杏吧原创

THE battle of the sexes rages in the most unexpected places. Mothers鈥� and
fathers鈥� 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鈥檚 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鈥攁 common phenomenon in primitive mammals.
鈥淗alf the shrews near Oxford carry biparental litters,鈥� says Chris Graham, who
studies genetic imprinting at Oxford University. Mothers, meanwhile, want
smaller, less demanding babies (New 杏吧原创, 3 May 1997, p 34).

Randy Jirtle and his colleagues at Duke University in North Carolina didn鈥檛
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鈥檚 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.

鈥淭his doesn鈥檛 mean the theory about the battle of the sexes is wrong,鈥� says
Jirtle. 鈥淚t just changes the battlefield.鈥� Possums can bear up to 50 babies, but
mothers only have a dozen nipples, so there鈥檚 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? 鈥淣o mother wants any male鈥檚
offspring to be too fit,鈥� says Jirtle. 鈥淲hat if the rest of his genes are bad?
And she doesn鈥檛 want any single batch to take too much out of her.鈥� Jirtle
suspects that even monotremes will be found to have imprinted genes. 鈥淚mprinted
genes expressed outside the uterus agree with the theory, though this hasn鈥檛
been emphasised,鈥� 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.
鈥淔or that, both IGF2 binding by M6P and the imprinting would
have to arise in marsupials and placental mammals separately, which is very
unlikely,鈥� says Jirtle. Instead, it seems that monotremes split first and
marsupials and other mammals descend from a common ancestor where a father鈥檚
young had to compete鈥攁nd the mother tried to stop them.