Kinetochores Associated with the Nuclear Envelope in the Mitosis of a Dinoflagellate

Mitosis in free-living dinoflagellates is unusual in that the nuclear envelope remains intact but a number of cytoplasmic channels containing mi-crotubules run completely through the nucleus (8, 9, 13). The chromosomes are known to be attached to the nuclear envelope (1, 8, 13) and during division this connection appears to take the form of a narrow stalk of chromosome material which fits into a depression in the nuclear membrane lining the channels. No centrioles or basal bodies have been observed adjacent to the dividing nuclei and until recently no connection had been seen between the microtubules and any other structure. It has been suggested that the microtubules merely provide a structural framework for mitosis and that the segregation of daughter chromatids might be brought about by movement of the nuclear envelope (8, 10, 12, 13). More recently it has also been suggested (6) that there are probably connections between the microtubules and the chromosomes , just as there are in most eucaryotic organisms and also in the parasitic dinoflagellate Oodinium (2). In the present note we report evidence for the FIGURE 1 Late anaphase-teiophase stage of Amphidinium showing the two daughter nuclei. Each contains scattered chromosomes and a single channel: that in the left-hand nucleus is sectioned obliquely and that in the right-hand nucleus, longitudinally. The right-hand nucleus also has a nucleolus, x 12,000. FIGURE 2 Enlargement of the lower half of the right-hand nucleus in Fig. 1. Note the microtubules in the channel and the three "kinetochores" (arrowheads) at the points where chromosomes are attached to the nuclear envelope, x 60,000. FIGURE 3 Two kinetochore-like structures (arrowheads) from another dividing nucleus. Note the arrowed portion of chromosome which contacts the nuclear envelope, x 100,000. FIGURE 4 High magnification of a kinetochore (arrowhead), showing the area of electron-dense material between the nuclear envelope and the microtubule, x 140,000.