Adaptive introgression in subdivided populations leaves distinct genomic footprints under global and local selection: a case study in Ciona intestinalis
This study demonstrates that the absence of a correlation between adaptive allele frequency and introgressed tract length in *Ciona intestinalis* supports a model of local adaptation rather than an ongoing adaptive travelling wave, highlighting distinct genomic footprints of adaptive introgression in subdivided populations.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Nature has a remarkable way of borrowing solutions. When a species encounters a new challenge, such as a sudden change in climate or a toxic environment, it does not always have to wait for a random genetic mutation to appear within its own family tree. Sometimes, it can acquire a life-saving trait by mixing with a different, closely related species. This process, known as adaptive introgression, allows a population to grab a beneficial gene from a neighbor and use it to survive. For decades, scientists have understood how this works when a single, isolated population adopts a new trait. The gene arrives, spreads, and eventually takes over the entire group. But the natural world is rarely so simple. Most species live in a patchwork of different environments, connected by the movement of individuals between them. In these complex landscapes, a helpful gene might be a lifesaver in one location but a burden in another. The big question for evolutionary biologists is how to tell the difference between a gene that is currently sweeping across a whole species range and one that is being held in check by local conditions.
A team of researchers set out to solve this puzzle by looking at the genetic footprints left behind by these events. They focused on a specific case involving two types of sea squirts, marine animals that look like small, leathery sacs. One species, native to the Northwest Pacific, has been introduced to the Atlantic Ocean, where it now lives alongside a native European species. Despite being distinct species that usually do not mix, they have begun to hybridize in human-made ports. In this contact zone, the native sea squirt has acquired a specific stretch of DNA from the invasive one. Previous studies showed that this borrowed DNA helps the native animal survive in polluted harbor waters. However, the frequency of this helpful gene varies wildly from one port to another. In some places, it is common; in others, it is rare. The researchers wanted to know why. Is the gene currently spreading like a wave, moving from one port to the next and taking time to reach everyone? Or is it a case of local adaptation, where the gene is useful in some specific spots but actively rejected in others?
To answer this, the scientists turned to computer simulations to model how these two scenarios would look genetically. They imagined a beneficial gene spreading through a chain of populations. In the first scenario, the gene was helpful everywhere. As it moved from one population to the next, it carried with it a long, unbroken chunk of DNA from the original donor. Over time, as the gene spread further, random shuffling of genetic material would break these chunks into smaller and smaller pieces. The researchers found that in this "traveling wave" scenario, there is a clear pattern: populations that received the gene recently have a low frequency of the gene and very short DNA chunks, while populations that received it long ago have a high frequency and long chunks. The length of the DNA chunk and how common the gene is should rise and fall together.
In the second scenario, the gene was only helpful in half of the populations and harmful in the other half. Here, the gene does not sweep across the whole map. Instead, it stays at high levels where it is needed and low levels where it is not, held in a delicate balance by the movement of individuals between the two types of environments. The simulations showed that in this case, the neat relationship between gene frequency and DNA chunk length disappears. A population might have a low frequency of the gene not because it just arrived, but because the local environment is actively pushing it down. Consequently, the size of the DNA chunk tells you nothing about how long the gene has been there.
The team then applied this logic to the real sea squirts. They collected samples from eight different ports along the European coast and used a modern sequencing technique to reconstruct the full genetic history of each animal's chromosomes. This allowed them to see exactly which parts of the DNA came from the invasive species and which parts remained native. They focused on the specific region of the genome where the pollution-tolerance gene sits. If the gene were spreading as a traveling wave, they would expect to see a correlation: ports with low levels of the gene should have very short DNA chunks, while ports with high levels should have long ones.
The results were clear and decisive. The researchers found no such correlation. In some ports, the gene was rare, yet the DNA chunks carrying it were surprisingly long. In other ports, the gene was common, but the chunks were short. This pattern ruled out the idea of a slow-moving wave sweeping across the region. Instead, the data strongly supported the local adaptation model. The variation in the gene's frequency is not a sign of how recently it arrived, but a reflection of how well it fits the specific conditions of each harbor. In some ports, the environment favors the borrowed gene, allowing it to thrive. In others, it is likely a disadvantage, keeping its numbers low. The fact that the gene persists in a stable, mixed state across the region suggests that the sea squirts are not in the middle of a rapid takeover, but have reached a steady state where the gene is kept in check by the local environment.
This finding changes how we understand the speed and nature of evolution in a changing world. It shows that when species adapt to human-altered landscapes, the genetic signatures they leave behind can be subtle and complex. A gene that looks like it is struggling to spread might actually be perfectly adapted to a specific, patchy environment. By combining the frequency of a gene with the size of the DNA chunk it travels on, scientists can now distinguish between a gene that is still on the move and one that has settled into a local niche. For the sea squirts in these ports, the story is not one of a global invasion, but of a fine-tuned, local response to the unique challenges of the harbor.
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