The Dictyostelium cell cycle and its relationship to differentiation

Abstract

The dictyostelium vegetative cell cycle is characterized by a short mitotic period followed immediately by a short S-phase (less than 30 min) and a long and variable G2 phase. The cell cycle continues during differentiation despite a decrease in cell mass: DNA replication and mitosis occur early in development and also at the tipped aggregate stage. Cells that are in mitosis, S-phase or early G2, when starved differentiate into prestalk cells and cells that are in the middle of G2 differentiate into prespore cells. We postulate that there is a restriction point late in the G2 phase, about 1-2 h before mitosis, where the cells can be arrested either by starvation and the initiation of development, by growing into stationary phase, or by prolonged incubation at low temperature. During development, this block persists to the tipped aggregate stage, where it is specifically released in prespore cells, and these cells then go through one more round of cell division. Genes encoding components of the cell cycle machinery have recently been isolated and attempts to specifically block the cell cycle by reverse genetics to study the effects on differentiation have been initiated.

Tissue-specific G1-phase cell-cycle arrest prior to terminal differentiation inDictyostelium

“…In addition, dominant-negative cyclin B mutants that block the cell cycle in mitosis also block spore production (Q. Luo, PhD Thesis, The University of British Columbia, 1996) (Luo et al, 1994). However, these mutants also block the production of prespore cells, as has been pointed out for the cyclin B mutant (Weeks and Weijer, 1994). These genetic tests provide some evidence that mitosis is under control of the developmental program, but they leave unanswered the question of whether mitosis is strictly required for terminal cell differentiation.…”

“…The regulatory events governing cell cycle exit are becoming increasingly understood, as in the case of myocyte differentiation where inhibitors of cyclin-dependent kinases control the nature and pattern of muscle development in animals (Halevy et al, 1995;Zhang et al, 1999). It has been thought that Dictyostelium cells differ from most other organisms by carrying out terminal differentiation in G2 (Weeks and Weijer, 1994). However, the currently accepted model of cell cycle progression during Dictyostelium development has been challenged in recent years .…”

“…The currently accepted model posits that all cells that divide during development replicate their chromosomes after mitosis and undergo terminal differentiation as G2 cells (Weeks and Weijer, 1994). Flow cytometry studies have suggested that all the developmental cells have the same nuclear DNA content as G2 vegetative cells, implying that developmental cells are in the G2 phase Weijer et al, 1984a).…”

Myosin Heavy Chain Phosphorylation Sites Regulate Myosin Localization during Cytokinesis in Live Cells

“…It may involve the timed destruction of a ubiquitinated species, as has been postulated for exit from mitosis (King et al, 1994). However, given that Dictyostelium has a very short G 1 phase (Weeks and Weijer, 1994), it may also reflect the onset of activation of the G 1 cyclins.…”

Live imaging of theDictyosteliumcell cycle reveals widespread S phase during development, a G2 bias in spore differentiation and a premitotic checkpoint

“…The cell cycle status of differentiating cells is less clear. Several reports imply the decision to become stalk or spore is influenced by cycle phase before development, and that terminal differentiation occurs in G2 (Araki et al, 1994;Gomer and Firtel, 1987;MacWilliams et al, 2006;Maeda, 2005;McDonald and Durston, 1984;Weeks and Weijer, 1994;Weijer et al, 1984a;Weijer et al, 1984b). Cells grown on bacterial or glucose-free media as a food source are biased towards the stalk fate when mixed with cells grown in normal media (Leach et al, 1973;Thompson and Kay, 2000).…”