Mitotic cell cycle control in Physarum. Unprecedented insights via flow-cytometry

Area of Science:

• Cell Biology
• Molecular Biology
• Mycology

Background:

• The regulation of nuclear division and DNA replication in eukaryotic cells is complex.
• Physarum polycephalum, a myxomycete, offers a unique model for studying cell cycle dynamics due to its multinucleated macroplasmodium.

Purpose of the Study:

• To investigate the synchrony of DNA replication and mitosis in Physarum polycephalum.
• To explore the factors regulating the cell cycle in the presence of mixoploidy.
• To provide experimental evidence for cell cycle control models.

Main Methods:

• High-resolution flow cytometry of isolated macroplasmodial nuclei.
• Analysis of DNA replication and progression through G2 phase.
• Interference with DNA and protein synthesis using cycloheximide.

Main Results:

• Nuclear division and DNA replication remain strictly synchronous in Physarum polycephalum, irrespective of genome size variations (mixoploidy).
• A constant S phase duration is maintained through differential regulation of replication rates.
• Evidence for a brief G1 phase and a protein synthesis-dependent transition point initiating DNA synthesis was found.

Conclusions:

• The findings challenge existing models of cell cycle regulation based solely on nuclear size or nuclear/cytoplasmic ratios.
• The study strongly supports the transition point concept for cell cycle control.
• Results are consistent with the replicon-set hypothesis in Physarum.