Independent evolution of holocentric chromosomes in an early branching apicomplexan parasite.

During its asexual cycle, Cryptosporidium parvum (C. parvum) amplifies through three rounds of nuclear division before undergoing cytokinesis to form infectious merozoites. However, chromosome organization and segregation during nuclear division remain unstudied. Here, we visualized H3-histones, including the centromeric histone H3 (CENH3), telomeres, and centrosomes during mitosis. Nuclear division was accompanied by centriole duplication and elongated microtubules that spanned synchronously dividing nuclei. Surprisingly, C. parvum centromeres detected by CENH3 showed diffuse staining throughout nuclear division, that overlapped with telomeres at the apical nuclear end. Analysis of centromeres based on CENH3 capture and DNA sequencing revealed numerous distinct binding sites scattered across all eight chromosomes, including both coding regions and intergenic regions, that were typified by GA rich repeat sequences. The unique holocentric chromosome structure of C. parvum is unlike the single regional centromeres found in related apicomplexans, suggesting it arose independently from other known examples in plants, insects and worms.