In this next installment of SciFiber we will be exploring some of the genetic factors affecting the fleece color of sheep. This past quarter I was enrolled in a Biology class focusing on genetics and evolution and learned about epigenetics for the first time. Being the diligent fiber nerd that I am, I just had to know how to apply this new found knowledge to deepen my understanding of the science behind why the fiber we love works the way it does. Don't think that I have forgotten about my promise for a post on the makeup of keratinous fibers; it's in my blog queue and you'll be hearing from me again very soon!
We have all known a black sheep, and perhaps some of us have even identified as one. Black sheep of the family go against the grain, they embody traits seen in no other members. The phrase has permeated culture as the way to describe the oddball of the gene pool, but have you ever thought about where it came from?
The literal black sheep of the herd is the singular black ovine in a sea of white (protip: just as cows are bovine and horses are equine, sheep are ovine). But why is the black sheep the odd man out? Why isn’t Axel here considered the white sheep of the family?
Knitters, this is where your intuition should be sparking. As Josh knows, you can’t overdye black. But you can dye white wool, and therefore white fleeces are far more desirable for dyers, knitters, and anyone who likes colorful clothing. So, in order to maximize profits, sheep farmers have been trying to reduce the number of black sheep in their flock for centuries. Every once in awhile a black sheep would pop out of a white/white pairing, but farmers are an intelligent breed, and were not surprised because they understood the role of heterozygotes in Mendelian inheritance (if that sounded like gibberish check out this primer, but don’t worry, you can save it for later, the rest of this post makes sense without it). But every few generations something different would come along.
The agouti sheep.
Also known as "badger face," sheep with agouti color patterns are rare and their fleece is desired by knitters, spinners and weavers who enjoy working with 100% natural, undyed wool. The richly colored fiber of agouti sheep has a natural variegation to it that spins up into drool-worthy handspun yarn. Angora rabbit breeders long ago recognized the beauty of agouti rabbits and began selecting for the trait during breeding. The result is a line of rabbits with incredibly beautiful, naturally variegated coats. This selection has taken hundreds of generations to stabilize, however, and you can bet your britches that an agouti sheep revolution won't happen overnight. In fact, the few breeders who have tried their hand at an agouti-based business have become frustrated by the inconsistency of the coloration of their sheep. From year to year and even from season to season sheep farmers have noticed drastic color changes in their agouti flock with seemingly little pattern1. If they were able to predict and control the color palatte of their flock we might see a boom of this beautiful wool. But what control can mere humans exert over ovine biological processes? The answer may lie in a developing subfield of genetics called epigenetics.
Before we unpack epigenetics
let's quickly go over the genetic factors affecting the coloration of sheep.
Genetics of Color in Sheep
Agouti-colored hair occurs in a wide range of mammals, most notably cats, mice, rabbits and, of course, sheep. Agouti hair is characterized by a banded structure of black and yellow pigments, and is a result of competition between two different signalling proteins at the Agouti Signalling Protein (ASIP) location in DNA. One of the signalling proteins creates pheomelanin, or yellow hair pigment, and the other creates eumelanin, or black hair pigment. Geneticists studying the ASIP location in both sheep and mice have found that the Agouti allele (or variation of the gene), is semi-recessive. That is, if you breed a white sheep and an agouti sheep you are most likely to produce a white offspring (agouti is recessive to white), but if you breed a black sheep and an agouti sheep you are most likely to get an agouti offspring (agouti is dominant to black).
Now, here’s where it gets interesting: researchers studying mice displaying agouti coloration have found that, through controlling the environment, they can affect whether mice display agouti or black coloration, and they've been able to hit just about every coloration in-between2. They have been able to exert this control using environmental factors that affect the epigenetics in these mice. Epigenetics describes a biological system of gene regulation where chemical reactions at the DNA level determine whether or not certain areas of the genetic code are turned "off" or "on." In the example below, all mice carry the agouti allele, however in the case of the black mouse it has been completely silenced (has been turned "off").
Just how have scientists been able to control the expression of agouti? One answer is through diet. Because the "offness" or "onness" of genes is related to the amount of methlyation at the gene location, increasing the amount of methyl in the diet of a pregnant mouse mom has shown to decrease the expression of the agouti allele in her offspring3,4. This means that, even in an agouti/agouti pairing, moms that eat diets rich in methyl can produce black babies. Even more interesting, there is some evidence that the amount of methylation, that is, the "onness" or the "offness" of the agouti gene, is heritable, meaning that it can be passed on to future generations of mice5.
So what does this mean for sheep?
Unfortunately, while geneticists have been studying the DNA of mice for decades, interest in the genetic code of sheep has only recently surfaced. There is very little information about epigenetic effects in sheep coloring, but only because research has just begun! In just a few short years researchers have not only successfully located the ASIP gene in merino sheep but have also determined the role of the agouti gene in fleece color expression6. The gene duplication mechanism seen at the ASIP location in sheep is similar to that in the genome of mice, indicating that sheep farmers might soon be able to exert epigenetic control over their flock's fleeces through maternal diet alteration.
So keep your ear to the ground because I predict that within my lifetime we will have understood the epigenetics of sheep thoroughly enough that we might start seeing custom, boutique sheep colorations appearing on the market.