carotenoid-based pigmentation. If we want to understand, for example, why some salmonid fish have pink muscles while others have white muscles, we now have a good clue to begin with. This is a good example of how fundamental research into our domestic animals can provide completely new basic biological knowledge of general interest.
The wild origins of chickens charted
The wild origins of chickens have been the subject of debate since the middle of the 19th century, and Charles Darwin made a pioneering contribution in this area also. Creationists believed that each species of domestic animal came from its own extinct original species. In contrast, Darwin claimed that these animals originate from one or possibly a few wild original forms and that the different species have been developed by humans. Obviously, Darwin was right on this issue also.
Darwin considered that the red junglefowl is the only species that contributed to the development of the domestic chicken. But when we determined the DNA sequence for the yellow leg trait, we were puzzled because it was so unlike the corresponding sequence in the red junglefowl. The difference between the two sequences was almost 1%, which can be compared with an average 1.2% difference in DNA between humans and chimpanzees. How is this possible?
We decided to study some species closely related to the junglefowl which, like the red junglefowl, live in southern Asia. These studies clearly showed that the yellow leg trait must have come from a different species, and most probably from the grey junglefowl which occurs in India. The most probable scenario is that, on one or several occasions, the grey junglefowl was crossed with the domestic fowl during the domestic fowl's early history. This is the first time it has been possible to demonstrate that more than one species has contributed to the development of the domestic chicken.
Yellow legs – good health
But why have humans so strongly preferred domestic chickens with yellow legs? Billions of chickens the world over carry this trait. One possible explanation could be that the yellow leg signals good nutritional status and good health. We have perhaps used the yellow coloured leg as an indicator of a chicken with good properties.
In studies of blackbirds, it has been possible to show that blackbird females prefer males with strongly orange beaks (which is of course due to carotenoids!) since this reflects how good the male is at getting food. So, if the blackbird female can judge the quality of the male on the basis of the colour of his beak, it is reasonable to believe that humans have used the colour of the leg to judge the quality of the chicken!
Author
:
Leif Andersson
is Professor at Department of Medical Biochemistry and Microbiology, Uppsala University and Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences.
Literature:
Eriksson,J., Larson, G., Gunnarsson, U., Bed'hom, B., Tixier-Boichard, M., Strömstedt, L., Wright, D., Jungerius, A., Vereijken, A., Randi, E., Jensen, P., and Andersson, L., 2008, Identification of the Yellow Skin Gene Reveals a Hybrid Origin of the Domestic Chicken. PLoS Genetics4: e1000010.