Darker black sheep's decline is in the genes
By Nic Fleming, Science Correspondent
Last Updated: 12:01pm GMT 18/01/2008

Scientists have cracked the mystery of the gradual disappearance of the
black sheep of St Kilda.

Soay sheep on St Kilda: Numbers of the darker sheep have been falling since
1985
A group of primitive Soay sheep on the Scottish island archipelago are among
the best understood mammals in the world because their isolated home
provides the ideal conditions to study natural selection.

Researchers were perplexed when they noticed the proportion of darker sheep
on Hirta, the largest of St Kilda's four islands, was falling - despite
their larger size suggesting they should been more likely to survive and
reproduce.

Now however, in a study published yesterday in the journal Science, they
reveal genetic variations responsible for the dark brown colour of their
coats is inherited alongside other genes that undermine reproductive
success.

St Kilda lies 110 miles west of the Scottish mainland. It was the most
remote inhabited part of the UK until the last remaining 36 islanders were
evacuated in 1930.

The sheep on Hirta originated on the nearby island of Soay - a Norse word
meaning "island of sheep".

The researchers noticed two years ago that the proportion of dark brown to
lighter animals had been dropped from approximately three quarters by an
average of 0.4 per cent per year since 1985.

Dr Jake Gratten, of the Department of Animal and Plant Sciences, at
Sheffield University, said: "The periodic population crashes provide
opportunities for natural selection to operate, giving us the ideal
conditions to study the process.

"We have known for some time that the dark sheep are larger than lighter
sheep, and we also know large size is usually associated with an increased
probability of survival and reproductive success.

"Our expectation was the darker sheep would perform better and that
eventually there would be no lighter sheep left on the island. The discovery
that the reverse was true was counter intuitive.

Dr Gratten and other colleagues in Sheffield, Edinburgh and Brisbane,
Australia, analysed the region of the genome - or genetic code - of the Soay
sheep which contains the coat colour gene.

They identified the existence of other genes in the same region which are
inherited alongside the one that determines coat colour and which, in the
darker sheep, reduce their chances of surviving into adulthood - outweighing
the benefits of larger body size.

Their findings support the view that relationships between traits such as
colouring and natural selection in all species are more complex than
previously believed.

See www.telegraph.co.uk and search for Black Sheep for original article and
picture.

It's just as well that our little flock of black Soay are thriving here in
sunny (yes, well) Scotland!

Gordon & Juliet.





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