Molecular configuration of amylose and its complexes in aqueous solutions. Part II. Relation between the DP of helical segments of the amylose–iodine complex and the equilibrium concentration of free iodine

The iodine which is added to an aqueous amylose solution is bound only partly by the amylose while forming the blue complex and partly remains free. The equilibrium normality of the free and the bound iodine at half-saturation of amylose by iodine is designated as [I_f]_v and [I_b]_w, respectively. The stability of the poly iodine chain formed within the axis of amylose helices depends on its length, i.e., indirectly on the DP of the amylose helices: the greater this stability, the lower the [I_f]_v value. The amylose molecule consists of helical segments. Such a molecule may behave as a random coil. The average length of the helical segments in freshly prepared amylose-iodine complexes depends on temperature, pH, iodide concentration, the presence of other complex-forming agents, and the DP of the amylose. This latter factor is investigated in the present paper. By the aid of an automatically recording photometrictitrating device the coherent values of [I_b] and [I_f] were determined. Plotting these values against DP _n for mechanochemically degraded as well as for periodateo-xidized amyloses resulted in curves consisting of two linear sections. The break of the curves occurred between DP _n 110 and 130. It was concluded that below DP _n = 100 the DP of helical segments (= sDP _n) is identical to the DP _n of the total molecule, i.e., the molecule consists of only a single, relatively stiff helix. Above this limit the molecule contains several helical segments. The DP of these helical segments can be calculated as follows: sDP _n = 141.1 ? 10.2 × 10⁵[I_f]_v. This equation is considered to be valid for 0.5–0.6 mg. amylose in 100 ml. 0.1N HCl at 20°C., λ = 650 mμ, euuvet diameter 3.4 cm., the feed rate of the iodate-iodide titrating solution (in acid medium resulting in a 5 × 10^?3N I₂ solution with a molar iodide to iodine ratio of 1.5) is 0.4ml./min. Amylose molecules of, e.g., DP ⁿ = 1380 consist of an average of 11.4 segments having a DP of about 120 and consisting of an average of 15–18 helical turns.     

Non‐GMO potato lines,synthesizing increased amylose and resistant starch,are mainly deficient in isoamylase debranching enzyme

Solanum tuberosum potato lines with high amylose content were generated by crossing with the wild potato species Solanum sandemanii followed by repeated backcrossing to Solanum tuberosum lines. The trait, termed increased amylose (IAm), was recessive and present after three generations of backcrossing into S. tuberosum lines (6.25% S. sandemanii genes). The tubers of these lines were small, elongated and irregular with small and misshaped starch granules and high sugar content. Additional backcrossing resulted in less irregular tuber morphology, increased starch content (4.3%–9.5%) and increased amylose content (29%–37.9%) but indifferent sugar content. The amylose in the IAm starch granules was mainly located in peripheral spots, and large cavities were found in the granules. Starch pasting was suppressed, and the digestion‐resistant starch (RS) content was increased. Comprehensive microarray polymer profiling (CoMPP) analysis revealed specific alterations of major pectic and glycoprotein cell wall components. This complex phenotype led us to search for candidate IAm genes exploiting its recessive trait. Hence, we sequenced genomic DNA of a pool of IAm lines, identified SNPs genome wide against the draft genome sequence of potato and searched for regions of decreased heterozygosity. Three regions, located on chromosomes 3, 7 and 10, respectively, displayed markedly less heterozygosity than average. The only credible starch metabolism‐related gene found in these regions encoded the isoamylase‐type debranching enzyme Stisa1. Decreased expression of mRNA (>500 fold) and reduced enzyme activity (virtually absent from IAm lines) supported Stisa1 as a candidate gene for IAm.     

