Wednesday, February 04, 2009

A Comeback for Lamarckian Evolution?

Two new studies show that the effects of a mother's early environment can be passed on to the next generation.

By Emily Singer

Silencing DNA: Adding methyl groups to specific spots in the genome can alter the expression of marked genes. The process, known as DNA methylation, is one mechanism of epigenetic change, heritable change that does not alter the sequence of DNA itself. In this image, colored bars represent the bases that make up a strand of DNA, while the green circles represent methylation. Credit: Technology Review

The effects of an animal's environment during adolescence can be passed down to future offspring, according to two new studies. If applicable to humans, the research, done on rodents, suggests that the impact of both childhood education and early abuse could span generations. The findings provide support for a 200-year-old theory of evolution that has been largely dismissed: Lamarckian evolution, which states that acquired characteristics can be passed on to offspring.

"The results are extremely surprising and unexpected," says Li-Huei Tsai, a neuroscientist at MIT who was not involved in the research. Indeed, one of the studies found that a boost in the brain's ability to rewire itself and a corresponding improvement in memory could be passed on. "This study is probably the first study to show there are transgenerational effects not only on behavior but on brain plasticity."

In recent years, scientists have discovered that epigenetic changes--heritable changes that do not alter the sequence of DNA itself--play a major role in development, allowing genetically identical cells to develop different characteristics; epigenetic changes also play a role in cancer and other diseases. (The definition of epigenetics is somewhat variable, with some scientists limiting the term to refer to specific molecular mechanisms that alter gene expression.) Most epigenetic studies have been limited to a cellular context or have looked at the epigenetic effects of drugs or diet in utero. These two new studies are unique in that the environmental change that triggers the effect--enrichment or early abuse--occurs before pregnancy. "Give mothers chemicals, and it can affect offspring and the next generation," says Larry Feig, a neuroscientist at Tufts University School of Medicine, in Boston, who oversaw part of the research. "In this case, [the environmental change] happened way before the mice were even fertile."

In Feig's study, mice genetically engineered to have memory problems were raised in an enriched environment--given toys, exercise, and social interaction--for two weeks during adolescence. The animals' memory improved--an unsurprising finding, given that enrichment has been previously shown to boost brain function. The mice were then returned to normal conditions, where they grew up and had offspring. This next generation of mice also had better memory, despite having the genetic defect and never having been exposed to the enriched environment.

The researchers also looked at a molecular correlate of memory called long-term potentiation, or LTP, a mechanism that strengthens connections between neurons. Environmental enrichment fixed faulty LTP in mice with the genetic defect; the fixed LTP was then passed on to their offspring. The findings held true even when pups were raised by memory-deficient mice that had never had the benefits of toys and social interaction. "When you look at offspring, they still have the defect in the protein, but they also have normal LTP," says Feig. The findings were published today in theJournal of Neuroscience.

"If the findings can be conveyed to human, it means that girls' education is important not just to their generation but to the next one," says Moshe Szyf of McGill University, in Montreal, who was not involved in the research.

In a second study, researchers found that rats raised by stressed mothers that neglected and physically abused their offspring showed specific epigenetic modifications to their DNA. The abused mice grew up to be poor mothers, and appeared to pass down these changes to their offspring.