We investigated genome folding across the eukaryotic tree of life. We find two types of three-dimensional (3D) genome architectures at the chromosome scale. Each type appears and disappears repeatedly during eukaryotic evolution. The type of genome architecture that an organism exhibits correlates with the absence of condensin II subunits. Moreover, condensin II depletion converts the architecture of the human genome to a state resembling that seen in organisms such as fungi or mosquitoes. In this state, centromeres cluster together at nucleoli, and heterochromatin domains merge. We propose a physical model in which lengthwise compaction of chromosomes by condensin II during mitosis determines chromosome-scale genome architecture, with effects that are retained during the subsequent interphase. This mechanism likely has been conserved since the last common ancestor of all eukaryotes.

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A 10‐month‐old male king penguin (Aptenodytes patagonicus) presented with an acute episode of collapse. Complete blood count and serum biochemistry revealed a severe lymphocytic leukocytosis (white blood cell count 122.76 × 103/µl; lymphocytes 117.8 × 103/µl), elevated creatinine kinase (1414 IU/L) and mild anaemia (packed cell volume 33%). Marked hepatomegaly was noted on radiographs of the coelom. Blood smears contained an extremely high number of lymphocytes with frequent bizarre forms and a high mitotic count. Despite good initial response to treatment with steroids and prophylactic antimicrobials, the bird's lymphocytic leukocytosis increased, and the animal spontaneously died 2 weeks after initial presentation. Postmortem examination revealed infiltration of multiple organs, including bone marrow, with neoplastic lymphocytes consistent with T‐cell phenotype.

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