Helical conformation of amylose in aqueous solution. I. Viscosity measurements

Two amylose samples, amylose V (DP_w = 2300) and amylose HE 15 (a low-substituted hydroxyethylamylose, DP_W = 1600) were studied. The intrinsic viscosity of the polymers in aqueous solution was measured with regard to its dependence on the alkalinity (0 to 5MNaOH), the ionic strength (0 to 5M), and the temperature (0 to 75°C). Additionally, the temperature dependence of the viscosity of the amylose-iodine complex was measured. It was found that the two amylose samples show the same dependence on the studied parameters. Therefore, it was concluded that the conformation is unchanged by the hydroxyethylation in the present case. In a discussion, steric, hydrodynamic, and thermodynamic data on amylose in solution are compared with the corresponding data on cellulose and dextran. The comparison leads to the conclusion that amylose in a neutral solution must have a helical conformation, corresponding to the well-accepted rod conformation of cellulose. The helical conformation also explains several viritual anomalies in the behavior of amylose. The results of the experiments support the helix model for amylose. The conclusion of the whole work, therefore, is that the amylose molecule in neutral aqueous solution can be regarded as a random coil, built up by helical segments. The average number of monomers per segment exceeds 100. This value decreases with increasing alkalinity.     

Molecular cloning of cDNA from foot-and-mouth disease virus C1-Santa Pau (C-S8). Sequence of protein-VP1-coding segment

cDNA segments copied from the RNA of foot-and-mouth disease virus (FMDV) C₁-Santa Pau (isolate C-S8) have been cloned in plasmid pBR322. A 998-bp DNA fragment, that includes the region coding for capsid protein VP1, the carboxy terminus of VP3, and the amino terminus of precursor protein p52 has been sequenced. Comparison of the nucleotide sequence with those from FMDV O₁K, A₁₀61, a₁₂ and C₃ Indaial (Kurz et al., Nucl. Acids Res. 9 (1981) 1919–1931; Kleid et al., Science 214 (1981) 1125–1129; Boothroyd et al., Gene 17 (1982) 153–161; Makoff et al., Nucl. Acids Res. 10 (1982) 8285–8295) indicates extensive variability between the corresponding gene segments, including short insertions and deletions. Base transversions are more frequent than transitions within the VP1 coding segment, but not in the sequence coding for the amino-terminal end of p52. The nucleotide sequence divergence is reflected in variability in both the primary and the predicted higher-order structures of the encoded VP1s.     

Stabilization of the H,K-ATPase M5M6 membrane hairpin by K+ ions. Mechanistic significance for p2-type atpases.

The integral membrane protein, the gastric H,K-ATPase, is an alpha-beta heterodimer, with 10 putative transmembrane segments in the alpha-subunit and one such segment in the beta-subunit. All transmembrane segments remain within the membrane domain following trypsinization of the intact gastric H,K-ATPase in the presence of K+ ions, identified as M1M2, M3M4, M5M6, and M7, M8, M9, and M10. Removal of K+ ions from this digested preparation results in the selective loss of the M5M6 hairpin from the membrane. The release of the M5M6 fragment is directed to the extracellular phase as evidenced by the accumulation of the released M5M6 hairpin inside the sealed inside out vesicles. The stabilization of the M5M6 hairpin in the membrane phase by the transported cation as well as loss to the aqueous phase in the absence of the transported cation has been previously observed for another P2-type ATPase, the Na, K-ATPase (Lutsenko, S., Anderko, R., and Kaplan, J. H. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7936-7940). Thus, the effects of the counter-transported cation on retention of the M5M6 segment in the membrane as compared with the other membrane pairs may be a general feature of P2-ATPase ion pumps, reflecting a flexibility of this region that relates to the mechanism of transport.     

UV-induced interstrand cross-linking of d(GT)n.d(CA)n is facilitated by a structural transition

