Domestication is an incredibly central process throughout civilization, with widespread effects on the target animal populations. Species like the modern domesticated chicken have experienced enormous selection over the past 8 000 years for everything from size, to color, to anxiety behavior.
Associate professor Dominic Wright, Ph.D., is an evolutionary biologist specializing in domestication, feralization and the genetic basis of animal behavior. As part of AVIAN, the Behavioural Genomics and Physiology research group at Linköping University, Assoc. Prof. Wright has managed to find evidence for the genetic basis of various forms of anxiety behavior in chickens and mice, with extrapolation into the genome of fruit flies and even humans.
QTL Analysis and Gene Mapping
Assoc. Prof. Wright's research uses a combination of QTL and eQTL mapping with an advanced intercross line to map the genetic basis of behavior, weight, bone density, color and other domestication traits. These can then be used as a model to explain externally observable factors, giving researchers insight into how these different traits are controlled.
The term QTL is a combination of Quantitative Trait, that is a continuous trait like height, weight, or behavior, and Loci, meaning location. QTL mapping as a technique enables geneticists to find the regions in the genome which control for specific traits. For example: there are lots of genes which affect height, meaning there are lots of QTLs that affect height. Each single gene mutation in one of these QTLs would increase or decrease the subject's height and the cumulative effect of all the QTLs would determine the subject's predisposition towards a particular height.
Because the domestic phenotype is so far removed from the original wild state, studying domestic by wild crosses gives researchers a lot of variation to work with. For example a modern broiler chicken is six or seven times heavier than the original Red Jungle fowl chicken. This large variation in continuous traits make it easier to map individual QTLs and their effects.
Increasingly Fine Scale
Assoc. Prof. Wright's research focuses on comparing highly selected domestic animals to their original state by using QTL crosses to genomically cross the wild and domestic animals, essentially using domestication as a genetic model, of domestic by wild. By continuing to cross the test subjects Assoc. Prof. Wright and the AVIAN group has been able to create a very fine scale mosaic of QTL variations.
Using eQTLs, expression QTLs, Assoc. Prof. Wright has managed to add another layer of complexity by looking at gene expressions. Due to the extensive facilities available in-house at Linköping University, the AVIAN group has been able to measure complete gene expressions, for example the hypothalamus expression in the brain, and measure which genes are up-regulated or down-regulated.
The Genetics of Anxiety Behavior
It has been postulated that the first thing to change in domestic animals is their fear of humans as domestic animals cannot be too afraid of humans without impairing our ability to breed them. This enables researchers to use a domestic by wild cross to discover which genes affect anxiety behavior.
Assoc. Prof. Wright has found that some of the genes that are central for anxiety selection in chicken intercrosses are also present in a mouse cross, and were affecting the same behavior in the mice as in the chickens. When comparing these particular genes with gene sets in humans with bi-polar disorder and schizophrenia, Assoc. Prof. Wright has found evidence that the same genes may be related to anxiety disorders in humans. Recently his group and their collaborators have also found evidence to support this in the fruit fly where, by down-regulating some of the genes found in chickens, they were able to affect the same type of open field behavior in transgenic flies.
That gives the idea that the genes found in the AVIAN group's domestic by wild paradigm can be transferred to practical applications in other fields. For example, the Wright Lab has found genetic links to schizophrenia and has used the chicken as a model for osteoporosis.
Sexual ornaments, such as comb size, where males and females chose mates with the larger comb, are very common in evolutionary biology. The classical example of this is the peacock's tail, which appears to be a purely sexual ornament and quite useless in terms of natural selection – these are cumbersome, expensive to produce and actually impede the peacocks flight. But because of the strong female preference they always maintain this otherwise useless trait.
Assoc. Prof. Wright has found a strong correlation in chickens between the size of their comb and their reproductive potential, governed by the HAO1 and BMP2 genes and their pleiotropic effects on fecundity traits. This is because females with larger combs have more medullary bone, which they use as a calcium store for making egg shells, which is in turn a strong limiting factor in their reproduction. Thus it makes sense for males to choose females with larger combs.
Assoc. Prof. Wright's sexual ornamentation research also encompasses color, which has been extensively studied in all types of animals due to the ease of mapping it in the classical Mendelian way, as well as the amount of selection acting on the color. In the case of colour it is possible to map the intensity of colours in a similar fashion to other quantitative traits. Using this approach and in collaboration with colleagues at Linköping's MIT dept., the group has identified QTL and genes associated with the intensity of red coloration in chickens.
In the late 1980's and early 1990's several powerful hurricanes struck Hawaii, destroying thousands of chicken coops. The domesticated chickens that escaped started becoming feral and interbreeding with the locally present Red Junglefowls. This has given researchers a fantastic opportunity to study large-scale feralisation, the process by which previously domestic animals become feral, and most specifically the genomic changes associated with feralisation.
Assoc. Prof. Wright has, in collaboration with Dr. Eben Gering from Michigan State University, studied the process of feralisation amongst Hawaii's escaped chickens. This has enabled them to study which regions of the chicken genome responds to feralisation and what genes change frequency, in order to start mapping the corresponding traits and creating a model for the ‘reverse of domestication’.
Genetic Basis for Brain-to-Body Ratio
The predominant view on brain-to-body ratio has for a long time been that there is an allometric relationship between brain and body size, i.e. that you need a certain brain size to maintain a certain body size. However, Assoc. Prof. Wright's recent research has shown that in chickens it is possible to independently select genes for brain size and body size. For example, domestic animals have often been thought of as somewhat less smart, and when one looks at their brain-to-body ratio they have a lower ratio than their wild progenitor.
By looking at the underlying genetic architecture for brain mass and body mass, however, researchers at the Wright Lab have discovered that domestic chickens actually have larger, not smaller, brains than their wild counterparts. Thus brain size and body size can be decoupled, and it is possible to select for a larger birds without selecting for a larger brain. This can have large repercussions in terms of evolutionary biology, as relative brain size (that is brain size over body size) is often used to compare different species.
If it is possible to select for increased body size without selecting for increased brain size, this method may be flawed. Further to this, by looking at individual substructures of the brain, it is possible to pinpoint the regions that are most associated with domestication selection – in the case of the chicken, the cerebellum seems to be particularly enlarged in the domestic bird.