Distribution of intramembranous particles in the plasmalemma of Physarum polycephalum during sclerotization, spore-formation and spore-germination

Abstract

The distribution and density of intramembranous particles (IMP) in the plasma membrane of the acellular slime mold Physarum polycephalum during sclerotization, sporulation and germination was investigated by means of freeze-etching. The total number of IMP on both the PF and EF of young as well as dry spores (2,100–2,200 IMP/μm 2), wetted spores (1,550 IMP/μm 2) and sclerotia (2,300 IMP/μm 2) is remarkably reduced in comparison to the IMP-density in the plasma membrane of migrating plasmodia (3,680 IMP/μm 2). The most obvious decrease in IMP-density can be observed in EF, i. e., the asymmetry of the cell membrane as expressed by the PF: EF ratio shifts from about 2.5 : 1 to 10 : 1. The cell surface of spores and sclerotia shows a regular pattern of elevations on the PF and corresponding indentations on the EF. On the PF the elevations display a distinct accumulation of IMP. The significance of the fine structural changes of the cell membrane during sclerotization, sporulation and the early stage of spore germination is discussed with regard to altered physiological conditions.