Kinetochore size correlates with chromosome size in Star of Bethlehem (Ornithogalum kochii Parl., Asparagaceae).

Abstract

Chromosome segregation during cell division relies on the kinetochore, a multiprotein complex that mediates attachment of chromosomes to spindle microtubules. Biophysical considerations predict that kinetochore size should scale with chromosome size to meet increasing mechanical demands during segregation, yet empirical evidence for such scaling within karyotypes remains limited, particularly in species with modest chromosome size variation. Here, we investigated the relationship between chromosome size and kinetochore size in Star of Bethlehem (Ornithogalum kochii, Asparagaceae), a species from a genus with moderate intra-karyotype chromosome size variation (2.4-4.5-fold). Using immunolabelling of the conserved outer kinetochore protein KNL1 and linear mixed-effects modelling, we analysed 172 chromosomes across multiple metaphases, slides and individuals. We detected a strong positive scaling relationship between chromosome size and kinetochore size, demonstrating that cytogenetic approaches can resolve chromosome-kinetochore scaling even across relatively narrow size gradients. Our results demonstrate that kinetochore size scales with chromosome size even in species with modest intra-karyotype chromosome size variation, supporting the generality of chromosome-kinetochore scaling. Together with evidence from taxa exhibiting more extreme karyotype heterogeneity, this pattern suggests that centromere and kinetochore architecture is shaped by both functional constraints on chromosome segregation and long-term evolutionary processes such as centromere drive. Viewed in a broader context, chromosome-kinetochore scaling may also be relevant for processes beyond segregation, as evolutionary changes in centromere and kinetochore architecture could indirectly influence recombination rates through their effects on chromosome size.