HETEROTHALLISM AND HOMOTHALLISM IN TWO MYXOMYCETES

Collins , O'Neil Ray . (Queens Coll., New York City.) Heterothallism and homothallism in two Myxomycetes. Amer. Jour. Bot. 48(8): 674–683. Illus. 1961.—Single-spore studies of 2 Myxomycetes, Didymium iridis and Fuligo cinerea, revealed that the former is heterothallic and the latter is homothallic. In D. iridis, 256 single-spore isolations were made from sporangia which developed in mass-spore cultures. Of these, 101 germinated and 22 yielded plasmodia that later fructified in most cases. The remaining 79 single-spore cultures produced clones of myxamoebae and swarm cells only. When 18 of the 79 clones were mated in all possible combinations, plasmodia developed in a pattern which showed that the clones were either (+) or (–) with regard to mating type. Fructifications were readily obtained from these plasmodia. Fifty-three single spores of the F₁ generation were isolated. Of the 44 that germinated, 9 yielded plasmodia in monospore cultures, and 35 produced clones of myxamoebae and swarm cells only. Twenty-five of the F₁ clones were back-crossed with their parents. Results of the back crosses show that each F₁ clone is capable of yielding plasmodia with either the (+) or the (–) parent, never with both. When 14 of the F₁ clones were mated among themselves, a (+) and (–) mating type system was again revealed. Most of the 22 original single-spore cultures which produced plasmodia, later formed sporangia. From these sporangia, 88 spores were isolated. Seventy-two of these germinated and yielded large populations of swarm cells and myxamoebae, but none produced plasmodia. Twenty of the 72 clones were then mated among themselves. Some matings resulted in plasmodial formation, but the pattern was difficult to interpret. However, when these 20 clones were mated with known (+) and (–) clones, the results appear to be in keeping with a (+) and (–) mating type system. In F. cinerea, 219 single spores were isolated from aethalia derived from mass-spore cultures. Of these, 144 germinated and the same number yielded plasmodia. Fructifications were easily obtained from such plasmodia. Thirty-five second-generation single spores were isolated, of which 15 germinated and 15 yielded plasmodia. These results indicate that F. cinerea is homothallic.     

Site of Fluoride Accumulation in Navel Orange Leaves

Fluoride-polluted navel orange leaves, Citrus sinensis (Linn.) Osbeck, were fractionated into the subcellular components in hexane/carbon tetrachloride mixtures having various densities. Fluoride was determined at each fraction. Analyses were also made for the subcellular distribution of chlorophyll, nitrogen, and DNA to assess the extent of cross-contamination of each component.The fraction containing cell wall, nuclei, and partly broken cells apparently contained a major amount of fluoride. However, if allowance was made for the cross-contamination of chloroplasts and chloroplast fragments, the fraction of chloroplasts was found to be the site of the highest fluoride accumulation. When each particulate component was washed with water after drying, the combined washings contained more than 50% of the total fluoride of the isolated fractions.The usual method of subcellular fractionation with aqueous solvent shifted the major site of fluoride accumulation from the fraction of chloroplasts to that of the supernatant.     

Aspects of sieve element ultrastructure in Primula obconica

Summary At maturity, the enucleate sieve element of Primula obconica is lined with a parietal layer of cytoplasm consisting of plasmalemma, one or more cisterna-like layers of endoplasmic reticulum, numerous mitochondria and plastids, and a membrane which apparently separates these cytoplasmic components from a large central cavity. The central cavity contains numerous longitudinally oriented slime tubules. We believe these tubules normally form strands which run the length of the cell and traverse consecutive cells through the sieve-plate pores. Developmental aspects are discussed.This research has been supported by NSF Grant GB 3193.     

Terminal Regions Confer Plasticity to the Tetrameric Assembly of Human HspB2 and HspB3

Heterogeneity in small heat shock proteins (sHsps) spans multiple spatiotemporal regimes—from fast fluctuations of part of the protein, to conformational variability of tertiary structure, plasticity of the interfaces, and polydispersity of the inter-converting, and co-assembling oligomers. This heterogeneity and dynamic nature of sHsps has significantly hindered their structural characterization. Atomic coordinates are particularly lacking for vertebrate sHsps, where most available structures are of extensively truncated homomers. sHsps play important roles in maintaining protein levels in the cell and therefore in organismal health and disease. HspB2 and HspB3 are vertebrate sHsps that are found co-assembled in neuromuscular cells, and variants thereof are associated with disease. Here, we present the structure of human HspB2/B3, which crystallized as a hetero-tetramer in a 3:1 ratio. In the HspB2/B3 tetramer, the four α-crystallin domains (ACDs) assemble into a flattened tetrahedron which is pierced by two non-intersecting approximate dyads. Assembly is mediated by flexible “nuts and bolts” involving IXI/V motifs from terminal regions filling ACD pockets. Parts of the N-terminal region bind in an unfolded conformation into the anti-parallel shared ACD dimer grooves. Tracts of the terminal regions are not resolved, most likely due to their disorder in the crystal lattice. This first structure of a full-length human sHsp heteromer reveals the heterogeneous interactions of the terminal regions and suggests a plasticity that is important for the cytoprotective functions of sHsps.     

Progesterone-induced Acrosome Exocytosis Requires Sequential Involvement of Calcium-independent Phospholipase A2β (iPLA2β) and Group X Secreted Phospholipase A2 (sPLA2)

Phospholipase A₂ (PLA₂) activity has been shown to be involved in the sperm acrosome reaction (AR), but the molecular identity of PLA₂ isoforms has remained elusive. Here, we have tested the role of two intracellular (iPLA₂β and cytosolic PLA₂α) and one secreted (group X) PLA₂s in spontaneous and progesterone (P4)-induced AR by using a set of specific inhibitors and knock-out mice. iPLA₂β is critical for spontaneous AR, whereas both iPLA₂β and group X secreted PLA₂ are involved in P4-induced AR. Cytosolic PLA₂α is dispensable in both types of AR. P4-induced AR spreads over 30 min in the mouse, and kinetic analyses suggest the presence of different sperm subpopulations, using distinct PLA₂ pathways to achieve AR. At low P4 concentration (2 μm), sperm undergoing early AR (0–5 min post-P4) rely on iPLA₂β, whereas sperm undergoing late AR (20–30 min post-P4) rely on group X secreted PLA₂. Moreover, the role of PLA₂s in AR depends on P4 concentration, with the PLA₂s being key actors at low physiological P4 concentrations (≤2 μm) but not at higher P4 concentrations (∼10 μm).