When it comes to genes, evolutionary biologists have traditionally favored seniority. Genes thought to be most essential to life must be ancient and conserved, the assumption goes, handed down from species to species as the basic instructions of life. That sharing is evident in early developmental stages, which 19th-century biologist Ernst Haeckel observed to be very similar between different organisms in his famed recapitulation theory. The genes that drive those early stages of development are also shared by creatures as different as flies, mice, and humans, lending support to the idea that the most important genes for life go a long way back on the evolutionary tree.
By comparison, new genes haven’t gotten nearly as much credit. Arising more recently in evolution’s history, rookies that only count their age in tens of millions of years were thought to be less important – providing new functions and features that were nice, but not essential. If old genes were the bread and butter of life…
“Maybe the new genes serve a function like vinegar or soy sauce,” said Manyuan Long, professor of ecology & evolution at the University of Chicago. “They make your life better, change behavior, help a male find females more efficiently, but that’s all.”
But that ageist perspective is shaken in this week’s Science, courtesy of an exciting new study from Long’s laboratory. Using the fly species Drosophila melanogaster, Long, graduate student Sidi Chen, and postdoctoral researcher Yong Zhang tested whether silencing a new gene would be as fatal as silencing an old one. With RNA-interference (RNAi), a method which interrupts the translation of genes into proteins, they silenced 195 new genes between the age of 3 and 35 million years, one at a time.
The tests found that these young, supposed “condiment” genes could be just as deadly when they were silenced. Thirty percent of the genes were fatal when knocked out, suggesting that new genes can quickly become an essential part of an organism’s survival. What’s more, new genes were nearly as likely to be essential as old genes – when RNAi experiments were repeated on a sample of older genes, a similar 35 percent of them were fatal.
“A new gene is as essential as any other gene; the importance of a gene is independent of its age,” Long said. “New genes are no longer just vinegar, they are now equally likely to be butter and bread. We were shocked.”
Knocking out an older essential gene typically causes an organism to die in its early embryonic stages, a clue that the gene is important for the first steps of development. Some of the newer genes also caused the flies to die very early in their development, but others were fatal at later stages, such as metamorphosis. That suggests that conserved genes shared between species are more likely to conduct early development, while newer genes unique to each species take over the more advanced steps.
“This may change the way we view the developmental program,” Long said. “Each species has a different species-specific developmental program shaped by natural selection, and we can no longer say that from Drosophila to humans the development of different organisms is just encoded by the same genetic program. The story is much more complicated than what we used to believe.”
One area that could be affected by this new perspective is animal research, which has long been built on the assumption that the most important biological components are shared between species. Those core similarities have allowed scientists to make discoveries about human disease in animals such as mice and rabbits, but the presence of essential, species-specific genes may mean that such studies are only getting part of the story.
“I think it has important implications on human health,” said Chen, a graduate student in ecology & evolution. “Animal models have proven to be very useful and important for dissecting human disease. But if our intuition is correct, some important health information for humans will reside in the unique parts of the human genome.”
Another surprise implication of the data is the speed with which new genes can become an indispensable part of a species’ genome. When a new gene appears in a species due to evolution, one would not expect it to be immediately crucial – otherwise, how did the species survive before its arrival? But like a new employee in an office, a new gene can make itself essential by forming relationships with older, essential genes through what are called gene-gene interactions. Before long, the new gene has become a key part of the species’ survival…and there’s no turning back. One name Long has come up with to describe this phenomenon is the “gene addiction hypothesis,” but Chen’s metaphor for the process is even better.
“New genes come in and quickly interact with older genes, and if that interaction is favorable by helping the organism survive or reproduce better, it is favored by natural selection and stays in the genome,” Chen said. “After a while, it becomes essential, and the organism literally cannot live without the gene any more. It’s something like love: You fall in love with someone and then you cannot live without them.”
Chen, S., Zhang, Y., & Long, M. (2010). New Genes in Drosophila Quickly Become Essential Science, 330 (6011), 1682-1685 DOI: 10.1126/science.1196380