At the beginning of the 19th century, the French naturalist Jean-Baptiste Lamarck put forward a theory of evolution according to which the adaptations acquired by an organism during its lifetime could be transmitted to its descendants. According to Lamarck, the lanky necks characteristic of modern giraffes, for example, would have emerged as a result of the constant stretching of their ancestors. Although widely praised and popularized in the first half of the century, the theory was thoroughly examined and debunked in the second half.
But two hundred years later, research has come full circle. Scientists reported in Development cell last month that parental adaptive responses to the environment may not only be passed down from generation to generation, but may even determine the course of evolution – at least in some roundworms.
Caenorhabditis elegans has become the favorite worm of geneticists over the past few decades. It is a very small roundworm, with a life cycle of only three days. It is also androdioecious, meaning its offspring can be either male or hermaphroditic. Hermaphroditic worms produce both sperm and eggs; for most of their short life they easily self-fertilize. Towards the end, however, as the sperm supply empties, C.elegans releases pheromones that attract male worms for cross-fertilization (the type of fertilization humans are more familiar with).
Now, new research reveals that the worm’s parental experiences can be passed down for up to 100 generations, says Joel Rothman, a developmental biologist at the University of California, Santa Barbara, who was not involved in the new study. Lamarck’s hypotheses [and explanations] were definitely naive when talking about the inheritance of acquired traits,” he says. “But now we realize he was definitely right to some extent.”
Oded Rechavi, a geneticist at Tel Aviv University in Israel, has long worked to determine how environmental change affects the functioning of genes (or “epigenetics”) in C.elegans. In their latest experiment, he and his team were motivated by a big question: how do ascarid offspring evolve when subjected to constant environmental stress over multiple generations?
To find out, they raised the roundworms in the lab at 77 degrees Fahrenheit — a temperature that’s just plain uncomfortable for small nematodes. Then the team sat and watched them carefully for generations. “In these verses you can even [observe] over ten generations quite quickly because there are only three days left before the next generation,” explains Rechavi.
For a few generations, the researchers found that the offspring behaved quite similarly to their stressed parents: the hermaphrodite offspring self-fertilized for most of their lives and only outcrossed at the end. But after nearly ten generations of exposure to stress, the team noticed that the offspring were in a hurry – they were secreting pheromones that attracted males and fertilized earlier in life.
The worms obviously didn’t know if this rapid cross-fertilization would work, but it was their best bet. Rothman explains that the stressed worms passed the strategy on to their children and grandchildren, almost as if to say, “Guys, we’ve been stressed. Unless you have more news, go ahead and watch. We have to fix things.
The biomolecules to blame
After delving into the cellular mechanisms behind this change in behavior, Rothman and his team discovered the biomolecules responsible: small ribonucleic acids. Small RNAs are secreted by every cell in the worm’s body, but one of its important functions is to regulate sperm genes. The researchers found that at higher temperatures, small RNAs did not work well. Because of this defect, Rechavi says, “they probably couldn’t fertilize their own eggs too, so they started producing the pheromones attracting the males earlier.”
Wondering why the worms waited a few generations before making the switch? Rechavi speculates that something in the biology of the worms must first accumulate: “Perhaps the disruption of the hereditary small RNA pool becomes so large that it crosses some sort of threshold.”
Shouhong Guang, a molecular geneticist from the University of Science and Technology of China who was not involved in the study, calls the research effort wonderful. Indeed, the findings could reshape our understanding of the role and mechanism of hermaphrodite behavior in animal evolution. “How each species chooses its reproductive strategy will determine whether the species could increase its own copies of genes in the animal kingdom, over millions of years of evolution,” Guang says.
While things aren’t crystal clear just yet, there’s plenty to be excited about. “Simple model organisms often provide the first evidence,” says Rechavi, describing how 19th-century mathematician and biologist Gregor Mendel discovered various rules of genetics while studying pea plants – long before science knew anything about them. either on DNA or chromosomes.
Of course, the prospects of epigenetics and its implications for human evolution are also fascinating. Rechavi hopes this kind of mechanism is conserved in humans as well, “but we have to say very explicitly that we don’t currently know that,” he says.
Meanwhile, Rothman, who is expecting a new grandchild in a few weeks, adds that the very idea of ”how my life experiences can [biologically] Touching them gives me chills.