2008 ‘Sheep don’t all look the same.’

In Biological Sciences Review, vol 21, p26

The text is reproduced here:

SHEEP - GENETIC VARIATION MEANS THEY DON’T ALL LOOK THE SAME

ANN LACKIE

There are about 50 different breeds of sheep in the UK, ranging from the tiny black Hebridean to the mighty dread-locked Wensleydale. But even within an all-white breed, not all sheep look the same – take them to a market and buyers will soon pick out the differences and pay different prices.

A sheep-breeder looks for good ‘conformation’ or shape, a butcher checks for meaty haunches, a wool-buyer wants to feel the quality of the fleece – and someone who keeps sheep as lawn-mowers might just be looking for an animal ‘with a nice face’. All these qualities are regulated by the sheep’s genes and can be selected for by careful breeding.

Woolly jumpers
In 2004 in New Zealand, Shrek the Merino sheep was finally captured; he had been living wild for 6 years and his fleece had grown to such monstrous size that the woman who found him said he “looked like a biblical beast” – 27 kgs of matted wool was taken off him when he was shorn. Merino wool is normally highly prized because it is white, and is thus easily dyed. The wool fibres have a diameter about 20μm, and are long, about 9-10cms. Because of this, a Merino pullover is soft and fine.
But you might not want to wear a Herdwick pullover - the wool of this ancient breed found in the Lake District is like the wire-wool used in pan-scrubbers, thick, coarse and with short fibres, its colour ranging from black through greys to white (see Figure 1). Herdwick sheep are well-adapted to the harsh environment of the Lake District fells. Their wool supports a small rug and loft-insulation industry but their meat is renowned for its distinctive taste and quality.

Colour change
At birth a Herdwick lamb is black all over but within a year its face is pure white, and the fleece on its body starts turning bluish-grey. Something interesting happens during the lamb’s development: the genes which are involved in making the black pigment melanin have been switched off in some of the hair-follicle cells - ‘white’ hairs are actually colourless. We know that environmental factors such as day-length switch off one of the genes that cause the fur of arctic foxes to turn white in winter – so does the same factor switch off the genes in sheep? As yet, even though sheep are much more readily available as experimental animals than are arctic foxes, it seems that nobody has done the research.

Patterns
Although the knitters of Fair Isle pullovers use brightly-dyed wool in striking, traditional patterns, there has more recently been a shift towards using the natural undyed colours of the local Shetland sheep. Shetland sheep, like Icelandic sheep, come in a variety of colours with white, brown, grey and black wool. Shetlands have 11 recognised colours (eg ‘moorit', red-brown and ‘shaela’, dark grey) and 30 different patterns (eg 'katmoget', dark belly), all of which have a genetic basis. Like the seasonal and developmental colour changes in arctic foxes and Herdwicks, the formation of fixed patterns in the skin of mammals is a fascinating problem.

Most research on such patterns has been done on mice and shows that there is a complicated set of molecular signals that switch colour-affecting genes on or off, and cause the colour-forming cells to move to different areas of the embryo’s skin. There are recessive and dominant forms of some of the controlling genes.

Black sheep
In Merino sheep like Shrek, white is the dominant colour: black is recessive. In the skin cells of black Merinos the gene for ‘Agouti signalling protein’ (ASIP) is switched off. ASIP inhibits production of melanocytes, the skin cells which make the black pigment, melanin. If  there’s no ASIP, there are more melanocytes in the groups of hair follicle cells that make the wool -- and more melanin. Hence we get black sheep.

Sheep-farmers can select for fleece colour and pattern by careful breeding programmes, and they can also select, to a limited extent, for wool quality – finer, or stronger, or longer fibres of this important protein. Wool quality is also influenced by environmental factors, especially nutrition. If you see a sheep with its fleece hanging in tatters in the Spring, the odds are that she has been pregnant with twin lambs, during which time she didn’t receive sufficient high-energy food. The lambs growing inside her would have taken most of the energy deriving from her food and so less was available for her fleece; fibre-growth is temporarily halted and the wool breaks at the point of weakness.

Lamb chops and legs of mutton
An apple-orchard bears a crop of apples, a sheep-flock bears a crop of lambs. The principle is the same, fertilisation followed by development of an embryo, which the parent nurtures with food while it is growing. Then, when the offspring is a suitable size, whether it’s an apple or a lamb, it is harvested by the farmer. The parents live on to produce another crop the following year.

Until about the 1960s ‘mutton’ - from older sheep – was the usual type of meat, but then the more tender meat from lambs took over. Mutton is becoming fashionable again, helped by high-profile campaigns by Jamie Oliver and the Prince of Wales, amongst others.

Whether lambs are slaughtered for meat or reared to adulthood for breeding, the bigger the crop of lambs from the sheep-flock, the greater the profit for the farmer. Breeding programmes select rams and ewes that produce more lambs. Although twins are normal, triplets and even quadruplets, are becoming common. Quality is important too. Meat-eaters want large fat-free loin chops, and chunky legs of lamb for their Sunday roasts, so farmers also want to breed fast-growing lambs with big backsides.

