Zinfandel’s Genetic Secrets: Genome Sequencing of Eight Tribidrag Clones Reveals a Puzzle

One of the world’s most celebrated wine grapes has just had its genetic secrets laid bare, and the results are as surprising as they are illuminating. Tribidrag, the ancient Croatian variety better known to most of the planet as Zinfandel, Primitivo, or Kratošija depending on the country pouring the glass, has long been a favorite subject for scientists studying how grapevines change over centuries of vegetative propagation. Now, a team of researchers from Croatia and China has sequenced the whole genomes of eight Tribidrag clones and compared those sequences with three years of field measurements of productivity and fruit chemistry. Their findings, published in BMC Plant Biology, reveal a striking disconnect: the clones are clearly and measurably different at the DNA level, but those genetic differences do not map neatly onto the visible and enological differences that growers and winemakers care about.

The story of Tribidrag is itself a tale of scientific detective work. For decades, the identity of Zinfandel was one of the great mysteries of viticulture, with competing claims about whether the variety originated in Italy, where it thrives as Primitivo, or elsewhere in the Mediterranean. Genetic fingerprinting eventually traced the cultivar to Croatia, where it survives under its oldest recorded name, Tribidrag, and where old vines still cling to terraced slopes along the Adriatic coast. Because grapevines are propagated vegetatively, by cuttings rather than seeds, every plant of a given variety is in principle a genetic copy of its parent. Yet over centuries of cloning, somatic mutations accumulate, and growers have selected vines that differ in yield, berry size, sugar accumulation, and acidity. These selected lines, called clones, are the raw material of clonal selection programs that supply nurseries and vineyards worldwide.

The research team, led by Goran Zdunić of the Institute for Adriatic Crops and Karst Reclamation in Split and including collaborators at the University of Zagreb and Yunnan Agricultural University in Kunming, set out to bridge a persistent gap in clonal research. Although previous studies had documented variation among Tribidrag clones both in the vineyard and at the molecular level, no one had systematically connected the two. The question was deceptively simple: if two clones of the same variety look and behave differently, can we find the genetic differences responsible? To answer it, the researchers evaluated productive traits in eight clones over three consecutive growing seasons, measuring yield, yield components, and basic must chemistry, the sugar and acid parameters that determine the character of the resulting wine.

The field data told a clear story of phenotypic divergence. The eight clones varied significantly in productivity and showed significant phenotypic plasticity, meaning their performance shifted from year to year depending on growing conditions. Statistical analysis revealed a significant clone-by-year interaction, confirming that no single clone dominates in every season. Two clones maintained under the Foundation Plant Services collection in California, designated FPS03 and FPS06 and known as Primitivo selections, consistently showed lower productivity, smaller berries, higher sugar content, and lower total acidity in the 2021 and 2022 seasons compared with the Zinfandel 1–24 clone. That latter clone displayed greater productivity in all three years of the study, making it the most reliable performer in the trial. For winemakers, these differences matter enormously: smaller berries with concentrated sugars tend to produce more structured, intense wines, while higher-yielding clones favor volume and freshness.

With the phenotypic landscape mapped, the team turned to the genome. Using short-read whole-genome resequencing, they generated high-coverage sequence data for each of the eight clones and called variants across the entire reference genome of grapevine. The scale of the variation was remarkable. Each sample carried approximately 3.8 million single nucleotide polymorphisms, or SNPs, which are positions where a single DNA letter differs from the reference, along with roughly 300,000 insertions and deletions, known as InDels, where short stretches of sequence are present or absent. That level of variation within a single vegetatively propagated cultivar underscores how much somatic mutation and historical mixing of plant material can accumulate over centuries of clonal transmission. Most of the structural variants were located in intergenic regions, the stretches of DNA between genes, which is consistent with the expectation that purifying selection keeps functional gene sequences relatively stable while neutral variation piles up elsewhere in the genome.

