2010高口阅读训练26-《蛋壳DNA帮助繁殖频危物种》

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本文来源：GUARDIAN
本文字数：597
发表日期：MARCH 10, 2010  
所属类别：SCIENCE

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DNA from fossilised eggshells could help reconstruct lives of extinct birds

Scientists have collected DNA from the fossilised eggshells of birds that died hundreds and in some cases thousands of years ago.

The oldest eggshell to yield DNA came from an Australian emu that died around 19,000 years ago. It is the first time that scientists have succeeded in extracting ancient DNA from the fossilised eggshells of a bird.

Genetic material from the Madagascan elephant bird, the heaviest bird that ever lived, was also recovered, along with DNA from Australian owls, New Zealand ducks and flightless moas.

Elephant birds were native to Madagascar but had gone extinct by the 17th century. The ostrich-like creatures grew to around 3 metres tall and weighed up to half a tonne. Their eggs were bigger than footballs.

Eggshells from two other extinct species, the little bush moa and the heavy footed moa, both from New Zealand's north island, were estimated to be more than 3,000 years old. Attempts to collect DNA from a 50,000-year-old flightless Australian bird from the genus Genyornis failed because the DNA had degraded too much.

The ancient DNA has yet to be sequenced, but researchers will soon be looking to draw up genetic profiles of long-lost birds by extracting genetic material from eggshells held in museums and excavated at archaeological and fossil sites.

Previously, they had little hope of reading DNA from species that lived in warm climates because the genetic material breaks down so quickly.

By sequencing the genomes of ancient birds, scientists hope to build up a better picture of their physiology and how they dispersed and split into different species. It may even be possible to surmise their diets from genes encoding the enzymes for digesting particular types of food.

Charlotte Oskam, who led the study at Murdoch University in Perth, Western Australia, is now analysing a large collection of eggshells from ancient sites in New Zealand and hopes that DNA profiles of the birds will help explain how the arrival of humans brought about the extinction of the giant moa around 500 years ago.

The researchers used a technique called confocal microscopy to see exactly where the DNA is located inside the egg shells of two of the extinct birds, the New Zealand giant moa and the Madagascan elephant bird.

From this they were able to say that the DNA almost certainly comes from the mother hen rather than the embryo growing inside the egg. When the egg moves away from the ovary, cells from the mother get mixed up in the calcium carbonate shell as it thickens.

The research, reported in Proceedings of the Royal Society B, does not mean scientists will soon be able to resurrect long-extinct birds. Although the DNA can be sequenced, scientists would need to know how to repackage it into chromosomes, the giant molecules that carry genes.

The same problem makes it unlikely that scientists will bring woolly mammoths back to life, even though their DNA has been sequenced from well-preserved specimens recovered from the Siberian permafrost.

"As with all ancient DNA, the DNA we isolated from eggshell is very fragmented," said Oskam. It will be possible to sequence extinct genomes from fossil eggshell, he said, "but it is a huge leap to imagine we can clone an extinct species."