If in biology there are easy questions to answer, this should be among the simplest: can memory be inherited? The answer, it would seem, is a resounding “no”. Memories are stored in the brain in the form of neuronal connections or synapses, and there is no way to transfer this information to the DNA of germ cells, the inheritance we receive from our parents; we do not inherit the French they learned at school, but we must learn it for ourselves. And yet, for years there have been more and more indications that are undermining this classic dogma of biology: at least in some species, certain forms of memory can be transmitted to descendants.
In its broadest sense, the idea of memory applies to a phenotypic trait—measurable or observable—that is acquired during the life of the individual. At the time of the gestation of evolutionary theory, French naturalist Jean-Baptiste Lamarck became the champion of a theory of inheritance of acquired characteristics: for example, the development of a muscle through exercise. Lamarck’s theses were eventually refuted, but at the end of the 20th century research began to reveal that there does exist an inheritance of acquired traits, which was given the name of epigenetics (literally, “on top of genes”).
Environmental influences such as food, contaminants and others can leave chemical marks on genes that do not alter their sequence, but do modify their activity. Because these epigenetic marks could be passed on to the next generation through germ cell DNA, they represent a form of inheritable memory: children’s genes remember the environmental factors their parents were exposed to. Although epigenetics still conceals many of its secrets, those that are becoming known show that we have not yet measured its limits.
Transfer memories from one animal to another
In 1962 biologist James V. McConnell published a controversial experiment in which he claimed to pass on a learning process from one worm to another by feeding the latter the ground up remains of the former. McConnell was convinced that the engram of a memory—a hypothetical term for the physical trace of memory—resided in RNA molecules, and that these could transfer that memory from one animal to another.
Although McConnell’s experiments were discredited by the scientific community, shortly thereafter other studies were published that appeared to show a transfer of memories from one rodent to another through injections of brain extracts. In these cases too, negative results from other researchers called into question a theory that was already difficult to digest. And yet, in recent decades it has become known that not only are RNA molecules among the most common epigenetic mechanisms, but they are also involved in the formation of long-term memory.
Recently, McConnell’s hypothesis has been reborn from its ashes, regardless of whether the effects he described in his experiments were real or not. In May 2018, a study conducted by David Glanzman of the University of California, Los Angeles showed that the injection of an RNA extract from specimens of the sea slug Aplysia californica—a classic animal in memory research—trained to respond to an electrical stimulus is capable of transferring this learning to other untrained individuals. Moreover, Glanzman also showed that the RNA of the first animal stimulates the neurons of the second animal isolated on a Petri dish. “It’s as though we transferred the memory,” says Glanzman. “If memories were stored at synapses, there is no way our experiment would have worked.”
For now, other experts have been cautious in their assessment of Glanzman’s study. But it’s not the only one that points to mechanisms capable of transferring epigenetic engrams—that is, memories—from one individual to another. Moreover, other experiments have dispensed with the artificial method of injection, which is unlikely to occur in nature, showing that the presumed RNA memory could be inherited by children from their parents.
For this to be possible, a pathway must first be opened for the transfer of RNA from neurons to germ cells, something that, according to the dogma, does not exist. But this is precisely what a team of researchers at the University of Maryland did in 2015 when they discovered that in the worm Caenorhabditis elegans certain strands of RNA produced in neurons can travel to germ cells and silence genes in the offspring, even up to 25 generations later.
In June 2019, a study published by scientists at Tel Aviv University (Israel) extended these results, demonstrating that RNA produced in worm neurons affects their offspring’s foraging behaviour through germ cell transmission, and that this learning is transferred over several generations. According to the study’s director, Oded Rechavi, “these findings go against one of the most basic dogmas in modern biology.”
The following month, a study conducted by Giovanni Bosco of the Geisel School of Medicine at Dartmouth (USA) showed that Drosophila melanogaster fruit flies can inherit from their parents egg-laying behaviour induced in parents by contact with wasps that parasitize their larvae; their descendants adopt the same behaviour without having experienced the threat themselves.
However, it should be stressed that what is observed in worms or flies does not necessarily apply to humans. But it could. As early as 2013, a study by Emory University showed that the fear induced in mice of a particular smell can also be transmitted to their offspring by epigenetic mechanisms. And a mouse is already much more like us.
“Does this happen in animals other than fruit flies and worms?” asks Bosco. “Yes, I am convinced that it does, and we just need someone clever to think of the right experiment to do in order to actually test it, in humans for example,” he tells OpenMind. “A new avenue is now finally becoming accessible to experimentation in a manner where we can start to understand molecular mechanisms and specific molecules that allow animals to inherit particular types of behaviour and memory.”