Germplasm characterization and genetic structure of Ukrainian sweet cherry (Prunus avium L.) assessed by microsatellite and S-locus markers

In the orchards of southern Ukraine, sweet cherry trees have been cultivated for centuries, producing fruit that is both a local staple and a source of national pride. These trees are not wild plants that grow from seed; they are carefully bred varieties, or cultivars, that have been selected over generations for specific traits like sweetness, size, and ripening time. Because a cherry tree grown from a seed will not produce fruit identical to its parent, farmers preserve these varieties by taking cuttings from a specific tree and grafting them onto rootstock. This process creates a genetic clone, ensuring the new tree is an exact copy of the original. Over time, however, names get mixed up. A tree might be mislabeled in a nursery, or a name might be applied to two different trees by mistake. When this happens, the history of the fruit is lost, and the genetic identity of the collection becomes a puzzle. For scientists working to preserve these trees, knowing exactly which tree is which is not just a matter of record-keeping; it is essential for breeding new varieties and ensuring that the right trees are planted to pollinate one another.

A researcher recently turned their attention to the sweet cherry collections held in Ukraine, a region with a long and significant history of fruit breeding. They set out to verify the true identity of ninety-four different cherry accessions, which are individual trees or groups of trees held in gene banks. The scientist used two main tools to solve the mystery of these trees. First, they looked at the DNA of the trees using markers called microsatellites, which act like a unique genetic fingerprint for each individual. Second, they examined the S-locus, a specific part of the genome that determines whether a tree can accept pollen from another tree. In sweet cherries, a tree cannot fertilize itself or another tree with the same S-locus type, so knowing this genetic code is vital for orchard management. By comparing the DNA of the Ukrainian trees against a standardized European database, the researcher aimed to clear up decades of confusion regarding names, parentage, and genetic relationships.

The investigation revealed that the labels attached to these trees were often unreliable. When the researcher compared the genetic fingerprints of trees that shared the same name, they found that five different names actually referred to pairs of trees that were not identical. In some cases, two trees with the same name were completely different varieties, essentially homonyms that had been confused over time. In other instances, trees with different names turned out to be genetic clones of each other, meaning they were the same variety growing under two different labels. The study also uncovered that some trees were mislabeled propagations, where a branch from one variety had been mistakenly grafted and labeled as another. After correcting for these duplicates and misidentifications, the researcher was left with ninety unique genetic profiles, providing a much clearer picture of the actual diversity within the collection.

One of the most striking discoveries involved two famous yellow-fruited varieties known as 'Drogans gelbe Knorpelkirsche' and 'Dönissens gelbe Knorpelkirsche'. These two trees are considered foundational ancestors for many modern Ukrainian cherries. However, the researcher found that the trees currently held in Ukrainian collections labeled as 'Drogans' actually carried the genetic fingerprint of 'Dönissens', and vice versa. This mix-up was not limited to Ukraine; the same error appeared in European reference collections, suggesting a widespread confusion that had been passed down through generations of propagation. Despite having swapped identities, the two trees remained distinct at the genetic level, particularly in their S-locus markers, which place them in different pollination groups. This means that while the names were swapped, the actual genetic material of the original founders was preserved, just attached to the wrong labels.

The study also shed light on the parentage of many Ukrainian cultivars. By comparing the genetic markers of offspring with their recorded parents, the scientist could confirm some family trees while rejecting others. They verified that a variety called 'Drogans gelbe Knorpelkirsche' was indeed the parent of seventeen different accessions, confirming its role as a major founder in the region's breeding history. However, they also found that the famous variety 'Valerii Chkalov' was a source of significant confusion. The records suggested that many trees were its offspring, but the researcher could not find a single authentic 'Valerii Chkalov' tree to serve as a reference. Instead, they found that two distinct genetic trees were being propagated under this single name. One version was compatible with the trees from the Melitopol breeding program, while another, different version was used in the Bakhmut program. This discovery means that many recorded pedigrees involving 'Valerii Chkalov' are actually based on two different, unidentified trees, making the family history more complex than previously thought.

Beyond the specific identities of individual trees, the research painted a picture of the genetic structure of the entire Ukrainian sweet cherry collection. The analysis showed that the breeding history was built on a narrow foundation, relying heavily on a small number of founder varieties that were repeatedly crossed to create new cultivars. While the collection contains a wide array of modern and early-selection varieties, the genetic diversity is largely shaped by these few ancestors. The study also expanded the known genetic data for the region, determining the S-locus types for thirty-three cultivars that had never been recorded before. This information is critical for growers, as it allows them to match trees with compatible pollinators, ensuring a successful harvest.

The work underscores a vital lesson for anyone managing plant collections: a name on a label is not enough to guarantee the identity of a tree. The researcher concluded that for conservation to be effective, records must be tied to the individual tree itself, supported by genetic evidence rather than just historical documents. In a world where collections are at risk from conflict and changing borders, having a verified genetic profile for every tree ensures that the material can be recognized and recovered, even if the original labels are lost or incorrect. By aligning Ukrainian data with European standards and correcting long-standing errors, this study provides a reliable map for the future of sweet cherry breeding and conservation in the region.