Asynchronous nuclear division cycles in multinucleated cells

Synchronous mitosis is common in multinucleated cells. We analyzed a unique asynchronous nuclear division cycle in a multinucleated filamentous fungus, Ashbya gossypii. Nuclear pedigree analysis and observation of GFP-labeled spindle pole bodies demonstrated that neighboring nuclei in A. gossypii cells are in different cell cycle stages despite close physical proximity. Neighboring nuclei did not differ significantly in their patterns of cyclin protein localization such that both G1 and mitotic cyclins were present regardless of cell cycle stage, suggesting that the complete destruction of cyclins is not occurring in this system. Indeed, the expression of mitotic cyclin lacking NH2-terminal destruction box sequences did not block cell cycle progression. Cells lacking AgSic1p, a predicted cyclin-dependent kinase (CDK) inhibitor, however, showed aberrant multipolar spindles and fragmented nuclei that are indicative of flawed mitoses. We hypothesize that the continuous cytoplasm in these cells promoted the evolution of a nuclear division cycle in which CDK inhibitors primarily control CDK activity rather than oscillating mitotic cyclin proteins.

Mitotic domains reveal early commitment of cells in Drosophila embryos

In embryos of Drosophila melanogaster all the nuclei in the syncytial egg divide with global synchrony during the first 13 mitotic cycles. But with cellularization in the 14th cycle, global mitotic synchrony ceases. Starting about one hour into the 14th interphase, at least 25 ‘mitotic domains’, which are clusters of cells united by locally synchronous mitosis, partition the embryo blastoderm surface into a complex fine-scale pattern. These mitotic domains, which are constant from one embryo to the next, fire in the same temporal sequence in every embryo. Some domains consist of a single cell cluster straddling the ventral or dorsal midline. Most consist of two separate cell clusters that occupy mirror-image positions on the bilaterally symmetric embryo. Others comprise a series of members present not only as bilateral pairs but also as metameric repeats. Thus a domain can consist of either one, two, or many (if metamerically reiterated) clusters of contiguous cells. Within each cluster, mitosis starts in a single cell or in a small number of interior cells then spreads wave-like, in all directions, until it stops at the domain boundary. Each domain occupies a specific position along the anteroposterior axis—as determined by the expression pattern of the engrailed protein, and along the dorsoventral axis—as determined by cell count from the ventral midline. The primordia of certain larval structures appear to consist solely of the cells of one specific mitotic domain. Moreover, cells in at least some mitotic domains share specific morphogenetic traits, distinct from those of cells in adjacent domains. These traits include cell shape, spindle orientation, and participation by all the cells of a domain in an invagination. The specialized behaviors of the various mitotic domains transform the monolayer cell sheet of the blastoderm into the multilayered gastrula. I conclude that the fine-scale partitioning of the newly cellularized embryo into mitotic domains is an early manifestation of the commitment of cells to specific developmental fates.

Cytokinin-Induced Mitosis in Cultured Explants of Helianthus tuberosus L. Tuber Tissue

Freshly excised explants of H. tuberosus tuber parenchyma tissue were cultured for up to 42 h in aqueous solutions of 0.6 �M indole-3-acetic acid (IAA; 0.6 �M) or one of four different cytokinins (0.4 or 2.0 �M) in the absence of IAA. All phytohormone treatments were effective in inducing partially synchronous mitosis, but mitosis was not observed in control explants cultured in water alone. IAA induced mitosis earlier and in a greater proportion of cells than did any cytokinin treatment. In most cases, cytokinins were more effective at the higher concentration. The relative effectiveness of the different cytokinins appeared to be benzyladenine > zeatin ≥ kinetin > zeatin riboside. Microdensitometric measurements of relative nuclear DNA levels indicated that DNA synthesis had not occurred in control explants, but was induced by both IAA and cytokinins before the mitotic activity that was observed in the later stages of culture. These responses of freshly excised explants were compared to the effect of zeatin on IAA-induced mitosis in prewashed explants, in which zeatin increased the synchrony of IAA-induced mitosis but not the proportion of cells undergoing mitosis.

The occurrence of nuclear fusion in the amoebal phase of the slime mold Echinostelium minutum: a reinterpretation of the significance of this phenomenon

Nuclear fusion occurs in less than 1% of the myxamoebae of Echinostelium minutum de Bary, isolate D-3, sublines 1965 and 1971. Binucleate amoebae undergo synchronous mitosis, the two nuclei fuse, the fusion nucleus divides, cytokinesis occurs, and uninucleate daughter cells are formed. Failure to find a haploid–diploid alternance between the amoebal and plasmodial phases using Feulgen cytophotometry suggests that nuclear fusion is not a prerequisite for plasmodial formation in this isolate. Nuclear fusion may be one of the mechanisms which has led to the polyploid condition of the 1971 subline. This phenomenon may also represent a parasexual process.