CELL DIVISION IN CYANOPHORA PARADOXA

Abstract

SummaryMitosis in the nucleus of Cyanophora paradoxa is shown to be generally typical of other eukaryotic cells. Basal bodies and the flagella apparatus appear to replicate before nuclear division. At preprophase, microtubules approach the nucleus from the periphery of the cell to form the usual extranuclear spindle by prophase; at this stage the nucleolus begins its breakdown, which is soon complete. The spindle poles, clearly defined by the orientation of microtubules, never contained any obvious structural component, and basal bodies were never associated with the spindle. By prometaphase, breakdown of the nuclear envelope is accompanied by invasion of the nucleus by the extranuclear microtubules. During anaphase, two plates of chromosomes separate inside the spindle containing both continuous and chromosomal microtubules. Remnants of the nuclear envelope often coat the chromosomes. Cleavage, initiated by early anaphase, is achieved by an asymmetric, longitudinal constriction of the cell membrane, which passes between the replicated flagellar apparatuses. The plane of cleavage was always oriented at right angles to the spindle axis, and did not involve any detectable microfibrillar elements. Telophase nuclei reform as normal. These observations indicate that the host cell of the symbiotic pair cannot be considered as a dinoflagellate as had been previously suggested.Division of the cyanelles is independent of nuclear division in the host, which therefore contains variable numbers of these symbionts. ‘Wall’ material is secreted into the furrow of the cleaving cyanelles. Furthermore, a very characteristic organelle was discovered in many cells invariably appressed to the cyanelles. Its dense granular contents were contained within a single unit membrane, and it was also frequently associated with endoplasmic reticulum. It was always inserted into the cleavage of dividing cyanelles and apparently it divided along with them, so that each daughter cyanelle remained associated with one of these inclusions. This organelle is thought to be a peroxisome and its possible significance in the host‐cyanelle relationship (and also in the evolution of chloroplasts) is briefly indicated.