Sorocarp

Abstract: Three classes of temperature-sensitive mutants of the cellular slime mold Dictyostelium discoideum have been isolated. One class contains strains able to grow at 22 C but not at 27 C. Cells of these strains can develop into sorocarps at both temperatures. Another class contains strains which can grow at both temperatures but can only develop at the lower temperature. The third class contains strains unable to grow or develop at 27 C. Those strains whose development is temperature-sensitive appear to carry mutations which affect the cells only during the period of aggregation before the construction of a multicellular sorocarp. When pairs of growth-temperature-sensitive (GTS) strains develop in mixed aggregates, temperature-resistant (TR) cells are formed at a frequency of about 10(-4). These TR cells transmit the phenotype in a relatively stable hereditary fashion. However, temperature-sensitive segregants can be isolated from TR strains even after six clonal passages. Mixed incubation of pairs of morphologically aberrant GTS strains was found to give rise to TR progeny which develop normally. These progeny clones independently segregate morphologically aberrant strains and temperature-sensitive strains. The results indicate that several temperature-sensitive and morphological mutations are recessive and nonidentical.

Abstract: SUMMARY: Changes in the structure of the myxamoebae involved in the formation of the stalk of the fruiting body, or sorocarp, of Dictyostelium discoideum were investigated. Observed changes in the ultrastructure relating to cell differentiation include: (1) food vacuoles disappear and autophagic vacuoles take their place after cell aggregation; (2) an inclusion in the mitochondria disappears during the vegetative feeding stages; (3) the decrease in number of an electron-clear vacuole is coincident with the appearance of the ‘prespore’ vacuole in cells of the hind portion of the migrating pseudoplasmodium; (4) while the prespore cells diminish in size and their cytoplasm becomes electron dense during culmination, the myxamoebae destined to form cellular elements of the stalk swell and their cytoplasm finally disintegrates. The slime coat that encircles the entire pseudoplasmodium remains after fixation and dehydration as a triple-layered membrane resembling cell membranes. At the start of culmination, the initial core of unwalled stalk-forming cells descends through the then vertically oriented cell mass. The polarization of cytoplasm in the horizontally oriented prestalk cells of the apical papilla is described, as well as the formation of the two-layered sheath around the stalk. The mass of so-called rearguard cells of the migrating pseudoplasmodium separates during culmination and a fraction of them form the basal disc while the remainder rise on the stalk just below the prespore mass.

Abstract: Extracts from Dictyostelium discoideum at the sorocarp stage of development catalyzed the degradation of acid-swollen cellulose to d-glucose at an optimal pH of about 4.5; cellobiose and cellotriose were the major products from this degradation at pH 7.0. The optimal temperature at pH 7.0, when swollen cellulose was used as substrate, was about 45 to 48 C, and was somewhat lower at pH 4.2 to 4.5, i.e., about 40 C. To account for this degradation pattern, two types of enzymes have been characterized, cellulase(s) and β-glucosidase. With carboxymethyl cellulose as substrate, cellulase activity was not found in extracts prior to the sorocarp stage of development, but increased rapidly during aging of the sorocarp. Evidence is presented that at earlier stages of development the cellulase(s) is present in an inactive form. An inhibitor found in these extracts is heat-labile and probably of protein nature. The β-glucosidase is present at all stages of development and the specific activity changes about fourfold, the highest activity occurring during the culmination and sorocarp stages of development.