Meiosis in barley: Mixing it up

Project Information

MEIOBARMIX

Grant agreement ID: 949618

DOI

10.3030/949618

Project closed

EC signature date 23 October 2020

Start date 1 February 2021

End date 31 January 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 497 875,00

EU contribution

€ 1 497 875,00

Coordinated by

LEIBNIZ - INSTITUT FUER PFLANZENGENETIK UND KULTURPFLANZENFORSCHUNG
Germany

Periodic Reporting for period 4 - MEIOBARMIX (Meiosis in barley: Mixing it up)

Reporting period: 2025-08-01 to 2026-01-31

Summary of the context and overall objectives of the project

Ensuring food security in the future is a major global challenge, particularly in the context of climate change and a growing world population. Achieving this goal requires rapid and targeted crop improvement in frame of breeding. Traditional plant breeding relies on natural genetic variation that is generated during meiosis, a special type of cell division in which chromosomes exchange genetic material. This genetic exchange occurs through a process called meiotic recombination. During recombination, programmed DNA breaks are repaired either without leaving a genetic trace or by exchanging DNA between parental chromosomes, thereby creating new combinations of traits. In many crop plants, including barley (Hordeum vulgare), this exchange is however unequal: recombination occurs mainly at chromosome ends, while large interstitial chromosome regions remain largely inaccessible to breeding. As a result, natural available genetic diversity cannot be efficiently exploited, limiting progress in crop improvement. Furthermore, plant breeding has traditionally focused almost exclusively on one visible outcome of recombination, known as crossovers. However, most recombination events are repaired in other ways that do not lead to obvious reciprocal chromosome exchanges. In particular, short, non‑reciprocal DNA exchanges between parental chromosomes (non‑crossover gene conversions) have been largely overlooked, mainly because their frequency, length, and value for plant breeding were poorly understood.
The overall objective of the MEIOBARMIX project was to overcome these limitations by uncovering basic principles underlying meiotic recombination and developing new strategies to control it. The main objectives were to (i) understand previously untapped forms of genetic variation generated during meiosis, (ii) establish novel tools to direct recombination to predefined genomic regions, and (iii) to modify recombination outcome in barley.
The project showed that meiotic recombination outcomes can be deciphered, manipulated, and even targeted to specific genomic sites. In detail, it showed that an underestimated form of genetic exchange, known as gene conversion, is in fact very frequent and represents a major, previously untapped source of natural variation. Importantly, the project also showed that meiotic recombination can be targeted to defined genomic regions in plants, including regions where genetic exchange normally does not occur.
Together, MEIOBARMIX significantly advances our understanding of how genetic diversity during meiosis is generated and provides new approaches to make better use of this diversity. By enabling more efficient breeding strategies, the project contributes to sustainable agriculture and long‑term food security.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Over the course of the project, MEIOBARMIX combined genetic and technological approaches to decipher how meiotic recombination generates genetic diversity and how this process can be redirected for crop improvement. The work was carried out in the model plant Arabidopsis thaliana and translated to the crop barley, with a strong focus on identifying previously overlooked forms of genetic variation and developing tools to direct recombination outcomes.
First, the project established genetic systems to dissect local meiotic recombination outcomes. These analyses revealed that short, non‑reciprocal DNA exchanges between parental chromosomes, so-called non‑crossover gene conversions, are far more frequent than previously assumed. At many genomic regions, gene conversions occurred much more often than crossovers and varied in length from only a few to thousands of base pairs of DNA. Hence, a major, previously untapped source of natural genetic variation, that has largely been ignored in plant breeding, was revealed.
Second, MEIOBARMIX showed that meiotic recombination can be deliberately induced at predefined genomic sites in plants. By triggering targeted DNA breaks during meiosis, genetic exchange was activated even in chromosome regions that normally do not recombine. Importantly, these targeted events produced both crossovers and gene conversions, enabling precise genetic exchange without unwanted side exchanges. This represents a major conceptual and technological advance toward targeted and more efficient breeding strategies.
Third, the project developed and applied novel tools to study and manipulate meiotic recombination in barley. These included high‑throughput methods to measure recombination directly in pollen, eliminating the need to grow large segregating populations, and rapid virus‑based genome editing approaches that allow functional analysis of genes without conventional genetic transformation. While the applicability of some of these approaches proved limited, the focus consequently shifted toward identifying genetic factors and sex‑specific differences that can be exploited to modify recombination patterns in breeding.
The results of MEIOBARMIX have been disseminated through scientific publications, presentations at international conferences, and outreach activities. Some findings are currently being prepared for publication or intellectual property protection. Together, the work performed establishes a new framework for understanding, measuring, and controlling meiotic recombination and provides powerful tools to unlock previously inaccessible genetic variation for sustainable crop improvement.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Plant breeding accesses genetic variation generated during meiosis through recombination. In many crops, including cereals such as barley, this process is limited in frequency and skewed toward chromosome ends, leaving large genomic regions inaccessible to breeding. This represents a major bottleneck for the development of improved crop varieties.
MEIOBARMIX has increased our understanding of how to decipher, understand, and manipulate meiotic recombination in plants. The project demonstrated that non‑reciprocal DNA exchanges between parental chromosomes, known as non-crossover gene conversions, are not rare exceptions but instead represent the most frequent outcome of meiotic recombination in plants. This finding extends the long‑standing focus on crossovers as the primary source of meiotic variation and reveals a major, yet untapped reservoir of natural genetic diversity with direct relevance for plant breeding. Technologically, MEIOBARMIX established that plant meiotic recombination landscapes are programmable. Homologous recombination could be induced at predefined genomic loci, including regions that normally do not recombine. Importantly, targeted recombination events produced both crossovers and gene conversions, enabling precise genetic exchange without unwanted linkage effects. This represents a major step toward more efficient breeding strategies. In addition, the project delivered a versatile and transferable toolbox to study and manipulate plant meiosis. Novel genetic systems and sequencing‑based approaches allow precise detection of local recombination frequency and outcome, including gene conversion tract lengths and the identification of genetic factors controlling recombination. Virus‑based genome editing and high‑throughput pollen‑based analyses provide practical ways to assess and modify recombination directly in breeding‑relevant material.
Several findings were unexpected and opened new research directions, including the high incidence of gene conversions and the feasibility of inducing targeted meiotic recombination even in genomic regions that are normally recombination-inactive. Moreover, meiotic recombination in barley proved rather stable across developmental stages. This shifted the focus toward genetic and sex‑specific factors that can be exploited to reshape recombination patterns through breeding design rather than environmental manipulation.
The approaches developed in MEIOBARMIX provide a foundation for future efforts to harness meiotic recombination in crop improvement. Given the strong conservation of meiosis, the established strategies are expected to be transferable to other crops. Future research will focus on refining targeted recombination approaches, exploiting gene conversion‑based variation, and integrating these tools into practical breeding. Ultimately, accessing previously untapped genomic regions and directing recombination with precision has the potential to accelerate crop improvement and contribute to sustainable agriculture and long‑term food security.

Male meiosis in Hordeum vulgare (barley) and Arabidopsis thaliana