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An important question in cell biology is how cellular organelles partition during cell division. In organisms undergoing closed mitosis, the elongation of an intranuclear spindle drives nuclear division, generating two identically sized nuclei [1, 2]. However, how the site of nuclear division is determined and the underlying mechanism driving nuclear envelope (NE) fission remain largely unknown. Here, using the fission yeast, we show that the microtubule bundler Ase1/PRC1 at the spindle midzone is required for the local concentration of nuclear pore complexes (NPCs) in the region of the NE in contact with the central spindle. As the spindle elongates during anaphase B, components of these NPCs are sequentially eliminated, and this is accompanied by the local remodeling of the NE. These two events lead to the eventual removal of NPCs and nuclear division. In the absence of importin α, NPCs remain stable in this region and no event of NE remodeling is observed. Consequently, cells fail to undergo nuclear division. Thus, our results highlight a new role of the central spindle as a spatial cue that determines the site of nuclear division and point to NPC removal as the triggering event., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 The Authors. Published by Elsevier Inc. All rights reserved.)

Although nuclei are the defining features of eukaryotes, we still do not fully understand how the nuclear compartment is duplicated and partitioned during division. This is especially the case for organisms that do not completely disassemble their nuclear envelope upon entry into mitosis. In studying this process in Drosophila neural stem cells, which undergo asymmetric divisions, we find that the nuclear compartment boundary persists during mitosis thanks to the maintenance of a supporting nuclear lamina. This mitotic nuclear envelope is then asymmetrically remodeled and partitioned to give rise to two daughter nuclei that differ in envelope composition and exhibit a >30-fold difference in volume. The striking difference in nuclear size was found to depend on two consecutive processes: asymmetric nuclear envelope resealing at mitotic exit at sites defined by the central spindle, and differential nuclear growth that appears to depend on the available local reservoir of ER/nuclear membranes, which is asymmetrically partitioned between the two daughter cells. Importantly, these asymmetries in size and composition of the daughter nuclei, and the associated asymmetries in chromatin organization, all become apparent long before the cortical release and the nuclear import of cell fates determinants. Thus, asymmetric nuclear remodeling during stem cell divisions may contribute to the generation of cellular diversity by initiating distinct transcriptional programs in sibling nuclei that contribute to later changes in daughter cell identity and fate., Competing Interests: Declarations of interests The authors declare no competing interests., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

