Meiotic self-pairing of the Psalidodon (Characiformes, Characidae) iso-B chromosome: A successful perpetuation mechanism

Abstract B chromosomes are non-essential additional genomic elements present in several animal and plant species. In fishes, species of the genus Psalidodon (Characiformes, Characidae) harbor great karyotype diversity, and multiple populations carry different types of non-essential B chromosomes. This study analyzed how the dispensable supernumerary B chromosome of Psalidodon paranae behaves during meiosis to overcome checkpoints and express its own meiosis-specific genes. We visualized the synaptonemal complexes of P. paranae individuals with zero, one, or two B chromosomes using immunodetection with anti-medaka SYCP3 antibody and fluorescence in situ hybridization with a (CA)15 microsatellite probe. Our results showed that B chromosomes self-pair in cells containing only one B chromosome. In cells with two identical B chromosomes, these elements remain as separate synaptonemal complexes or close self-paired elements in the nucleus territory. Overall, we reveal that B chromosomes can escape meiotic silencing of unsynapsed chromatin through a self-pairing process, allowing expression of their own genes to facilitate regular meiosis resulting in fertile individuals. This behavior, also seen in other congeneric species, might be related to their maintenance throughout the evolutionary history of Psalidodon.

摘要：B染色体（B chromosome）是存在于多种动物和植物物种中的非必需附加基因组元件。在鱼类中，脂鲤属（*Psalidodon*，脂鲤目、脂鲤科）物种具有丰富的核型多样性，多个种群携带不同类型的非必需B染色体。本研究以帕拉奈脂鲤（*Psalidodon paranae*）的额外非必需B染色体为对象，分析其在减数分裂过程中的行为，以阐明其如何克服减数分裂检查点并表达自身的减数分裂特异性基因。我们采用抗青鳉SYCP3抗体免疫检测法，结合(CA)₁₅微卫星探针的荧光原位杂交（fluorescence in situ hybridization, FISH）技术，对携带0个、1个或2个B染色体的帕拉奈脂鲤个体的联会复合体（synaptonemal complex）进行了可视化分析。结果显示，在仅携带1个B染色体的细胞中，B染色体会发生自身联会；在携带2个相同B染色体的细胞中，这些B染色体元件在细胞核区域内以独立的联会复合体或紧密的自身联会结构形式存在。综上，本研究揭示B染色体可通过自身联会过程逃脱未联会染色质的减数分裂沉默，从而得以表达自身基因，以促进正常减数分裂并产生可育个体。这一在其他同属物种中也被观测到的行为，可能与脂鲤属物种在演化历史中B染色体的持续保留密切相关。