Asymmetric inter-kingdom gene transfer and a horizontal transposon invasion in a powdery mildew fungus
This study reveals that recent horizontal DNA transfer between plants and fungi is rare and strongly asymmetric—predominantly involving fungal-to-plant gene flow and a single plant-to-fungal transposon invasion—yet these acquired sequences subsequently drive significant genome diversification through mechanisms like duplication, loss, and transposon amplification.
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 This is an AI-generated explanation of a preprint that has not been peer-reviewed.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
Life on land began with a quiet partnership. More than 450 million years ago, the first plants to leave the water did not do it alone; they formed intimate alliances with fungi. These microscopic partners helped the early colonizers survive the harsh, dry conditions of the new world, trading nutrients for shelter in a relationship that has persisted ever since. Over deep time, these two kingdoms have lived so close together that their genetic material has occasionally crossed the boundary between them. This process, known as horizontal transfer, is like a genetic loan where an organism picks up a piece of DNA from a neighbor rather than inheriting it from a parent. While scientists have long known that this happens, they have struggled to see how often it occurs between complex, multicellular life forms like plants and fungi, and what happens to that borrowed DNA once it arrives.
A team of researchers has now taken a fresh look at this question by scanning the entire genetic blueprints of hundreds of species. Instead of relying on pre-existing maps of genes, which can miss new or unusual sequences, they compared the raw DNA of 418 different plant species against 1,081 fungal species. This massive, unbiased search allowed them to spot recent instances where DNA had jumped from one kingdom to the other. They found that while such transfers are rare, they are not random. The flow of genetic material is heavily lopsided: plants are much more likely to receive DNA from fungi than the other way around. Furthermore, the researchers discovered that once this foreign DNA lands in a new host, it does not simply sit there. It can be copied, reshaped, lost, or even explode in number, depending on the specific family of plants or fungi that received it.
The study revealed that in the grasses known as Pooideae, which include wheat and barley, fungi have donated several genes that the plants have kept and modified. These transfers came specifically from a group of fungi called Epichloë, which live inside the grasses as harmless endophytes. The researchers found that the grasses did not just accept these genes; they duplicated them. Some of these borrowed genes now exist in multiple copies within a single plant, and the copies have started to behave differently. While one copy might remain active in the roots, another might be silent, or they might have changed their structure by swapping out internal parts. This suggests that the grasses are actively experimenting with this foreign genetic material, using it as raw material to evolve new traits. In contrast, in non-vascular plants like mosses, the story is different. When mosses acquired fungal genes, they tended to keep them as single copies. However, these single copies are unstable; the researchers found that within different subspecies of the liverwort Marchantia polymorpha, some individuals have kept the gene while others have lost it entirely, showing that the genetic landscape is still shifting.
Perhaps the most surprising discovery was a rare event in the opposite direction: a piece of plant DNA jumping into a fungus. The researchers identified a specific type of mobile genetic element, a transposon, which they named the Gnawa element. This element, which originated in plants, was found integrated into the genome of the tomato powdery mildew fungus, a pathogen that attacks tomatoes. Unlike the plant genes that arrived in fungi and disappeared, this fungal invader did not just survive; it thrived. After entering the fungus, the Gnawa element woke up and began to copy itself aggressively. It multiplied until there were 172 copies of a smaller, derived version of itself scattered throughout the fungal genome. This massive expansion happened so recently that the copies are nearly identical to one another. The researchers confirmed that this was not a mistake in their data or a contamination from the tomato plant itself, but a genuine case of a plant transposon taking over a fungal genome.
The findings paint a picture of a dynamic, albeit uneven, exchange of genetic information. The study rules out the idea that these transfers are common, widespread events happening equally across all plants and fungi. Instead, they appear to be rare, specific occurrences tied to close ecological relationships, such as the symbiosis between grasses and their internal fungal partners. The research also clarifies that the fate of transferred DNA is not uniform. In some lineages, the DNA is preserved and duplicated, potentially offering new functions. In others, it is fragile and prone to being lost. The discovery of the plant-to-fungus transfer of the Gnawa element adds a new chapter to this story, showing that the flow of genetic material can move in both directions, and that when it does, it can trigger rapid and dramatic changes in the recipient's genome. This work highlights that the boundaries between kingdoms are not as rigid as once thought, and that the history of life on land is written not just in the genes we inherit, but also in the genes we borrow.
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