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1. The Arabidopsis anaphase‐promoting complex/cyclosome subunit 8 is required for male meiosis

   https://doi.org/10.1111/nph.16014  

   Xu, Rong‐Yan ; Xu, Jing ; Wang, Liudan ; Niu, Baixiao ; Copenhaver, Gregory P. ; Ma, Hong ; Zheng, Binglian ; Wang, Yingxiang ( July 2019 , New Phytologist)

   Faithful chromosome segregation is required for both mitotic and meiotic cell divisions and is regulated by multiple mechanisms including the anaphase‐promoting complex/cyclosome (APC/C), which is the largest known E3 ubiquitin‐ligase complex and has been implicated in regulating chromosome segregation in both mitosis and meiosis in animals. However, the role of theAPC/C during plant meiosis remains largely unknown. Here, we show that ArabidopsisAPC8is required for male meiosis.

   We used a combination of genetic analyses, cytology and immunolocalisation to define the function of AtAPC8 in male meiosis.

   Meiocytes fromapc8‐1plants exhibit several meiotic defects including improper alignment of bivalents at metaphase I, unequal chromosome segregation during anaphaseII, and subsequent formation of polyads. Immunolocalisation using an antitubulin antibody showed thatAPC8 is required for normal spindle morphology. We also observed mitotic defects inapc8‐1,including abnormal sister chromatid segregation and microtubule morphology.

   Our results demonstrate that ArabidopsisAPC/C is required for meiotic chromosome segregation and thatAPC/C‐mediated regulation of meiotic chromosome segregation is a conserved mechanism among eukaryotes.

    

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2. Putative Condition-Dependent Viability Selection in Wild-Type Stocks of Drosophila pseudoobscura

   https://doi.org/10.1159/000522585  

   Altindag, Ulku H. ; Taylor, Hannah N. ; Shoben, Chelsea ; Pownall, Keeley A. ; Stevison, Laurie S. ( June 2022 , Cytogenetic and Genome Research)

   Meiotic recombination rates vary in response to intrinsic and extrinsic factors. Recently, heat stress has been shown to reveal plasticity in recombination rates in Drosophila pseudoobscura. Here, a combination of molecular genotyping and X-linked recessive phenotypic markers were used to investigate differences in recombination rates due to heat stress. In addition, haplotypes from the genetic crosses were compared to test if they deviated from equal proportions, which would indicate viability selection. To avoid this potential bias, SNP genotyping markers overlapping the regions assayed with mutant markers were used to further investigate recombination rate. Interestingly, skews in haplotype frequency were consistent with the fixation of alleles in the wild-type stocks used that are unfit at high temperature. Evidence of viability selection due to heat stress in the wild-type haplotypes was most apparent on days 7–9 when more mutant non-crossover haplotypes were recovered in comparison to wild type (p < 0.0001). Recombination analysis using SNP markers showed days 9–10 as significantly different due to heat stress in 2 pairs of consecutive SNP markers (p = 0.018; p = 0.015), suggesting that during this time period the recombination rate is most sensitive to heat stress. This peak timing for recombination plasticity is consistent with Drosophila melanogaster based on a comparison of similarly timed key meiotic events, enabling future mechanistic work of temperature stress on recombination rate. 

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3. The mop1 mutation affects the recombination landscape in maize

   https://doi.org/10.1073/pnas.2009475118  

   Zhao, Meixia ; Ku, Jia-Chi ; Liu, Beibei ; Yang, Diya ; Yin, Liangwei ; Ferrell, Tyshawn J. ; Stoll, Claire E. ; Guo, Wei ; Zhang, Xinyan ; Wang, Dafang ; et al ( February 2021 , Proceedings of the National Academy of Sciences)

