Chromosome‑scale genome assembly of Verticillium longisporum CVL1: validation of species‑specific genomic fragment VLspe2 and establishment of a one‑tube visual RPA‑CRISPR/Cas12b detection assay

In the hidden world beneath our feet, a silent war is waged against the crops that feed us. Among the many invisible enemies, a fungus known as Verticillium longisporum stands out as a particularly destructive foe. This soil-dwelling organism invades the roots of oilseed rape and other related vegetables, clogging the plant's internal waterways and causing them to wither and die. Because the fungus can lie dormant in the soil for a decade, waiting for the right moment to strike, farmers face a persistent threat that is difficult to manage. To fight back effectively, scientists need two things: a detailed map of the enemy's genetic code to understand how it works, and a fast, reliable way to spot its presence before it spreads. For years, the maps available were drawn from samples found in Europe, leaving a gap in knowledge about the specific strains causing trouble in China. Without a local genetic blueprint, it has been difficult to design detection tools that are perfectly tuned to the specific variations of the fungus found in Chinese fields.

A team of researchers in Beijing has now filled this gap by creating the first complete, chromosome-scale genetic map of a Chinese strain of this fungus, isolated from Qinghai Province. By piecing together DNA sequences from different types of reading technologies, they reconstructed the entire genome of the fungus, which is a complex hybrid made of two distinct ancestral parts. This high-resolution map allowed them to confirm the location of a specific genetic fragment that acts as a unique fingerprint for this species. Using this confirmed fingerprint, they developed a new method to detect the fungus that is both incredibly sensitive and simple enough to use outside a laboratory. The result is a diagnostic tool that can identify the pathogen in a single tube within forty minutes, producing a result that can be seen with the naked eye under a blue light, without the need for expensive machines.

The journey began with the creation of a reference genome for the strain named CVL1. The researchers combined short DNA reads, long DNA reads, and a technique that maps how DNA folds inside the cell to assemble the genetic material into 16 distinct chromosomes. This assembly was remarkably complete, capturing nearly all the essential genetic markers expected in a fungus of this type. A key challenge with this particular fungus is that it is a hybrid, meaning its genome is a mosaic of two different parent lineages. The researchers successfully untangled this mixture, assigning specific sections of the DNA to one parent lineage and others to the second, while also identifying a small portion of the genome that could not be confidently assigned to either. This level of detail provided a solid foundation for the next step: finding a target that is unique to this fungus and does not appear in its close relatives.

With the genome in hand, the team turned their attention to a genetic fragment called VLspe2, which had been previously identified as a potential marker for the species. Using their new, detailed map, they verified that this fragment is indeed present in the Chinese strain and exists as a single copy on one of the chromosomes. Crucially, they confirmed that this specific sequence is missing from other closely related fungi that often grow in the same fields. This validation was essential because it meant the fragment could serve as a reliable target for a detection test that would not accidentally flag harmless neighbors as dangerous. The researchers then designed a set of molecular tools to hunt for this specific sequence. They created short pieces of DNA to amplify the target and a guide molecule to direct a molecular scissors enzyme to the site.

The detection system they built combines two powerful technologies into a single, streamlined process. First, a method called RPA rapidly copies the target DNA at a constant, warm temperature, making millions of copies if the fungus is present. Second, a CRISPR-based enzyme, guided by the specific RNA molecule, waits for those copies to appear. Once the enzyme finds its target, it snaps into action and cuts a nearby fluorescent reporter molecule. This cutting action releases a bright signal that can be seen directly. The researchers optimized this system by testing different combinations of guides and primers until they found the pair that produced the strongest and most consistent signal. They then tested the final tool against a variety of samples, including DNA from the target fungus, DNA from other similar fungi, and samples with no fungus at all.

The results showed that the new assay is highly specific, lighting up only when the target fungus is present and ignoring all other tested species. It is also remarkably sensitive, capable of detecting as few as ten copies of the fungal DNA in a single microliter of liquid. The entire process takes about forty minutes, and the final result is visible to the naked eye when the tube is held under a blue light, turning from clear to bright. This means that the test does not require complex laboratory equipment or highly trained personnel to interpret the results. The researchers noted that while the test was validated using pure DNA from cultured samples, its performance in real-world soil or plant tissue still needs to be confirmed. Nevertheless, the combination of a high-quality genetic map and a portable, visual detection method offers a powerful new resource for monitoring and managing this destructive pathogen, potentially helping to protect vegetable crops from devastating yield losses.