To hunt for signatures of clonal selection, the researchers took a windowed approach, dividing the genome into 100-kilobase segments and counting pairwise SNP differences among the eight clones within each window. This method highlights genomic neighborhoods where particular clones stand out from the rest, suggesting that a mutation or set of mutations arose on a specific vine lineage and was then propagated along with it. The analysis revealed candidate regions that distinguish individual clones, providing a catalog of genomic intervals that future studies can mine for genes influencing berry size, sugar transport, acid metabolism, and other traits of viticultural interest. Because the windows are small enough to pinpoint candidate loci but large enough to be robust to sequencing noise, the approach offers a practical middle ground between whole-genome association studies and targeted candidate-gene sequencing.

Then came the twist. When the researchers performed a principal component analysis on the genome-wide SNP data, the eight clones resolved into three principal genetic clusters. If clonal selection had driven the phenotypic differences, one might expect the low-yielding, sugar-rich Primitivo clones to group together genetically, apart from the productive Zinfandel selections. Instead, the genetic groupings showed no clear geographic association among the samples and, more strikingly, no direct association with the phenotypic or productive traits of individual clones. The clones that look most different in the vineyard are not necessarily the ones that are most different in their DNA, at least not at the resolution of genome-wide SNP patterns. Measurable genetic differentiation is real, but it is not a simple mirror of measurable phenotypic divergence.

This disconnect is not a failure of the study; it is arguably its most valuable finding, and it echoes a pattern seen in other clonally propagated crops. Phenotypic differences among clones can arise from a small number of mutations with large effects, from epigenetic changes such as DNA methylation that whole-genome resequencing of the kind performed here does not capture, from structural rearrangements too large or too complex for short-read sequencing to resolve, or from interactions between genotype and environment that three field seasons, however carefully measured, only begin to sample. The significant clone-by-year interaction observed in the trial reinforces this last point: the same clone can rank differently for yield and must composition depending on the season, so any attempt to link genes to traits must account for environmental noise. The candidate SNP signatures identified in 100-kilobase windows now provide a starting list of genomic regions where deeper investigation, including long-read sequencing, epigenomic profiling, and functional validation, is most likely to pay off.

For the wine industry, the implications are practical as well as scientific. Clonal selection programs rely on the assumption that the clones they distribute are genetically distinct and predictably different in performance. The new study confirms that Tribidrag clones carry abundant genomic variation and that some of it marks individual clonal lineages, which strengthens the case for DNA-based tools to authenticate and track clonal material. At the same time, the weak link between genome-wide differentiation and field performance is a caution against assuming that a genetic profile alone can predict how a clone will behave in a given vineyard. Growers choosing between the consistently productive Zinfandel 1–24 and the smaller-berry, higher-sugar Primitivo selections FPS03 and FPS06 will still need multi-season field data, but they now have a far richer genomic map to guide the search for the mutations that ultimately matter in the glass.

The work also carries a conservation message. Tribidrag survives in Croatia as a heritage cultivar of limited distribution, and the study was supported by the Croatian Science Foundation through the RemainGrape project, which is dedicated to the prospection and genomic analysis of endangered Croatian grapevine cultivars and wild grapevine. By documenting both the phenotypic richness and the genomic depth of variation within a single ancient variety, the research makes the case that preserving old vineyards and clonal collections preserves an irreplaceable archive of somatic evolution. Every century-old vine carries mutations accumulated over generations of vegetative transmission, and each clone is a living record of both natural mutation and human selection. Sequencing eight clones has revealed how much of that record remains to be read, and how much of the connection between the grapevine genome and the wine in the bottle is still waiting to be decoded.

Subject of Research: Whole-genome sequencing and phenotypic analysis of clonal variation in the grapevine cultivar Tribidrag (Zinfandel)

Article Title: Whole-genome sequencing of eight grapevine clones of cv. Tribidrag (Vitis vinifera L.) reveals measurable differentiation not linked directly to phenotypic divergence

Article References: Zdunić, G., Duan, S., Wang, C., Lukšić, K., Marinov, L., Mucalo, A., Ozretić Zoković, M., Maletić, E., Pejić, I., & Chen, W. (2026). Whole-genome sequencing of eight grapevine clones of cv. Tribidrag (Vitis vinifera L.) reveals measurable differentiation not linked directly to phenotypic divergence. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10110-x

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10110-x

Keywords: Tribidrag, Zinfandel, grapevine, clonal variation, whole-genome sequencing, somatic mutation, Vitis vinifera, viticulture, SNP, phenotypic plasticity, plant genetics, clonal selection