Photochemical alterations following ultraviolet irradiation of the alternating copolymer d(GT)n.d(CA)n were studied. We found that in solution conditions which produced circular dichroism spectra compatible with B-form or A-form DNA, no interstrand cross-linking or photoproduct formation could be demonstrated. Zimmer et al. (Zimmer, C., Tymen, S., Marck, C., and Guschlbaumer, W. (1982) Nucleic Acids Res. 10, 1081-1091) and Vorlickova et al. (Vorlickova, M., Kypr, J., Sotkrova, S., Sponar, J. (1982) Nucleic Acids Res. 10, 1071-1080) have reported a number of solution conditions which produce a structural transition of this polymer characterized by a negative deviation of the circular dichroism spectrum in the region of 280 nm. The nature of this transition has not yet been elucidated. Following ultraviolet irradiation of d(GT)n.d(CA)n under two conditions which produce this transition (manganese solution or ethanol plus trace salts solution) we found ultraviolet dose-dependent interstrand cross-linking as well as dose-dependent formation of thymine-containing photoproduct. Interstrand cross-linking is demonstrated by two criteria: increase in polymer size as detected by alkaline agarose gel electrophoresis, and generation of intermediate density material in alkaline cesium sulfate isopycnic gradients. The thymine-containing photo-product was demonstrated by thin layer chromatography of acid hydrolysates of the polymer. The photo-product is at least partially photoreversible. These findings suggest that the geometry of the alternative conformation is such that pyrimidines from different strands are closely approximated, allowing for photodimerization.     

Morphological changes during postembryonic development in two species of neotropical harvestmen (Opiliones,Laniatores, Cranaidae)

Morphological changes during postembryonic development in the Cranaidae are described on the basis of the examination of an incomplete series of larvae, nymphs, and adults of Phareicranaus calcariferus and Santinezia serratotibialis. The life histories of these species are hypothesized to consist of six nymphal stages, featuring the appearance of secondary male sexual characteristics in the antepenultimate nymph (N5). Color and body shape change dramatically during development. Growth rates for nymphs based upon leg measurements were similar for both species. In S. serratotibialis, the greatest increase in leg size occurred from larva to 1st nymph. The tarsomeres of legs I–IV varied by 1–2 segments per leg for each nymph stage, with the number of tarsal segments increased by 1–2 segments at each stage. Adults had nearly twice as many tarsomeres on leg II than other legs. Ontogenetic changes were observed in the armature of the proximal cheliceral segment, ocularium, pedipalp, opisthosoma, distitarsus III and IV, and leg IV. Morphological changes in postembryonic development in cranaid harvestmen are similar to those reported for other Laniatores. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

The segments of influenza viral mRNA.

Influenza viral mRNA, i.e., complementary RNA (cRNA), isolated from infected cells , was resolved into six different species by electrophoresis in 2.1% acrylamide gels containing 6 M urea. The cRNA''s were grouped into three size classes: L (large), M (medium-size), and S (small). Similarly, when gels were sliced for analysis, the virion RNA (vRNA) also distributed into six peaks because the three largest vRNA segments were closely spaced and were resolved only when the gels were autoradiographed or stained. Because of their attached polyadenylic acid [poly(A)]sequences, the cRNA segments migrated more slowly than did the corresponding vRNA segments during gel electrophoresis. After removal of the poly(A) by RNase H, the cRNA and vRNA segments comigrated, indicating that they were approximately the same size. One of the cRNA segments, S2, was shown by annealing to contain the genetic information in the vRNA segment with which it comigrated, strongly suggesting that each cRNA segment was transcribed from the vRNA segment of the same size. In contrast to the vRNA segments, which when isolated from virions were present in approximately 1:1 molar ratios, the segments of the isolated cRNA were present in unequal amounts, with the segments M2 and S2 predominating, suggesting that different amounts of the cRNA segments were synthesized in the infected cell. The predominant cRNA segments, M2 and S2, and also the S1 segment, were active as mRNA''s in wheat germ extracts. The M2 cRNA was the mRNA for the nucleocapsid protein; S1 for the membrane protein; and S2 for the nonstructural protein NS1.     

