NEWS NATURE|Vol 439|23 February 2006 Y chromosome fuels dynastic dilemma TOKYO Politicians and legal experts debating whether to allow females to inherit the Japanese impe- rial line have consulted geneticists for advice. A government advisory committee considered KAMBAYASHI AP/S. whether the passage of an intact Y chromo- some down a male line of descent might be a scientific argument for the male-only descent system. They concluded, however, that assign- ing rights to the throne on the basis of the Y chromosome would give a claim to many ordi- nary citizens. The incident shows how desperate Japanese politicians are to resolve the problem of Japan’s emperor, the latest in what may be the world’s longest unbroken imperial line, having no male heir. When Emperor Akihito dies, the throne will pass to his two sons. But it’s unclear what will happen after that. His grandchildren are all Desperate for a boy: Emperor Akihito (second from left) is still without a grandson to inherit the throne. female, and at ages 39 and 42, the princes’ wives are running out of time to produce a boy. Analogous pairs of all the other chromosomes average citizen and the imperial household Prime Minister Junichiro Koizumi has swap genetic elements, which disrupts the would become uncertain,” he says. Isao thrown his weight behind an amendment to integrity of the DNA sequences. But only 5% Tokoro, an expert in Japanese legal history at allow the oldest child of either sex to inherit of the Y chromosome combines with the X Kyoto Sangyo University, Kyoto City, agreed: the throne. This would allow the four-year-old chromosome; the rest is conserved. This “I don’t think the ‘stamp of the Y chromosome’ daughter of the eldest prince to be next in line. means the Y chromosome has unmatched is very meaningful here.” Last year, Koizumi vowed to propose the genetic integrity and forms an ideal marker for So the Y-chromosome argument did not amendment during the parliament’s current the male line (see ‘Tracing a Y chromosome make it into the committee’s final report. After session, which began on 20 January. through 100 generations’). receiving the advice in November, Koizumi Nature has learned that in a series of cabinet “In a male-descent system, even a distant rel- promised to push for a gender-neutral meetings held ahead of Koizumi’s pledge, ative will have inherited the same Y chromo- amendment in this season’s parliament — a politicians, historians and legal experts called some,” said Yagi. “Of course our ancestors didn’t measure apparently supported by 80% of the on scientists for advice. Some experts, vigor- have this knowledge of genetics, but they would population. ously opposed to the idea of allowing females have known that blood can be inherited.” The situation was confused earlier this to pass on the line, used the Y chromosome to But the argument backfired, when Akinori month, however, by the announcement that support their argument. Takamori, a historian of ancient Japan at the second prince’s wife was 6-weeks pregnant. Government records show that committee Takushoku University in Tokyo, told the meet- With the possibility of a male heir within member Hidetsugu Yagi, an expert in consti- ing that if the Y chromosome were so well con- the imperial line, conservatives are pushing tutional law at Takasaki City University of served, it would also have been passed down a Koizumi to postpone the issue, something he Economics, referred to the “stamp of the Y countless number of splinter lines over the looks set to do. ■ chromosome”, and the need to preserve it. past 1,000 years. “The distinction between the David Cyranoski Tracing a Y chromosome through 100 generations Those justifying a male-only If that’s true, the imperial Y analyse their DNA could show how very lucky to get good Y data from system routinely refer to an chromosome should be largely far the Y chromosome stretches your ancient emperor.” unbroken male line of 125 conserved, say geneticists. back, how much it has changed in An alternative to digging up generations since Japan’s first Accumulated point mutations are that time, and whether the line has graves would be to trace a tree of emperor Jimmu. Although the first probably negligible, and although ever been broken. But he says he the imperial family by comparing ten or so generations are probably inversions and deletions might have doesn’t want to be named, much the Y-chromosomes of distant living mythical, and there may have been built up, the sequence should still less involved in such a project, relatives to estimate when the a break after the 25th emperor, be recognizable, says geneticist for fear of being “assassinated”. And oldest family member would have Japanese historians say they are Yutaka Nakahori of Tokushima Mark Jobling of Leicester University, lived. Nakahori says he would love to confident that the line has been University. UK, who studies Y-chromosome do this, but realizes it would never be continuous (if not always directly A genetics expert consulted by evolution, says that most nuclear allowed by the Imperial Household father-to-son) for 100 generations, the government committee told DNA from 1,500 years ago would Agency, which keeps a jealous guard or since about 500 AD. Nature that digging up emperors to have deteriorated: “You would be on such matters. D.C. 898 © 2006 Nature Publishing Group.