A critically endangered flightless parrot in New Zealand has survived as an inbred population for so long that it has fewer harmful mutations than expected.
This may be because many of the birds affected by the mutations died out over the past few thousand years. 鈥淎s far as inbreeding goes, at a population level, what doesn鈥檛 kill you makes you stronger,鈥� says at the Centre for Palaeogenetics in Stockholm, Sweden.
The findings come from the first genomic study of the kakapo (Strigops habroptilus), only 204 of which are alive. There were once hundreds of thousands of the birds in New Zealand, but their numbers began falling when settlers arrived from Polynesia about 700 years ago and declined more steeply with the arrival of Europeans, who introduced cats, rats and stoats.
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In the 1970s, kakapo were thought to be extinct, but then 50 were discovered on Stewart Island and were moved to a few smaller sanctuary islands that had been cleared of predators. An intensive conservation programme has turned around their fortunes.
Dal茅n鈥檚 team compared the genes of 35 of the sanctuary birds with those of 13 old museum specimens from the mainland plus one found living on the mainland in 1975.
The birds on Stewart Island were thought to have been deliberately taken there in the past 500 years, based on historical accounts and an absence of kakapo fossils. In fact, the genetic analysis shows that the birds on Stewart Island and the mainland diverged about 10,000 years ago, suggesting they were already present in the area when rising sea levels cut off the island at that time.
As expected, the kakapos living on Stewart Island have less genetic diversity than the mainland birds. But surprisingly, they have an average of 18 potentially harmful genetic mutations per bird, compared with 34 in the museum specimens.
This is probably because of a process called 鈥減urging鈥�, when communities become so inbred that harmful mutations accumulate and individuals with two mutated copies of a particular gene have fewer offspring.
鈥淭he Stewart Island kakapo have been suffering from inbreeding for 10,000 years,鈥� says Dal茅n. 鈥淐onsequently, when the population size decreased even further in the last centuries, they were likely less affected than they would have been without going through this earlier purging of harmful mutations.鈥�
There was bad news too, though: the lone mainland bird that was taken to Stewart Island in the hope of boosting genetic diversity had 32 mutations. This may mean his offspring shouldn鈥檛 be chosen for the ongoing breeding programme, says Dal茅n. 鈥淗e might be going to break the inbreeding but there鈥檚 a threat that the mutations he carried will make things worse.鈥�
Cell Genomics
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