Loose-Bunch Trait in Grapevine Somatic Variant VP11 - Reduced Gamete Viability Through Balanced Reciprocal Translocation

Aug 27, 2026

Clonally propagated grapevine cultivars preserve varietal attributes, yet extended propagation histories can generate somatic mutations with useful phenotypes. This work characterizes VP11, a loosebunch somatic variant of ‘Tempranillo Tinto’, by combining phenotyping, genomics, and genetic analyses to uncover how bunch looseness arises. VP11 shows ~50% reduced pollen viability and fewer seeds per berry versus a control. Long-read whole-genome sequencing identifies eleven large somatic structural variants, including a heterozygous balanced reciprocal translocation (Tra1-3) whose unbalanced gametes are non-viable, lowering gamete fitness while producing the loose-bunch trait.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages27

Reading Time1 h 8 min

AccessFree to download

Background and Rationale

Clonal propagation and somatic mutation in grapevine

Importance of bunch compactness

Study Design

Phenotyping and genetic analyses of VP11

Long-read whole-genome sequencing

Structural Variants and Validation

Identification of somatic structural variants

Validation and inheritance of Tra1-3

Mechanistic Conclusion

Reduced gamete viability as a cause of the loose-bunch trait

Instituto de Ciencias de la Vid y del Vino (ICVV; CSIC, Universidad de La Rioja) Instituto de Ciencias de la Vid y del Vino (ICVV; CSIC, Universidad de La Rioja) Max Planck Institute for Biology Tübingen Max Planck Institute for Biology Tübingen Instituto de Ciencias de la Vid y del Vino (ICVV; CSIC, Universidad de La Rioja) Instituto de Ciencias de la Vid y del Vino (ICVV; CSIC, Universidad de La Rioja) bunch compactness, clonal variation, gamete viability, genome structural variation, grapevine, long-read sequencing, reciprocal translocation, somatic genome rearrangement October 6th, 2025 [https://doi.org/10.21203/rs.3.rs-7594260/v1](https://doi.org/10.21203/rs.3.rs-7594260/v1) 􀁱 􀅏 This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Competing interest reported. D.W. holds equity in Computomics, which advises plant breeders. DW previously consulted for KWS SE, a globally active plant breeder and seed producer. All other authors declare no competing interests. A version of this preprint was published at BMC Plant Biology on January 27th, 2026. See the published version at [https://doi.org/10.1186/s12870-026-08212-7](https://doi.org/10.1186/s12870-026-08212-7) . Because grapevine (Vitis vinifera L.) cultivars are highly heterozygous, they must be clonally propagated to preserve their varietal attributes. Over extended cultivar propagation histories, somatic mutations arise and can generate new phenotypes useful for intra-varietal improvement. Somatic variants with looser bunches – associated with more uniform berry ripening and reduced bunch rot incidence – are particularly valuable in compact-bunch cultivars. To understand the basis of this trait, we combined phenotyping, genomics, and genetic analyses to study VP11, a loosebunch somatic variant clone of the wine grape cultivar ‘Tempranillo Tinto’. Pollen viability and the number of seeds per berry were reduced by ~50% in VP11 compared to a control clone of‘Tempranillo Tinto’. Long-read whole-genome sequencing identi«ed eleven large somatic structural variants (SVs) in VP11, including three inter-chromosomal events. These consisted of one «xed reciprocal translocation (Tra1-3), with duplications spanning tens of kilobases at the translocation breakpoints, and two segmental duplications (one «xed and one likely L2 meristem cell layer-restricted) . All three SVs were molecularly validated, including the phasing and exchange of distal chromosome segments in Tra1-3 . In VP11 self-cross progeny, pollen viability was signi«cantly reduced among individuals carrying Tra1-3, and the two translocation chromosomes were always inherited together, indicating that gametes with an unbalanced chromosomal content are non-viable. This study identi«es reduced gamete viability caused by a heterozygous balanced reciprocal translocation as a mechanism underlying the loose-bunch trait in grapevine. We conclude that even if a genetic defect reduces gamete viability, it can still be useful to decrease seed and fruit set in vegetatively propagated crops where these traits are desirable. Grapevine (Vitis vinifera L.) is one of the most important fruit crops, covering more than 7.3 million hectares worldwide [ 1] . Grapevine is the foremost basis of winemaking but also of table grapes, raisins and juices. Grapevine cultivars are vegetatively propagated to maintain their varietal attributes, and many cultivars have been cultivated for centuries. Throughout their history of cultivation, spontaneous somatic mutations have occurred, and these have been maintained in speci«c clonal lines during cycles of vegetative multiplication [2] . Some of the associated phenotypes improve traits of interest, offering opportunities for the improvement of traditional grape cultivars [3–5] . Unlike genetic crosses, which in highly heterozygous organisms like grapevine cultivars disrupt varietal identity, somatic variants allow variety imp

FAQs

What trait does this study focus on in grapevine?

The study focuses on the loose-bunch trait, specifically how looser bunches contribute to more uniform berry ripening and reduced bunch rot incidence in compact-bunch cultivars.

How was VP11 characterized compared with a control clone?

VP11 showed about a 50% reduction in pollen viability and a lower number of seeds per berry compared with a control clone of ‘Tempranillo Tinto’.

Which genomic change is linked to reduced gamete viability?

A heterozygous balanced reciprocal translocation (Tra1-3) is identified as the key structural variant associated with reduced gamete viability, where unbalanced gametes are non-viable.

Why are the results useful for crop improvement despite reduced gamete fitness?

Vegetatively propagated crops can still benefit from defects that reduce seed and fruit set, because the trait can be maintained while achieving desired agronomic outcomes.

Aug 27, 2026

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