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Mum versus Dad: A genetic battle is being fought inside marsupial pouches

By Deborah MacKenzie, New Scientist

4 April 2000: THE battle of the sexes rages in the most unexpected places. Mothers' and fathers' genes fight for supremacy even outside the womb, say American researchers. They have discovered that marsupials, like placental mammals, have imprinted genes for growth, although their young spend only a short time in the uterus before crawling into their mother's pouch.

Imprinted genes are expressed only if they are inherited from a particular parent. For instance, IGF2, the gene for a growth factor that leads to bigger babies, is only expressed in humans if it comes from the father. However, M6P, which opposes IGF2, is only expressed if it comes from the mother.

Genes such as these are thought to have evolved because fathers want big babies that hog nutrients, especially if they share womb space with siblings that may have different fathers-a common phenomenon in primitive mammals. "Half the shrews near Oxford carry biparental litters," says Chris Graham, who studies genetic imprinting at Oxford University. Mothers, meanwhile, want smaller, less demanding babies (New Scientist, 3 May 1997, p 34).

Randy Jirtle and his colleagues at Duke University in North Carolina didn't expect genes to be imprinted in marsupials, whose embryos spend a very short time in the womb. After only a few days of initial development, the tiny offspring crawl to the mother's pouch and latch onto nipples, where they stay until birth. Nor should the genes appear in monotremes, where after a brief spell inside, the young are laid in eggs.

The team, together with Barry Munday at the University of Tasmania, measured gene expression in two monotremes, the platypus and echidna. Neither animal had imprinted copies of the M6P gene, which did not affect the actions of IGF2. But in the American opossum, a marsupial, M6P does oppose IGF2, and is maternally imprinted as in people, the team found. The possum IGF2 is also paternally imprinted.

"This doesn't mean the theory about the battle of the sexes is wrong," says Jirtle. "It just changes the battlefield." Possums can bear up to 50 babies, but mothers only have a dozen nipples, so there's a race to latch on first. In a group of embryos with different fathers, the biggest should win.

But why does the mother blunt that advantage? "No mother wants any male's offspring to be too fit," says Jirtle. "What if the rest of his genes are bad? And she doesn't want any single batch to take too much out of her." Jirtle suspects that even monotremes will be found to have imprinted genes. "Imprinted genes expressed outside the uterus agree with the theory, though this hasn't been emphasised," says Graham.

The findings may also help resolve a dispute over whether marsupials and monotremes are more closely related to each other than to placental mammals. "For that, both IGF2 binding by M6P and the imprinting would have to arise in marsupials and placental mammals separately, which is very unlikely," says Jirtle. Instead, it seems that monotremes split first and marsupials and other mammals descend from a common ancestor where a father's young had to compete-and the mother tried to stop them.