Decoupling of Nuclear Division Cycles and Cell Size during the Coenocytic Growth of the Ichthyosporean Sphaeroforma arctica

Abstract

SummaryCoordination 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, 3, 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.

A unicellular relative of animals generates a layer of polarized cells by actomyosin-dependent cellularization

“…Individual coenocytes exhibit growth in cell size until approximately 60 hr, after which they undergo release of newborn cells (Figure 1B, Video 1). This was consistent with previous results in bulk cultures (Ondracka et al, 2018)⁠, confirming that our experimental setup for long-term live imaging faithfully reproduces culture growth. However, by measuring the diameter of the coenocytes, we observed that newborn cell release occurred with somewhat variable timing (Figure 1—figure supplement 1A, Video 1).…”

“…This suggests two possible evolutionary scenarios. First, given the widespread presence of coenocytes in opisthokonts, we have suggested that the common ancestor of opisthokonts already had a coenocytic life cycle (Ondracka et al, 2018). In this scenario, cellularization of the coenocyte by this conserved actomyosin mechanism would have already been present in this common ancestor.…”

“…Sphaeroforma arctica cultures were grown and synchronized as described previously (Ondracka et al, 2018)⁠. Briefly, cultures were grown in Marine Broth (Difco BD, NJ, USA; 37.4 g/L) in flasks at 12°C until saturation, producing small cells.…”

“…All characterized ichthyosporeans proliferate through rounds of nuclear divisions within a cell-walled coenocyte, followed by release of newborn cells (Marshall and Berbee, 2011; Marshall and Berbee, 2013; Suga and Ruiz-Trillo, 2013; Whisler, 1968)⁠. We have previously suggested that they undergo cellularization (Ondracka et al, 2018; Suga and Ruiz-Trillo, 2013)⁠. However, at present nothing is known about the ichthyosporean cellularization, and whether it involves animal-like mechanisms.…”

“…Here, we comprehensively characterized cellularization in the ichthyosporean Sphaeroforma arctica, in which we have previously shown that coenocytic cycles can be synchronized (Ondracka et al, 2018)⁠. We used imaging, transcriptional profiling, and pharmacological inhibition, to study the gene expression dynamics, morphological rearrangements, and mechanisms of cellularization.…”