The evolutionary path from protists to multicellular animals remains a mystery. Recent work on the genomes of several unicellular relatives of animals has shaped our understanding of the genetic changes that may have occurred in this transition. 1-3 However, the specific cellular modifications that took place to accommodate these changes remain unclear. To address this, we need to compare metazoan cells with those of their extant relatives, which are choanoflagellates, filastereans, ichthyosporeans, and corallochytreans/pluriformeans. Interestingly, these lineages display a range of developmental patterns potentially homologous to animal ones. Genetic tools have already been established in three of those lineages. 4-7 However, there are no genetic tools available for Corallochytrea. We here report the development of stable transfection in the corallochytrean Corallochytrium limacisporum. Using these tools, we discern previously unknown biological features of C. limacisporum. In particular, we identify two different paths for cell division-binary fission and coenocytic growth-that reveal a non-linear life cycle. Additionally, we found that C. limacisporum is binucleate for most of its life cycle, and that, contrary to what happens in most eukaryotes, nuclear division is decoupled from cellular division. Moreover, its actin cytoskeleton shares characteristics with both fungal and animal cells. The establishment of these tools in C. limacisporum fills an important gap in the unicellular relatives of animals, opening up new avenues of research to elucidate the specific cellular changes that occurred in the evolution of animals., Competing Interests: Declaration of interests The authors declare no competing interests, (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Normal mitotic spindle assembly is a prerequisite for faithful chromosome segregation and unperturbed cell-cycle progression. Precise functioning of the spindle machinery relies on conserved architectural features, such as focused poles, chromosome alignment at the metaphase plate, and proper spindle length. These morphological requirements can be achieved only within a compositionally distinct cytoplasm that results from cell-cycle-dependent regulation of specific protein levels and specific post-translational modifications. Here, we used cell-free extracts derived from Xenopus laevis eggs to recapitulate different phases of the cell cycle in vitro and to determine which components are required to render interphase cytoplasm spindle-assembly competent in the absence of protein translation. We found that addition of a nondegradable form of the master cell-cycle regulator cyclin B1 can indeed induce some biochemical and phenomenological characteristics of mitosis, but cyclin B1 alone is insufficient and actually deleterious at high levels for normal spindle assembly. In contrast, addition of a phosphomimetic form of the Greatwall-kinase effector Arpp19 with a specific concentration of nondegradable cyclin B1 rescued spindle bipolarity but resulted in larger-than-normal bipolar spindles with a misalignment of chromosomes. Both were corrected by the addition of exogenous Xkid (Xenopus homolog of human Kid/KIF22), indicating a role for this chromokinesin in regulating spindle length. These observations suggest that, of the many components degraded at mitotic exit and then replenished during the subsequent interphase, only a few are required to induce a cell-cycle transition that produces a spindle-assembly-competent cytoplasm., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Coordination of the cell division cycle with the growth of the cell is critical to achieve cell size homeostasis [1]. Mechanisms coupling the cell division cycle with cell growth have been described across diverse eukaryotic taxa [2-4], but little is known about how these processes are coordinated in organisms that undergo more complex life cycles, such as coenocytic growth. Coenocytes (multinucleate cells formed by sequential nuclear divisions without cytokinesis) are commonly found across the eukaryotic kingdom, including in animal and plant tissues and several lineages of unicellular eukaryotes [5]. Among the organisms that form coenocytes are ichthyosporeans, a lineage of unicellular holozoans that are of significant interest due to their phylogenetic placement as one of the closest relatives of animals [6]. Here, we characterize the coenocytic cell division cycle in the ichthyosporean Sphaeroforma arctica. We observe that, in laboratory conditions, S. arctica cells undergo a uniform and easily synchronizable coenocytic cell cycle, reaching up to 128 nuclei per cell before cellularization and release of daughter cells. Cycles of nuclear division occur synchronously within the coenocyte and in regular time intervals (11-12 hr). We find that the growth of cell volume is dependent on concentration of nutrients in the media; in contrast, the rate of nuclear division cycles is constant over a range of nutrient concentrations. Together, the results suggest that nuclear division cycles in the coenocytic growth of S. arctica are driven by a timer, which ensures periodic and synchronous nuclear cycles independent of the cell size and growth., (Copyright © 2018 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

The impact of age on a woman's ability to produce normal eggs remains a great enigma of human biology. A new paper provides intriguing experimental evidence that age may cause a breakdown in the egg cell division machinery., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

A new study shows that differences in the regulation of lipin can account for the different strategies of nuclear division in two closely related fission yeast species., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Eukaryotes remodel the nucleus during mitosis using a variety of mechanisms that differ in the timing and the extent of nuclear envelope (NE) breakdown. Here, we probe the principles enabling this functional diversity by exploiting the natural divergence in NE management strategies between the related fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus [1-3]. We show that inactivation of Ned1, the phosphatidic acid phosphatase of the lipin family, by CDK phosphorylation is both necessary and sufficient to promote NE expansion required for "closed" mitosis in S. pombe. In contrast, Ned1 is not regulated during division in S. japonicus, thus limiting membrane availability and necessitating NE breakage. Interspecies gene swaps result in phenotypically normal divisions with the S. japonicus lipin acquiring an S. pombe-like mitotic phosphorylation pattern. Our results provide experimental evidence for the mitotic regulation of phosphatidic acid flux and suggest that the regulatory networks governing lipin activity diverged in evolution to give rise to strikingly dissimilar mitotic programs., (Copyright © 2016 The Authors. Published by Elsevier Ltd.. All rights reserved.)

