Utilising nuclear encoded plastid DNA to identify donors of grass-to-grass lateral gene transfer
This study demonstrates that nuclear-encoded plastid DNA (NUPTs) can effectively identify specific donor species of grass-to-grass lateral gene transfer, revealing that *Eremochloa attenuata* co-transferred a DNA fragment containing both a previously known LGT and a newly identified NUPT into *Alloteropsis semialata*.
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 This is an AI-generated explanation of a preprint that has not been peer-reviewed.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
In the vast library of life, DNA is usually passed down from parent to offspring, a vertical inheritance that builds the tree of evolution branch by branch. But sometimes, nature breaks the rules. DNA can jump sideways between two different species that are not related by direct ancestry, a phenomenon known as lateral gene transfer. While this was once thought to be a rare oddity limited to simple microbes, scientists now know it happens frequently in plants, particularly in grasses. When a grass species acquires a large chunk of DNA from a neighbor, it can gain new traits, but figuring out exactly which neighbor donated the gift is often a puzzle. The donor might be extinct, or the DNA might have changed so much over time that the original source is unrecognizable. To solve this, researchers need a way to trace the history of these transferred genes with high precision, looking for clues that survive the passage of time.
A team of scientists recently tackled this problem by looking for a specific type of genetic fossil hidden inside the nuclear genomes of grasses. They focused on the grass Alloteropsis semialata, a species known to have acquired several large DNA fragments from other grasses. Inside the nucleus of these plants, they searched for nuclear DNA of plastid origin, or NUPTs. These are pieces of chloroplast DNA—the genetic material responsible for photosynthesis—that have accidentally inserted themselves into the main nuclear genome over millions of years. Because grass chloroplast genomes are highly conserved and have been sequenced for thousands of species, they offer a massive, detailed map that nuclear genes often lack. The researchers hypothesized that if a plant acquired a large DNA fragment from a donor, it might have also picked up a NUPT that was sitting inside that donor's genome at the time. By finding these NUPTs and tracing their evolutionary history, they could pinpoint the donor species with much greater accuracy than by looking at the transferred genes alone.
The team began by scanning the genomes of four different accessions of Alloteropsis semialata. They compared the nuclear DNA of these plants against a database of chloroplast genomes from hundreds of other grasses to find matches that did not belong to the plant itself. They filtered out weak matches, keeping only those that were long and highly similar to foreign chloroplasts. This process revealed a small number of candidates that appeared to be foreign genetic material that had been integrated into the plant's nucleus. To confirm their suspicions, the researchers reconstructed the family trees for these specific DNA fragments. They found that in two of the plant accessions, the NUPTs did not sit where they should have if they had been inherited vertically from the plant's own ancestors. Instead, these fragments nested deeply within the evolutionary branches of other grass groups, suggesting they had been acquired laterally.
In one case, the researchers identified a NUPT that grouped closely with the Digitaria genus, a type of grass known as crabgrass. In another, more complex case, the NUPT pointed toward the Eremochloa genus. To narrow down the exact donor for this second transfer, the team sequenced the chloroplast genomes of twelve additional Eremochloa species from herbarium samples. When they added these new sequences to their analysis, the evidence became overwhelming: the foreign DNA fragment in the Alloteropsis plant was most closely related to a specific species, Eremochloa attenuata. The genetic match was so strong that the researchers could place the donor with high confidence, something that would have been difficult using only the nuclear genes involved in the transfer.
The study went a step further to see if the NUPT and the nearby transferred genes traveled together. The researchers looked at the region of the Alloteropsis genome where the NUPT was found and noticed it was located just downstream from a previously identified lateral gene transfer event. They mapped short DNA reads from the suspected donor, Eremochloa attenuata, onto this specific region of the Alloteropsis genome. The reads covered the area consistently, spanning the known transferred genes and extending right into the region containing the NUPT. This pattern suggests that the entire block of DNA, including both the functional genes and the chloroplast fragment, was transferred as a single, continuous piece from the donor to the recipient.
This approach highlights a powerful new way to trace the history of genetic exchange. By using the abundant and well-preserved records of chloroplast DNA hidden within the nucleus, scientists can identify the donors of lateral gene transfer events with much greater precision. The researchers found that these transfers can involve large, complex fragments of DNA that include both genes and non-coding regions, and that the same donor can contribute to multiple transfer events. While the method relies on the presence of these specific genetic fossils and may not work for every ancient transfer, it offers a clear path forward for understanding how grasses have swapped genetic material throughout their evolutionary history. The ability to identify the exact donor species opens the door to understanding the mechanisms that allow DNA to jump between species and the traits that grasses gain from these exchanges.
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