MITOSIS IN THE FLAGELLATE TRYPANOPLASMA-BORRELI (KINETOPLASTIDEA, BODONIDA)

The mitosis in both uninucleate and giant multinucleate T. borreli cul ture forms was studied using transmission electron microscopy. The int erphase nucleus is of the vesicular type with one prominent nucleolus and dense peripheral chromatin. Nucleosome-like fibres, occasionally w ith scattered nodules on them of 21-23 nm in diameter, are found in th e karyoplasm. The onset of mitosis is signalled by the decondensation of the peripheral chromatin. No individual chromosomes are visible dur ing the whole division. In the course of mitosis the nucleus elongates , the nucleolus becomes fragmented, the nuclear envelope remains intac t, and the kinetochore-like plaques and microtubules appear. At metaph ase, the spindle microtubules are connected with fibrillar discs that adhere to the inner envelope membrane near nuclear poles. At anaphase, three main bundles of about 40 microtubules each can be found in the spindle. In giant cells the mitotic figures often lie inside a single outer membrane of the envelope, but at different angles to each other. We infer that mitosis in T. borreli involves a combination of a primi tive membraneous mechanism of chromosome separation with a more advanc ed mode of chromosome transmission including spindle microtubules and associated structures. This type of nuclear division can be designated as intranuclear closed orthomitosis without discernible individual ch romosomes.