How do nuclear components, apart from chromosomes, partition equally to daughter nuclei during mitosis? In Schizosaccharomyces japonicus, the conserved LEM-domain nuclear envelope protein Man1 ensures the formation of identical daughter nuclei by coupling nuclear pore complexes to the segregating chromosomes., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Background: Current models of cell-cycle control, based on classic studies of fused cells, predict that nuclei in a shared cytoplasm respond to the same CDK activities to undergo synchronous cycling. However, synchrony is rarely observed in naturally occurring syncytia, such as the multinucleate fungus Ashbya gossypii. In this system, nuclei divide asynchronously, raising the question of how nuclear timing differences are maintained despite sharing a common milieu., Results: We observe that neighboring nuclei are highly variable in division-cycle duration and that neighbors repel one another to space apart and demarcate their own cytoplasmic territories. The size of these territories increases as a nucleus approaches mitosis and can influence cycling rates. This nonrandom nuclear spacing is regulated by microtubules and is required for nuclear asynchrony, as nuclei that transiently come in very close proximity will partially synchronize. Sister nuclei born of the same mitosis are generally not persistent neighbors over their lifetimes yet remarkably retain similar division cycle times. This indicates that nuclei carry a memory of their birth state that influences their division timing and supports that nuclei subdivide a common cytosol into functionally distinct yet mobile compartments., Conclusions: These findings support that nuclei use cytoplasmic microtubules to establish "cells within cells." Individual compartments appear to push against one another to compete for cytoplasmic territory and insulate the division cycle. This provides a mechanism by which syncytial nuclei can spatially organize cell-cycle signaling and suggests size control can act in a system without physical boundaries., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Condensins are conserved protein complexes that play integral roles in chromosome dynamics during mitosis and meiosis. Caenorhabditis elegans has been thought to be unusual in that it appeared to lack a typical condensin I complex. However, recent biochemical excavating in the nematode has unearthed the 'missing' condensin I complex as well as the worm homologs of long-lost canonical condensin subunits.

Background: Condensin complexes organize chromosome structure and facilitate chromosome segregation. Higher eukaryotes have two complexes, condensin I and condensin II, each essential for chromosome segregation. The nematode Caenorhabditis elegans was considered an exception, because it has a mitotic condensin II complex but appeared to lack mitotic condensin I. Instead, its condensin I-like complex (here called condensin I(DC)) dampens gene expression along hermaphrodite X chromosomes during dosage compensation., Results: Here we report the discovery of a third condensin complex, condensin I, in C. elegans. We identify new condensin subunits and show that each complex has a conserved five-subunit composition. Condensin I differs from condensin I(DC) by only a single subunit. Yet condensin I binds to autosomes and X chromosomes in both sexes to promote chromosome segregation, whereas condensin I(DC) binds specifically to X chromosomes in hermaphrodites to regulate transcript levels. Both condensin I and II promote chromosome segregation, but associate with different chromosomal regions during mitosis and meiosis. Unexpectedly, condensin I also localizes to regions of cohesion between meiotic chromosomes before their segregation., Conclusions: We demonstrate that condensin subunits in C. elegans form three complexes, one that functions in dosage compensation and two that function in mitosis and meiosis. These results highlight how the duplication and divergence of condensin subunits during evolution may facilitate their adaptation to specialized chromosomal roles and illustrate the versatility of condensins to function in both gene regulation and chromosome segregation.

Establishment of proper attachments between chromosomes and microtubules is essential for the accurate division of the genome. Two recent studies indicate that these attachments are facilitated by the geometry of chromosomes and the bipolar arrangement of spindle microtubules.

New research on lipid droplets in Drosophila embryos has led to the surprising conclusion that these poorly understood organelles have a novel role as a regulated storage depot of maternally supplied proteins, particularly histones.