Cell shape, motility and distribution of F-actin in amoebae of the mycetozoans Protostelium mycophaga and Acrasis rosea. A comparison with Dictyostelium discoideum

Abstract

Summary We investigated, by a computer-assisted video analysis, by F-actin cytochemistry, and by scanning electron microscopy (SEM), the dynamic morphology, the locomotory behavior, and the actin cytoskeleton of trophic amoebae of the mycetozoans Protostelium mycophaga and Acrasis rosea, and compared them with amoebae of the cellular slime mold Dictyostelium discoideum. The three species represent protist taxa marked by different reproductive systems and by differences of amoeboid motility in the non-reproductive phase of the life cycle. Live observations and the numerical analysis of video recordings revealed characteristic traits of cell shape changes and motility in each species. The leading edge of Protostelium amoebae is dominated by a large lamellipodium that is studded with pseudodigits. We computed a mean cell size (outline area) of 498 μm2 and a mean velocity of 26.5 μm/min (centroid displacement) for this species (N = 30, as for the other organisms). Most amoebae followed a sinuous path during the observation period, which resulted in a mean standard deviation of vector direction (SDV) of 0.64, but some hardly moved at all. Amoebae of Acrasis are essentially monopodial, with lobose, smooth pseudopodia. Their mean cell size was 759 μm2 and their mean velocity 71.6 um/min. They are thus among the fastest “crawling” eucaryotic cells, which may be related to the fact that they contain no cytoplasmic microtubules. Most amoebae followed a straight path, but some made one or two abrupt turns during the observation period (SDV = 0.58). The corresponding values for Dictyostelium amoebae with filose or lobose pseudopodia were 220 μm2, 10.3 μm/min, and SDV = 0.80. In amoebae of all the three species F-actin was localized in the cell cortex, especially in hyaline pseudopodia. As seen by SEM, Protostelium amoebae had the largest area of contact with the substratum, whereas Acrasis amoebae adhered with a foot-like structure, the leading edge and the uropod being detached from the support.