Kinetic studies of the complex formation in the ternary system of amylose,SDS, and iodine

Complex formation in the ternary system of amylose (degree of polymerization, DP, 1100), SDS, and iodine was studied statically by spectrophotometry and amperometric titration and kinetically by the pressure-jump method. It was clarified that (1) iodine (I₃^?) to some extent binds to amylose saturated with SDS to form an inclusion complex (ASI system); (2) the binding of SDS apparently transforms amylose of DP 1100 to that of much lower DP (less than 60) from the viewpoint of iodine binding; and (3) iodine binds to sites unoccupied by SDS in the center of the helical segment of amylose. Pressure-jump relaxation phenomenon was not observed in solutions in which iodine was dissolved prior to SDS (AIS system), but it was observed in the ASI system; it is ascribed to the association and dissociation of three molecules of iodine in the center of the amylose helix. Comparison of the rate constants in the ASI system with those in the amylose (DP 32) and iodine system indicates that iodine runs to and from the helical segment of amylose perpendicularly to the axial plane in the former, while it runs horizontally in the latter. We discuss the order of ligand mixing on the resulting structure of the ternary complexes of amylose, SDS, and iodine.     

A differential scanning calorimetric study of the thermal unfolding of mutant forms of phage T4 lysozyme.

In continuation of our earlier work on the effects of amino acid replacements on the thermodynamics of the thermal unfolding of T4 lysozyme [Kitamura, S., & Sturtevant, J. M. (1989) Biochemistry 28, 3788-3792; Connelly, P., Ghosaini, L., Hu, C.-Q., Kitamura, S., Tanaka, A., & Sturtevant, J. M. (1991) Biochemistry 30, 1887-1891; Hu, C.-Q., Kitamura, S., Tanaka, A., & Sturtevant, J. M. (1992) Biochemistry 31, 1643-1647], we report here a study by differential scanning calorimetry of the effects of five replacements at Ile3. Four of these replacements, those with Glu, Phe, Pro, and Thr, caused apparent destabilizations, while the replacement by Leu led to a small apparent stabilization. The largest observed destabilization (Ile3Pro) amounted to -3.0 kcal mol-1 in free energy at pH 2.00 and 38.8 degrees C (the denaturational temperature of the wild-type protein at this pH), and the largest stabilization amounted to +1.2 kcal mol-1 at pH 3.00 and 53.6 degrees C.     

Proton translocating ATPase in lysosomal membrane ghosts. Evidence that alkaline Mg2+-ATPase acts as a proton pump

Membrane ghosts were prepared from purified lysosomes (tritosomes) of rat liver by hypo-osmotic treatment. Mg2+-ATP-driven acidification was observed in the membrane ghosts using acridine orange as a fluorescent probe of the transmembrane pH gradient (delta pH). Its properties were the same as those of intact lysosomes reported previously (Ohkuma, S., Moriyama, Y., & Takano, T. (1982) Proc. Natl. Acad. Sci. U.S. 79, 2758-2762; Moriyama, Y., Takano, T., & Ohkuma, S. (1982) J. Biochem. 92, 1333-1336). The H+-pump was found to be electrogenic with use of bis(3-phenyl-5-oxoisoxasol-4-yl)pentamethine oxonol as a fluorescent membrane potential probe. Alkaline Mg2+-ATPase activity was also identified on the membranes. It showed a pH maximum of pH 8.0-8.5, a Km value for ATP of 0.36 mM and a Vmax of 0.41 units/mg protein at 30 degrees C. Its activity was inhibited by dicyclohexylcarbodiimide, tri-n-butyltin, azide and ADP, but not by ouabain or vanadate. It differed from mitochondrial F1F0-ATPase in sensitivities to N-ethylmaleimide, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, quercetin, and oligomycin. Since this alkaline Mg2+-ATPase activity is very similar to the H+-pump activity in its requirement for divalent cations, substrate specificity and sensitivities to various chemicals, it may act as a proton translocase (H+-pump). Possible mechanisms of action of some chemicals, such as 4-acetamide-4'-isothiocyanatostilbene-2,2'-disulfonic acid, that inhibited the H+-pump but not the alkaline Mg2+-ATPase, are discussed.     

