Independent evolution of holocentric centromeres in an early branching apicomplexan parasite
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Following host cell invasion by sporozoites, Cryptosporidium parvum ( C. parvum ) amplifies rapidly through three rounds of nuclear division prior to 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, centrosomes, microtubules, and nuclear pores during mitosis. Nuclear division was accompanied by centriole duplication and elongated microtubules that spanned dividing nuclei. Surprisingly, C. parvum centromeres showed diffuse staining throughout nuclear division, and telomeres overlapped with centromeres at the apical nuclear end. Analysis of centromeres based on CENH3 binding and DNA sequencing revealed ∼350 distinct binding sites scattered across all eight chromosomes, 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.
