SIX calves from Connecticut, cloning鈥檚 latest stars, should finally answer the question of whether clones age prematurely.
Dolly the sheep, the first animal to be cloned from an adult cell, appears healthy so far. But there are signs that the wear and tear accumulated by that donor cell could make her age before her time: Dolly鈥檚 chromosomes are shorter than normal sheep of the same age, which is a sign of cellular ageing (New 杏吧原创, 29 May 1999, p 12).
Even if Dolly dies young, however, it might just be that longevity doesn鈥檛 run in her family. Since Dolly鈥檚 relatives aren鈥檛 around, Jerry Yang at the University of Connecticut in Storrs and his colleagues are studying the way clones age more precisely. To see how cellular ageing might affect ageing of animals cloned from the cells, they took cells from the ear of a 17-year-old bull and allowed them to divide in culture for differing periods of time.
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Some cells divided five times, others as many as 30 times. 鈥淓ach division is like ageing the cells about one year,鈥� says Yang. So by comparing animals cloned from cells cultured for varying periods, it should be possible to tell if older cells yield clones that age faster.
Because the breed of bull used by Yang typically lives to about 25 years old, he suspected that the oldest cells wouldn鈥檛 be useful for cloning. But they actually seemed to work better. Only 30 per cent of embryos produced from cells that had divided 10 times established pregnancies, compared with 64 per cent for cells that had divided 20 times. And although the number of pregnancies that went to term was small, these tended to be from cells cultured for longer. 鈥淕etting healthy births from such old cells suggests that ageing might not be as big a concern as we thought,鈥� says Jim Robl of the University of Massachusetts at Amherst, who also works on cow cloning.
Yang intends to monitor the calves, looking for signs of accelerated ageing such as chromosome shortening and declining immunity. 鈥淭hese bulls will be watched closely day and night the rest of their lives,鈥� he says. Philip Cohen