Social competition among mice cause mothers to give birth to sexy sons who smell great and die young.
This is one of the first studies to demonstrate epigenetic effects contributing to increased mating success in offspring, report biologists in a paper published in PNAS Early Edition. Epigenetic effects are inherited changes in gene activity not caused by alterations in the DNA sequence. Instead, mechanisms such as adding methyl groups to the DNA (known as methylation) drive the changes.
The male offspring of female mice who mated in a “promiscuous” environment (free to choose their own mates) produced about 30 percent more sexually attractive pheromones in their urine than those whose parents bred in monogamous environments. No similar effects were seen in female offspring.
The sexually appealing sons from promiscuous mothers subsequently had greater success in mating and defending territories. But success comes at a price. The up-regulation of major urinary protein (MUP) pheromones (associated with reduction in DNA methylation) is costly. Producing a lot of MUPs reduces an animal’s lifespan.
“These results suggested that returning mice to sociality favored an attractive male-specific phenotype characterized by enhanced pheromone signaling,” write the authors, “but conspicuously lacking in vigor and viability, a result consistent with the sexy sons hypothesis of sexual selection.”
This hypothesis proposes females prefer mates whose male offspring will have the best chance of reproductive success, rather than mates who can directly offer benefits in the form of food or territory.
While the sons of promiscuous mothers produced more pheromones, the sons of promiscuous fathers actually produced fewer pheromones compared to the sons of monogamous fathers. (However, at a 5 percent reduction the effect was not great.) The authors propose this may point to a conflict of interest between mothers and fathers. Fathers don’t necessarily benefit from sexy sons.
“Fathers are competing with their sons and usually drive them out of the territory quickly, while they let daughters stay,” senior author Wayne Potts told the University of Utah. “If you’re worried about your sons impinging on your own reproductive success, then why make them sexy?”
Looking more closely at the pheromone gene Mup11, the authors found the sons of monogamous, domesticated mice had twice as much methylation as the sons of socially, promiscuous mice. Methylation is believed to play a role in repressing gene expression, and the difference between these two groups allowed the “promiscuous sons” to produce more pheromone.
Why should such dramatic changes be possible in just a generation? Mice naturally experience dramatic population fluctuations, say the authors. Female mice in high-density populations have more to gain by giving birth to MUP-rich sons than mice from low-density populations.
This research could, Potts suggests, help plan captive breeding programs. Animals bred and raised in zoos or sanctuaries often compete poorly for mates, food and territories when they are re-introduced to the wild.
“It’s amazing how often reintroduction of captive-breed endangered species fails – it’s estimated to be as high as 89 percent,” he says. “Domestication stimulates epigenetic mechanisms that make animals less fit for nature.”