Many of us love to keep chickens as pets.
And even those of us that don’t, are often impacted by them on a daily basis.
We may rely on them to produce eggs every day.
And we may use their meat for protein.
In fact, chickens are so profitable for the poultry market because of these traits.
But have you ever wondered why they are so desirable to our society?
It may surprise you to know that chickens never used to lay eggs daily.
Nor were they ever as large as many meat producing chickens are today.
We often refer to these egg producers as layers, and large meat producers as broilers.
These characteristics have only developed through domestication.
Scientists also like to study the genetics behind this domestication!
This is because it can tell us a lot about phenotypic evolution.
Unlike most other species, chickens still have their living ancestor, the red jungle
fowl.
As they have evolved, they have acquired new traits to become larger and lay more regularly
much like this Eevee might.
This means that scientists can compare the genomes of modern chickens against red jungle
fowl to examine the evolutionary genetic changes involved in this domestication.
Genetic studies of chickens have been around for years.
Sequencing of the chicken genome first began in 1992 with the publishing of the early draft
linkage map with 100 loci or gene positions.
A further two maps were generated in the following years.
In 2000 a group combined these to produce one linkage map totalling over 1800 loci.
Then, in 2004, an entire draft genome sequence for the red jungle fowl was published, which
would later be used as a reference by our researchers.
This all lead to the study performed by Rubin and Zody’s team in 2010.
They took four broiler populations, four layer populations and a population of red jungle
fowl to examine the genetics behind this domestication.
SOLiD technology was used to re-sequence their genomes.
SOLiD technology produces millions of short 25 to 50 base pair DNA sequences.
Since they are short, they are ideal for identifying single nucleotide polymorphisms, better known
as SNPs.
Besides from being a single nucleotide change, SNPs can often be used to describe a variation
in a specific allele across different individuals.
These SNPs – or variations - from layers, broilers, and red jungle fowl, were used to
find putative selective sweeps.
This essentially refers to a favourable region – or haplotype - which becomes fixed in
a population over time.
And these regions may contain a gene or genes that may potentially be involved in domestication.
Rubin and Zody’s team identified 3 potential sweeps in all domesticated chickens, which
combined broilers and layers.
One of these sweeps occurred at the locus for the thyroid stimulating hormone receptor
gene, also shortened to TSHR.
Further analysis of this gene copy demonstrated that it was a mutated version.
This resulted in just a single amino acid change within the protein.
Since the amino acid glycine is so structurally different to arginine, it has very different
properties.
This is why this cell surface receptor may lose its original function.
And, after they examined 271 domesticated chickens across the globe, it was found that
97% of them were homozygous for this sweep haplotype.
This meant that domesticated chickens appeared to carry two copies of this mutated gene.
So, this recessive gene seemed important in chicken domestication, as it had become fixed
in broilers and layers.
This was a really exciting find for the researchers!
This is because it’s been previously shown that TSHR is involved in reproductive control.
Since domesticated birds contained a different copy of the gene, they hypothesised that this
mutant TSHR may be responsible for loss of reproductive regulation.
This would result in egg production all year round like that seen in all domesticated chickens,
including layers and broilers.
The researchers did go on to identify other potential genes specific to broilers.
Many of these appeared linked to muscle growth, appetite, and potential obesity.
These genes may explain why the chicken you eat is so tender and juicy!
However, the TSHR locus discovered by Rubin and Zody’s team remained the one biologically
important difference between all domesticated birds and their common ancestor.
Over the following years after this paper’s release, similar studies were performed in
rabbits, pigs and dogs, just to name a few.
So, you could say this paper really revolutionised the field of domestication genetics along
with our understanding of chicken evolution.
And people will likely continue to study this evolution, as it is happening all around us!
So, the next time you see a chicken, imagine all the genetic evolution that has occurred
to produce the amazing bird before you.
Certain genes are why that chicken may lay eggs every day so that we can enjoy our morning
bacon and eggs.
Without genetics, the chicken you buy from the shop would taste no way near as good.
When you look at it that way, I’m sure you’ll be awed by the complexity of chickens and
the genetics involved!