Beautiful buttocks and big behinds
This is where the callipyge gene (cally-pie-jee, a Greek word meaning ‘beautiful buttocks’) comes in. Callipyge is one of three known genetic mutations in sheep that cause the muscles of the loin and pelvic regions to be very large. Unfortunately ‘beautiful buttocks’ make for tough Sunday roasts – but the ‘double-muscle’ mutation recently found in Texel sheep seems to be a winner.

Texels, from the Netherlands and Belgium, are now a common breed in the UK. They have ugly faces and enormous backsides  – indeed, I have occasionally seen Texels sitting upright on their haunches like dogs. There’s an interesting mutation in one of the genes involved in their muscle development. This gene normally codes for myostatin, a protein which regulates skeletal muscle growth. The mutation results in a lack of myostatin, and muscle growth is no longer controlled. Consequently the Texel sheep exhibit exaggerated muscle development with more muscle fibres rather than thicker fibres – and the buttocks get bigger and ‘meatier’.

Disease and losing genomes
Scrapie is a transmissible spongiform encephalopathy (TSE). It is an infectious disease of the nervous system like BSE in cattle and Creuzfeld-Jacob disease in humans. The disease develops when a protein (called PrP, the prion protein) which is normally present in a range of tissues, including brain tissue, nervous tissue and white blood cells, is converted into an abnormal shape and forms clumps. The diseased sheep lose control of their limbs, stop eating and eventually die.

Small changes in the gene for PrP also determine whether the sheep will be susceptible or resistant to the disease. Changes at 3 codons mean there are changes in the amino acids found at 3 specific positions in the PrP protein sequence. These changes have given rise to 5 different alleles coding for the prion protein in sheep. The alleles are identified by the amino acids coded by the 3 specific codons using a single letter of the alphabet to describe each amino acid (se Box 1). Since each sheep inherits one allele from each parent it will either be homozygous eg ARR/ARR, or heterozygous eg ARR/ARQ where the amino acids are A, alanine; R, arginine; Q, glutamine, in these examples. You can probably work out for yourself that 5 alleles give rise to 15 possible combinations. These result in varying degrees of resistance to scrapie, ranging from completely resistant lambs to completely susceptible (see Box 2).

The National Scrapie Plan (NSP) was started in 2001 with the aim of eradicating scrapie from British sheep, by increasing the proportion of resistant animals. The NSP provides Tables that are useful for farmers to calculate the results of mating sheep with the different alleles (see Further Reading). This is important because the NSP’s aim is to cull (kill) the sheep which have the ‘susceptible’ alleles VRQ, ARQ, AHQ and ARH. This will remove these alleles from the population.

How do farmers know which PrP alleles a ram has? A blood sample is taken, the DNA is extracted from the white blood cells, and a genetic test is used to check which of the 5 alleles are present. As susceptible sheep are culled a higher proportion of the remaining sheep will be resistant (see Box 2).

A genetic bottleneck
These alleles have been around a very long time, so it’s likely that they are linked in some way with other genes that are ‘useful’ – an example of a balanced polymorphism. So if all sheep with ‘scrapie-susceptible’ genes are culled, the genetic diversity of the remaining sheep population is greatly reduced.

This policy of culling has its dangers. Farmers argued that if an unusual disease were to strike in the future, the sheep remaining after the scrapie-cull might not have a large enough gene pool to evolve resistance to the new disease. Potentially ‘useful’ genes might have been removed from the gene pool too.

Luckily, the danger of creating this genetic bottleneck was spotted in time and the national Semen Archive was set up, to store frozen semen (and thus the genomes) of scrapie-susceptible rams before they are culled.

The disaster of Foot-and-Mouth disease
The 2001 foot-and-mouth epidemic led to a massive cull of millions of sheep, and farmers in the Lake District feared that the local Herdwick sheep, a breed highly adapted to the tough conditions on the hills, would become extinct. In a race against time to save the Herdwick genome, they helped scientists from York University to collect ova and sperm from the sheep. These, with genetic material from other rare breeds of sheep, are held frozen in a gene bank run by the Sheep Trust. When needed, the eggs and sperm can be thawed and used to create embryos which can be implanted into surrogate mother sheep to grow new lambs. The genomes have not been lost.

Ann Lackie formerly researched immunity to parasites, including sheep liver fluke in snails. Now a novelist (Ann Lingard), she also keeps sheep on a small-holding in Cumbria. www.annlingard.com
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Further reading:

NSP Table for calculating scrapie alleles www.defra.gov.uk/animalh/bse/othertses/scrapie/nsp/pdf/calculatorgrid.pdf

For excellent photos and descriptions of dozens of sheep breeds (including Texels) see The National Sheep Organisation
www.nationalsheep.org.uk

 


Box 1: Scrapie susceptibility: The different amino acids produced by the 5 alleles in the PrP gene

ARR, AHQ, ARH, ARQ, VRQ

(Key to amino acid abbreviations: A – alanine; H – histidine; R – arginine; Q – glutamine; V – valine)    


Box 2: Percentage of  scrapie-resistant (ARR allele) and scrapie-susceptible (VRQ allele) ram lambs in the test flocks since culling started (taken from NSP report)

                     Allele   2002   2006

                      ARR   50.4   68.8
                      VRQ    3.0   1.2