Feeding time. But how much pups get to eat may depend on which genes came from mom and which came from dad.

Milking Genes for All They're Worth

Among many animals, fathers and mothers have different interests when it comes to raising the next generation. Now a team of researchers says its new results support the idea that maternal and paternal genes compete for dominance in the offspring, a concept known as imprinting. But others aren't so sure.

Since the 1980s, biologists have been studying how genes in mammals inherited from the mother or the father may be selectively silenced, the result of genomic imprinting. Many link imprinting to sexual conflict: A father who mates with multiple females will want all the mothers to produce large offspring. The mother, on the other hand, needs to conserve her resources if she wants to produce additional litters.

To investigate this idea, Reinmar Hager and Rufus Johnstone of the University of Cambridge, U.K., crossbred two strains of mice, called CBA and B6. CBA litters contain roughly nine pups that are relatively small, while B6 litters contain about seven larger ones. Hager and Johnstone found that CBA males consistently sired more abundant litters, regardless of the mother's strain, suggesting that fathers influenced litter size more than mothers did.

But moms may get the last word. To see if genes played a role in how much mothers fed their young, the biologists farmed the crossbred pups out to CBA and B6 foster mothers. They then allowed each foster mother to feed pups for 2 hours before measuring her change in body weight. Intriguingly, B6 foster mothers gave up more milk to pups whose birth mothers were also B6 than they did to CBA pups, the researchers report in the 30 January issue of Nature. This suggests that the mothers can detect the genotype of pups. Moreover, the foster mother apparently can tell whether the pup's B6 genes came from its dad or mom, and they favor pups with genes that came from the mother. (Alternatively, pups may have genes that allow them to extract more milk from a mother that's the same strain.)

"I think imprinting is the most probable explanation" for all this, says Andrew Pomiankowski, a geneticist at University College London, although he believes the work needs to be repeated to solidify the results. "What one really would like to know is what are the genes involved?"

Related sites
Reinmar Hager's home page
Rufus Johnstone's home page
Information on genomic imprinting