Previous research has shown that bad rat mothering can be passed down through this kind of DNA modification–but those changes are thought to be triggered specifically by maternal behavior. In the new study, researchers also had healthy mothers raise the offspring of stressed mothers, and found that the problems were only partially fixed. That suggests that the changes “were not due to their neonatal experience,” says David Sweatt, a neuroscientist at the University of Alabama at Birmingham, who oversaw the study. “It was something that was already there when they were born.” The research was published online last month in Biological Psychiatry.
The results of both studies are likely to be controversial, perhaps resurrecting a centuries-old debate. “It’s very provocative,” says Lisa Monteggia, a neuroscientist at the University of Texas Southwestern Medical Center, in Dallas. “It goes back to two schools of thought: Lamarck versus Darwin.”
In contrast to natural selection, in which organisms that are born well adapted to their environment survive and reproduce, passing down those successful traits, Lamarckian evolution suggests that animals can develop adaptive traits, such as better memory, during their lifetimes, and pass on those traits to their offspring. The latter theory was largely abandoned as Darwin’s, and later Mendel’s, theories took hold. But the concept of Lamarckian inheritance has made a comeback in recent years, as scientists learn more about epigenetics.
“I didn’t set out to come up with findings that support neo-Lamarckian inheritance,” says Sweatt. “But the research now makes it more plausible that these things may be real and may be based in molecular mechanisms.”
Feig, on the other hand, argues that while the findings are “a Lamarckian kind of phenomenon it’s still Darwinian, because the changes don’t last forever.” In Feig’s study, the offspring of enriched mice lost their memory benefits after a few months.
Sweatt and others say that this type of inheritance may in fact be much more common than expected. Improving technologies are now providing a broader look at the epigenetic changes linked to environment and behavior. Scientists are starting to use DNA microarrays, which over the past few years have become widely employed in genetic studies of disease, to look at one specific type of change, known as DNA methylation. “The changes we see are not limited to a small number of genes,” says Szyf, who is using the technology to study epigenetics and cancer. “Whole circuitries are changed.”
DNA sequencing, which is rapidly dropping in price, can also be used to study DNA methylation. But epigenetics studies require high-volume sequencing, which has been prohibitively expensive. “In contrast to the genome, every epigenome is different in different types of cells,” says Sweatt. “A human epigenome project would be the equivalent of 250 human genomes, because there are at least 250 cell types in the body.” Cheap sequencing may soon make that type of study possible, he says.
The actual mechanism underlying these patterns of inheritance is somewhat mystifying to scientists. Feig theorizes that environmental enrichment triggers a long-lasting hormonal change: when the animal becomes pregnant, the hormone would somehow modify the DNA of the fetus, ultimately causing it to have improved memory and LTP as an adolescent. However, he cautions, there is no direct evidence of this, and no specific evidence that the behaviors are transmitted through epigenetic mechanisms.