Researchers at the Medical School have discovered a mechanism whereby organisms can acquire traits and pass them to their offspring—without using any DNA.
The results suggest that the classical Mendelian genetic theory may not provide a complete picture of how traits are inherited and have researchers reconsidering parts of the long-discredited evolutionary theory of Lamarckism.
“I think it is a very exciting result,” biology professor and Nobel laureate Martin Chalfie said.
Postdoctoral research scientist and lead author Oded Rechavi and his colleagues, Oliver Hobert and Gregory Minevich, studied the C. elegans roundworm species. The worms defend themselves against viruses using a system known as RNA interference, or RNAi. They found that worms that acquired immunity to a virus via RNAi passed that immunity on to their offspring for up to 100 generations—even when the researchers removed the genetic code for the RNAi system from those offspring.
The offspring did not receive the immunity via a change in their genomes, but rather through the transmission of small RNA molecules, known as virus-interferent RNAs, from parent cells to offspring.
“In this case, the effect segregates independently of DNA and the chromosome … The small RNAs are being inherited even when the DNA sequence that they affect, that is the template of them, is not even there,” Rechavi said.
The type of inheritance the researchers describe moves beyond the traditional Mendelian view of heredity, in which organisms’ traits are only transmitted through the generations as DNA sequences on chromosomes.
The results also suggest that evolution is not solely the product of the natural selection of random genetic mutations, but rather that organisms can also pass on to their offspring traits acquired as direct adaptations to their environments.
“People classically like to think of evolution according to Darwin as being random, which means that mutations accumulate in the DNA of the organism in a random way, and the organism that happens to have by complete accident, by chance, a mutation that makes it more fit, gets to produce progeny, and later these progeny take over the population,” Rechavi said.
By contrast, Rechavi said, “in this case, a direct encounter with a challenge from the environment—the challenge being a viral infection—affects the inheritance in a directed way.”
This view of evolution was famously advanced by 18th-century biologist Jean-Baptiste Lamarck. In a classic example, he suggested that giraffes developed their characteristic long necks by stretching to reach high leaves, and then passing that acquired stretch on to their offspring.
This vision was found to be entirely unsupported by the evidence, and was supplanted by Darwin’s and Mendel’s work.
The CUMC study joins a growing pool of evidence that documents the inheritance of acquired traits in organisms—from roundworms to humans—under much more limited circumstances.
According to Chalfie, the new results, published last December in the scientific journal Cell, are noteworthy because they demonstrate this kind of inheritance across many generations and lay a foundation for understanding a mechanism behind it.
“What is very nice about the paper is that they have been able to give a very clear-cut example of effects that are lasting through several generations,” he said.
“They have found a very interesting phenomenon,” he added. “This is something that opens the door to a lot more experimentation.”
The debates surrounding inheritance of acquired traits remain contentious. Rechavi is keen to emphasize the continuity of the new results with existing biology, and Chalfie pointed out that “it’s not a result that proves Lamarckism in all of its various guises.”
“It’s very, very important to emphasize that whatever we are showing blends into the normal mechanisms of genetics that we know,” Rechavi said.
Most of the time, he said, evolution progresses according to classic Darwinian natural selection. “This is sort of an additive effect … to add on to the known mechanisms of Mendelian inheritance.”
But he added that “these sorts of additions are very, very important, because although we have the genomic sequence of humans and other organisms, we are still struggling to really find the pattern of inheritance of certain diseases.”
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