Many families have heirlooms – special items that are passed down the generations, transferring precious memories of bygone times. For tiny nematode worms, this treasure takes the form of chemical marks in the genome, transmitting information about what life was like in the past. Impressively, these cellular memories can be passed down for at least 14 generations – although because worms live, breed and die in the space of a few days, that’s still only a few months.
The discovery, published by CRG group leader Ben Lehner in collaboration with researchers from the Josep Carreras Leukaemia Research Institute (IJC) and the Institute for Health Science Research Germans Trias i Pujol (IGTP), was initially made by accident. His PhD student Adam Klosin was studying C. elegans worms carrying a transgene array – a long string of repeated copies of a gene encoding a red fluorescent protein – when he noticed something strange.
If the worms were kept at 20˚C, the array of transgenes was less active, creating only a small amount of fluorescent protein. But shifting the animals to a warmer climate of 25˚C significantly increased the activity of the transgenes, making the animals glow bright red under ultraviolet light.
Then things got really weird.
When these worms were moved back to the cooler temperature, their transgenes were still highly active, suggesting they were somehow retaining the ‘memory’ of their warmer youth. Intriguingly, the bright fluorescence was passed on to their offspring and onwards for another seven generations living at the cooler temperature, even though the original animals only experienced the higher temperature for a brief time. Amazingly, keeping worms at 25 degrees for five generations led to the increased transgene activity being maintained for at least 14 generations once the animals went back to a colder life.
“It’s super cool!” Lehner jokes. “This is an artificial system, but the effect is really pronounced. We had to find out what was causing it so Adam abandoned his original PhD project and started working on this instead.”
To find out what was causing the strange inheritance pattern, Lehner and his team took a closer look at the transgene array itself, homing in on the ball-shaped proteins (histones) that package DNA inside the cell.
Histones can be modified with chemical ‘tags’ (epigenetic marks) in a number of different ways. Some epigenetic marks are associated with active genes, while others are linked to gene silencing. In particular, Lehner focused on a histone modification known as H3K9 trimethylation, which helps to shut down gene activity.
As might be expected, the researchers found that the transgenes in animals that had only ever been kept at 20 degrees had high levels of H3K9 trimethylation. Correspondingly, their transgenes were less active and they didn’t fluoresce very much. Worms that were then moved to 25 degrees lost the tags, switched on their transgene array and began to glow.
Surprisingly, these brightly fluorescent animals raised in the hotter climate still maintained this reduced histone methylation when they were moved back to the cooler temperature, suggesting that it is playing an important role in locking the memory about the environmental temperature into the genome.
Digging deeper, Lehner and his team found that a protein called SET-25 is responsible for maintaining the histone methylation patterns on the transgene arrays. But they still don’t know for sure exactly how the increase in temperature leads to the loss of histone methylation marks. And they also don’t know whether the histone methylation patterns themselves are responsible for transmitting the temperature memory down the generations, although they can be seen in eggs and sperm and are present at the earliest stages of worm development.
The fluorescent transgene array was put into the worms using genetic engineering techniques, so it might be expected that it acts strangely. But Lehner and his team also found that repeated parts of the normal worm genome that look similar to transgene arrays also behave in a similar way, suggesting that this is a potentially widespread phenomenon and not just restricted to artificially engineered genes.
“This isn’t entirely surprising,” Lehner says. “There are other repetitive elements in the worm germline that change their activity depending on the temperature, and we do seem to detect a whisper of inheritance a few generations later. But so far we haven’t found any ‘regular’ genes that behave like this.”
Although this phenomenon of epigenetic inheritance has been seen in a range of animals, including mammals, the evidence for long-term effects is lacking. Even the best examples, such as the impact of starvation during pregnancy, fade after a couple of generations. This makes Lehner’s worms the longest-running example of transgenerational environmental ‘memory’ ever observed in animals to date. But while it’s an intriguing result, it’s still not clear exactly how it might be useful to the worms themselves.
“We don’t know exactly why this happens, but it might be a form of biological forward-planning,” he explains. “Worms are very short-lived, so perhaps they are transmitting memories of past conditions to help their descendants predict what their environment might be like in the future.”
Ten generations for a worm is still less than a couple of months. We can predict fairly accurately what the temperature might be like next fortnight, so it makes sense for the worms to try and encode that information to help their great-great-great-great-great-great-great-great grandchildren (or, more accurately, grandworms) prepare for the environment they might hatch into. But it’s almost impossible to predict what the climate will be like after that many human lifetimes, so this kind of mechanism probably wouldn’t be useful for longer-lived species.
“At the moment this is all speculation,” Lehner says. “But biology is so weird that if something like this happens, it probably has been exploited for a purpose somewhere out there in nature.”
