Chromosome segregation in eukaryotes is driven by the kinetochore, a macromolecular complex that connects centromeric DNA to microtubules of the spindle apparatus. Kinetochores in well-studied model eukaryotes consist of a core set of proteins that are broadly conserved among distant eukaryotic phyla. By contrast, unicellular flagellates of the class Kinetoplastida have a unique set of 36 kinetochore components. The evolutionary origin and history of these kinetochores remain unknown. Here, we report evidence of homology between axial element components of the synaptonemal complex and three kinetoplastid kinetochore proteins KKT16-18. The synaptonemal complex is a zipper-like structure that assembles between homologous chromosomes during meiosis to promote recombination. By using sensitive homology detection protocols, we identify divergent orthologues of KKT16-18 in most eukaryotic supergroups, including experimentally established chromosomal axis components, such as Red1 and Rec10 in budding and fission yeast, ASY3-4 in plants and SYCP2-3 in vertebrates. Furthermore, we found 12 recurrent duplications within this ancient eukaryotic SYCP^2–3 gene family, providing opportunities for new functional complexes to arise, including KKT16-18 in the kinetoplastid parasite Trypanosoma brucei. We propose the kinetoplastid kinetochore system evolved by repurposing meiotic components of the chromosome synapsis and homologous recombination machinery that were already present in early eukaryotes.

真核生物中的染色体分离是由着丝粒驱动的，着丝粒是一种将着丝粒 DNA 连接到纺锤体微管的大分子复合物。在经过充分研究的模型真核生物中的动粒由一组核心蛋白质组成，这些蛋白质在远距离真核生物门中广泛保守。相比之下，Kinetoplastida 类的单细胞鞭毛虫具有一组独特的 36 种着丝粒成分。这些着丝粒的进化起源和历史仍然未知。在这里，我们报告了联会复合体的轴向元件成分与三种动质体着丝粒蛋白 KKT16-18 之间同源性的证据。联会复合体是一种拉链状结构，在减数分裂过程中在同源染色体之间组装以促进重组。通过使用敏感的同源检测协议，我们在大多数真核生物超群中鉴定出 KKT16-18 的不同直向同源物，包括实验建立的染色体轴组分，例如出芽酵母和裂殖酵母中的 Red1 和 Rec10、植物中的 ASY3-4 和脊椎动物中的 SYCP2-3。此外，我们在这个古老的真核 SYCP 中发现了 12 个反复出现的重复^{2-3基因家族，为新功能复合物的出现提供机会，包括动质体寄生虫}布氏锥虫中的 KKT16-18 。我们提出通过重新利用早期真核生物中已经存在的染色体突触和同源重组机制的减数分裂成分而进化的动质体着丝粒系统。