Ultrastructure of mitosis in the amoeboflagellate Naegleria gruberi.

Naegleria gruberi is an amoeboflagellate found in soil; mitosis is restricted to the amoeboid phase of its life-cycle. Ultrastructural examination of mitotic stages has confirmed some aspects of karyokinesis reported in earlier light-microscopic studies and expanded on other features of nuclear division described in electron-microscopic studies of Naegleria. The nuclear envelope remained intact throughout division, the nucleolus persisted, and centrioles were not found. Chromosomes were indistinguishable at the ultrastructural level, nor was any evidence detected of sites of microtubular attachment to possible chromosomes. An interzonal body, formed during separation in two of the nucleolus, was not an invariable feature of mitosis. The same was true of the polar caps, which appeared to be little more than the ends of the mitotic spindle. It is suggested that, in line with comparable situations in other protists, expansion of the nuclear envelope is chiefly responsible for separation of the nucleus into two daughter nuclei.