IT鈥橲 back to the drawing board for cloning pioneers trying to explain why only 1 to 5 per cent of pregnancies result in healthy, cloned animals.
Their previous explanation was that genes in cloned embryos get 鈥渕is-programmed鈥� during the process of making a cloned embryo. But Xiangzhong 鈥淕erry鈥� Yang at the University of Connecticut in Storrs and his colleagues found that the pattern of activated genes was almost identical in week-old cow embryos, regardless of whether the embryo was created through cloning or produced by insemination of a cow. Only 50 of the 5174 genes analysed differed in activation, less than 1 per cent of the total (Proceedings of the National Academy of Sciences, vol 102, p 17582).
Yang suggests that the glitches in cloned embryos that cause abnormalities such as excessive growth occur further into the pregnancy, perhaps when the genes for organs and tissue development are switched on. Alternatively, it may be that small initial differences between the cloned and natural embryos become magnified as the embryo develops. Of the genes studied, eight were activated only in the clones, and these could play a key role in the embryo鈥檚 fate.
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Yang鈥檚 research suggests that cloned embryos are actually more 鈥渘atural鈥� in their gene expression than IVF embryos created through test-tube fusion of egg and sperm, in which he found the expression of 198 genes to be different from that in normally conceived embryos.