   Meiotic recombination is a fundamental process that generates genetic diversity and ensures the accurate segregation of homologous chromosomes. While a great deal is known about genetic factors that regulate recombination, relatively little is known about epigenetic factors, such as DNA methylation. In maize, we examined the effects on meiotic recombination of a mutation in a component of the RNA-directed DNA methylation pathway,Mop1(Mediator of paramutation1), as well as a mutation in a component of thetrans-acting small interference RNA biogenesis pathway,Lbl1(Leafbladeless1). MOP1 is of particular interest with respect to recombination because it is responsible for methylation of transposable elements that are immediately adjacent to transcriptionally active genes. In themop1mutant, we found that meiotic recombination is uniformly decreased in pericentromeric regions but is generally increased in gene rich chromosomal arms. This observation was further confirmed by cytogenetic analysis showing that although overall crossover numbers are unchanged, they occur more frequently in chromosomal arms inmop1mutants. Using whole genome bisulfite sequencing, our data show that crossover redistribution is driven by loss of CHH (where H = A, T, or C) methylation within regions near genes. In contrast to what we observed inmop1mutants, no significant changes were observed in the frequency of meiotic recombination inlbl1mutants. Our data demonstrate that CHH methylation has a significant impact on the overall recombination landscape in maize despite its low frequency relative to CG and CHG methylation.

    

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4. Regulation of interference-sensitive crossover distribution ensures crossover assurance in Arabidopsis

   https://doi.org/10.1073/pnas.2107543118  

   Li, Xiang ; Zhang, Jun ; Huang, Jiyue ; Xu, Jing ; Chen, Zhiyu ; Copenhaver, Gregory P. ; Wang, Yingxiang ( November 2021 , Proceedings of the National Academy of Sciences)

   During meiosis, crossovers (COs) are typically required to ensure faithful chromosomal segregation. Despite the requirement for at least one CO between each pair of chromosomes, closely spaced double COs are usually underrepresented due to a phenomenon called CO interference. LikeMus musculusandSaccharomyces cerevisiae,Arabidopsis thalianahas both interference-sensitive (Class I) and interference-insensitive (Class II) COs. However, the underlying mechanism controlling CO distribution remains largely elusive. Both AtMUS81 and AtFANCD2 promote the formation of Class II CO. Using both AtHEI10 and AtMLH1 immunostaining, two markers of Class I COs, we show that AtFANCD2 but not AtMUS81 is required for normal Class I CO distribution among chromosomes. DepletingAtFANCD2leads to a CO distribution pattern that is intermediate between that of wild-type and a Poisson distribution. Moreover, inAtfancm,Atfigl1, andAtrmi1mutants where increased Class II CO frequency has been reported previously, we observe Class I CO distribution patterns that are strikingly similar toAtfancd2.Surprisingly, we found that AtFANCD2 plays opposite roles in regulating CO frequency inAtfancmcompared with either inAtfigl1orAtrmi1.Together, these results reveal that although AtFANCD2, AtFANCM, AtFIGL1, and AtRMI1 regulate Class II CO frequency by distinct mechanisms, they have similar roles in controlling the distribution of Class I COs among chromosomes.

    

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5. Cohesion and centromere activity are required for phosphorylation of histone H3 in maize

   https://doi.org/10.1111/tpj.13748  

   Liu, Yang ; Su, Handong ; Liu, Yalin ; Zhang, Jing ; Dong, Qianhua ; Birchler, James A. ; Han, Fangpu ( November 2017 , The Plant Journal)

   Haspin‐mediated phosphorylation of histone H3 at threonine 3 (H3T3ph) promotes proper deposition of Aurora B at the inner centromere to ensure faithful chromosome segregation in metazoans. However, the function of H3T3ph remains relatively unexplored in plants. Here, we show that in maize (Zea maysL.) mitotic cells, H3T3ph is concentrated at pericentromeric and centromeric regions. Additional weak H3T3ph signals occur between cohered sister chromatids at prometaphase. Immunostaining on dicentric chromosomes reveals that an inactive centromere cannot maintain H3T3ph at metaphase, indicating that a functional centromere is required for H3T3 phosphorylation. H3T3ph locates at a newly formed centromeric region that lacks detectable CentC sequences and strongly reducedCRMand ZmBs repeat sequences at metaphaseII. These results suggest that centromeric localization of H3T3ph is not dependent on centromeric sequences. In maize meiocytes, H3T3 phosphorylation occurs at the late diakinesis and extends to the entire chromosome at metaphase I, but is exclusively limited to the centromere at metaphaseII. The H3T3ph signals are absent in theafd1(absence of first division) andsgo1(shugoshin) mutants during meiosisIIwhen the sister chromatids exhibit random distribution. Further, we show that H3T3ph is mainly located at the pericentromere during meiotic prophaseIIbut is restricted to the inner centromere at metaphaseII. We propose that this relocation of H3T3ph depends on tension at the centromere and is required to promote bi‐orientation of sister chromatids.

    

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