Jixin Zhuang

The chromosome axis is a proteinaceous scaffold essential for crossover formation in meiosis. While the axis associated HORMA-domain protein ASY1 is known to be necessary for the localization of the recombinase DMC1 to the axis, the molecular mechanism linking ASY1 to DMC1 loading remains unclear. Here, we demonstrate that ASY1 directly recruits the plant-specific cyclin SDS to chromosomes, which ensures efficient DMC1 localization and interhomolog recombination. Live-cell imaging and immunodetection revealed that SDS localizes to chromosomes from leptotene to zygotene and disappears after pachytene resembling DMC1 dynamics. Biochemical assays demonstrated a direct interaction between ASY1 and SDS. Further analyses revealed that ASY1’s closure motif, a conformational regulatory element, is required for SDS recruitment despite being dispensable for ASY1’s own chromosome association. Moreover, unlike in sds mutants, depletion of the DMC1 inhibitor FIGL1-FLIP complex failed to restore DMC1 foci in asy1 mutants, positioning ASY1 upstream of both positive (SDS) and negative (FIGL1-FLIP) regulators of DMC1. These findings establish ASY1 as a key coordinator of interhomolog recombination, bridging chromosome axis architecture with recombinase dynamics to control crossover formation and genome stability.

Apomixis, or clonal reproduction through seeds, bypasses meiosis and fertilization to produce progeny that are genetically identical to the mother plant, thereby allowing fixation of hybrid vigor (heterosis) across generations. Synthetic apomixis has been established in Arabidopsis thaliana and rice (Oryza sativa) by combining 3 mutations into a Mitosis instead of Meiosis (MiMe) system where meiosis is transformed into a single mitosis-like division that produces clonal unreduced gametes, together with factors inducing parthenogenesis or haploidy. MiMe requires the simultaneous inactivation of 3 genes involved in different meiotic events, which poses challenges for its application in polyploids, which harbor multiple copies of each target gene. Here, we demonstrate that mutating 2 genes in the allotetraploid oilseed crop Brassica napus effectively converts meiosis into a mitosis-like division, producing over 95% viable clonal gametes, and fixing the genetic heterozygosity of F1 hybrids. This simplified MiMe system provides a practical framework for the production of clonal gametes, paving the way for engineering synthetic apomixis in Brassica napus and potentially other polyploid crops.

Seedlessness is a most valuable trait in fruit crops for fresh consumption and processing. The mutations in essential meiosis genes are known to confer sterility and seed abortion in plants. However, defects in meiosis have rarely been reported in fruit crops. Here, we found meiosis defects caused sterility in a seedless citrus bud sport cultivar, with massive unpaired univalents during diakinesis, indicating a disruption in crossover formation. A non‐functional CrMER3A^‐103 bp allele with a 103‐bp deletion in the gene body, together with the other non‐functional CrMER3a allele with a T deletion in exon, were identified in the seedless cultivar. The CrMER3 protein was undetectable at meiotic prophase I in the seedless cultivar, and knock out of CrMER3 resulted in sterility in precocious Mini‐citrus. Therefore, the natural variation in CrMER3 is responsible for sterility and seedlessness in this bud sport cultivar. The CrMER3a allele originated from the primitive wild mandarin and was passed to cultivated mandarins. A Kompetitive Allele‐Specific PCR (KASP) marker was developed to identify citrus germplasm with CrMER3a allele and to screen potential sterile and seedless hybrids in citrus cross breeding. Uncovering the natural mutations responsible for meiosis defects in citrus enhances our understanding of mechanisms controlling seedlessness in fruit crops and facilitates breeding of seedless varieties.

Summary: Polyploidy plays a key role in genome evolution and crop improvement. The formation of bivalents rather than multivalents during meiosis of polyploids is essential to ensure meiotic stability and optimal fertility of the species. However, the mechanisms preventing multivalent recombination in polyploids remain obscure. We studied the role of the synaptonemal complex in polyploid meiosis by mutating the transverse filament component ZYP1 in allotetraploid Brassica napus and autotetraploid Arabidopsis. In B. napus, a mutation of all four ZYP1 copies results in multivalent pairing accompanied by pairing partner switches, nonhomologous recombination, and interlocks, leading to severe chromosome entanglement and fertility abortion. The presence of only one functional allele of ZYP1 compromises synapsis and multivalent associations occur at nonsynaptic regions. Moreover, the disruption of ZYP1 causes a complete shift from predominantly multivalent pairing to exclusively multivalent pairing in pachytene cells of synthetic autotetraploid Arabidopsis thaliana, resulting in a dramatic increase in the frequency of multivalents at metaphase I. We conclude that the ZYP1‐mediated assembly of the synaptonemal complex facilitates the pairwise homologous pairing and recombination in both allopolyploid and autopolyploid species and plays a key role in ensuring a diploid‐like bivalent formation in polyploid meiosis.

Crossovers create genetic diversity and are required for equal chromosome segregation during meiosis. Crossover number and distribution are highly regulated by different mechanisms that are not yet fully understood, including crossover interference. The chromosome axis is crucial for crossover formation. Here, we explore the function of the axis protein ASYNAPSIS3. To this end, we use the allotetraploid species Brassica napus; due to its polyploid nature, this system allows a fine-grained dissection of the dosage of meiotic regulators. The simultaneous mutation of all 4 ASY3 alleles results in defective synapsis and drastic reduction of crossovers, which is largely rescued by the presence of only one functional ASY3 allele. Crucially, while the number of class I crossovers in mutants with 2 functional ASY3 alleles is comparable to that in wild type, this number is significantly increased in mutants with only one functional ASY3 allele, indicating that reducing ASY3 dosage increases crossover formation. Moreover, the class I crossovers on each bivalent in mutants with 1 functional ASY3 allele follow a random distribution, indicating compromised crossover interference. These results reveal the distinct dosage-dependent effects of ASY3 on crossover formation and provide insights into the role of the chromosome axis in patterning recombination.