Members of a herd of cattle that has been inbred for at least 300 years are almost clones, yet the herd remains as fertile as ever, says a British team.
This discovery supports suggestions that extreme inbreeding can sometimes result in a 鈥減urging鈥� of bad genes, rather than extinction. And it raises hopes that inbreeding or even cloning of extinct or endangered species might produce some successful new populations.
A team led by Peter Visscher of Edinburgh University studied DNA from members of the now 49-strong herd of Chillingham cattle. These cattle live in isolation in a park in northern England.
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Records indicate that no new cattle have joined the herd in over 300 years. They also reveal that the fertility of the herd has not declined over this time. This is despite a population crash in 1947, which left only eight bulls and five cows.
The new DNA analysis shows that the herd 鈥渋s almost a clonal organism,鈥� says Stephen Hall of De Montford University, UK, one of the researchers. 鈥淭his is unprecedented in mammals 鈥� no other population is so uniform.鈥�
鈥淭his study doesn鈥檛 alter the likelihood that inbreeding will lead to extinction. But it does suggest that purging can happen,鈥� says Hall.
So while inbreeding small numbers of endangered animals may often result in poor fertility and even extinction, Hall thinks that in some 鈥渓ucky鈥� cases, gene purging could lead to viable populations.
Lucky line
The team examined 25 鈥渕icrosatellite markers鈥� in DNA taken from 13 calves, cows and bulls. They found that 24 of the 25 markers were identical in all the cattle. Samples from a normal herd would have shown that around 70 per cent of the markers were different, says Hall.
The inbred herd鈥檚 survival may be due to harmful genetic mutations cropping up only infrequently. This would allow the mutations to be purged by cattle deaths without the herd being wiped out, says Hall.
But this successful purging probably happens infrequently, he says, as previous research suggests that inbreeding usually does weaken a population. One key study on mice in the 1960s found that 19 out of 20 inbred lines died out.
鈥淭here are other white cattle populations, but most would have died out if they hadn鈥檛 been cross bred,鈥� says Hall. 鈥淚t鈥檚 possible that we鈥檝e had 20 populations and we鈥檝e got one left.鈥�
Domestic bliss
Luck may have had a significant part to play in the Chillingham herd鈥檚 survival but other factors probably contributed to its success, says Hall. Most significantly, the herd was once domesticated.
鈥淭hey鈥檙e now running wild, and they鈥檙e feral, but there may have been some human help in the gene purging during domestication,鈥� says Hall. 鈥淭here is a theory that during domestication, you select for genes that tolerate inbreeding.鈥�
If genes that promote tolerance of inbreeding do exist, tracking them down may be important for the successful breeding of small numbers of endangered species in the future, he says.
More at: Nature (vol 409, p 303)
