Flow cytometric determination of nuclear DNA content during differentiation (spherulation and germination) of the myxomycete Physarum polycephalum

Using flow cytometry, spherulating nuclei of Physarum isolated at the beginning of spherule wall formation were found to exhibit a DNA content corresponding to the G2 phase of the cell cycle, although 8% lower. Before the first mitosis after spherule germination, a very slight incorporation of 3H thymidine into DNA was observed that was too weak to correspond to S phase, strongly suggesting that nuclei are stopped in G2 phase inside the spherules. The lower value of nuclear DNA content found using flow cytometry of germinating spherules may not be related to DNA quantity, but may be due to a difference in chromatin organization during growth or spherulation, resulting in interference with the staining.