Gastropod evolutionary rates and phylogenetic relationships assessed using partial 28S rDNA and histone H3 sequences

Sequence data for two segments of 28S and Histone H3 from 36 gastropod taxa, a chiton, two bivalves and Nautilus are used to test recently published morphology‐based phylogenetic hypotheses of gastropod relationships. Statistical results suggest that the accuracy of the available hypotheses could be improved. The data support the monophyly of the Patellogastropoda (true limpets), Euthyneura and the ‘higher’ vetigastropods and the polyphyly of the ‘Cocculiniformia’. The division of the gastropods into two major clades (Eogastropoda and Orthogastropoda) as has been proposed on morphological grounds is not supported, and neither the Caenogastropoda nor Heterobranchia is well supported. Within the Euthyneura, opisthobranchs are paraphyletic with respect to the pulmonates. The hot vent taxon, Depressigyra, groups with the lower vetigastropod Pleurotomaria in some analyses. Much of the variability in the 28S rDNA segments lies in discrete areas of the sequence. Forone of the segments, corresponding to positions 691–942 of the mosquito Aedes albopictus 28S sequence, the variable regions represent known expansion regions (D4 and D5). For the other segment, corresponding to positions 2259–2538 of the A. albopictus sequence, the variable area, which is found in the patellogastropods, vetigastropods and Nautilus, represents an unreported expansion region. The data show marked variability in the rate of evolution in both segments of the 28S rDNA, whether or not the expansion regions are included. The variability is largely clade specific. Rates are high in the patellogastropods, vetigastropods, the lower heterobranch ‘Heterostropha’ (Cornirostra and Philippea), Depressigyra and the deep sea cocculinid limpet Coccopigya and substantially lower in other taxa. Rate variation in the histone H3 data is less extreme. The correlation between evolutionary rates in the two 28S rDNA segments is very high, andis also significant for the the pairing of each of the 28S rDNA segments with H3. The rate variability may be due to differential selection but no causative factor has been identified. The histone H3 data have high codon usage bias. For all amino acids encoded by multiple codons, at least some triplets occur at a frequency of less than a quarter of their expected usage. For all three‐, four‐and sixfold degenerate amino acids, the most abundant triplet occurs at least twice as frequently as expected. Despite the usage bias, there is a large amount of apparent homoplasy in synonymous alternatives at both the first and third codon positions.     

The Interface Between an Alternating CG Motif and a Random Sequence Motif Displays Altered Nuclease Activity

Abstract

Previously we described the B-Z junctions produced in oligomers containing (5meCG)₄ segments in the presence of 5.0 M NaCl or 50 uM Co(NH₃)₆ ⁺³ [Sheardy, R.D. & Winkle, S.A., Biochemistry 28, 720–725 (1989); Winkle, S. A., Aloyo, M.C., Morales, N., Zambrano, T.Y. & Sheardy, R.D., Biochemistry 30, 10601–10606 (1991)]. The circular dichroism spectra of an analogous unmethylated oligomer containing (CG)₄, termed BZ-IV, in 5.0 M NaCl and in 50 uM CO(NH₃)⁺³ suggest, however, that this oligomer does not form a B-Z hybrid. BZ-IV possesses Hha I sites (CGCG) in the (CG)₄ segment and an Mbo I site (GATC) at the terminus of the (CG)₄ segment BZ-IV is equally digestible in the presence and absence of cobalt hexamine by Hha I, further indicating that the structure of BZ-IV is fully B-like under these conditions. The Mbo I cleavage site at the juncture between the (CG)₄ segment and the adjacent random segment displays enhanced cleavage by both Mbo I and its isoschizomer Sau3A I in the presence of cobalt hexamine. In addition, exonuclease IH digestion of BZ-IV is inhibited at this juncture. Actinomycin inhibits Mbo I activity in the presence of cobalt hexamine but not in the absence. Together, these results suggest that enzymes recognize the interfaces of (CG)n and adjacent random sequences as altered substrates even in the absence of a B-Z junction formation.     

KARYOTYPE ANALYSIS IN SOME GENERA OF COMPOSITAE. VIII. FURTHER STUDIES ON THE CHROMOSOMES OF ASTER

Huziwara , Y. (Kobe U., Mikage, Kobe, Japan.) Karyotype analysis in some genera of Compositae. VIII. Further studies on the chromosomes of Aster. Amer. Jour. Bot. 49 (2) : 116–119. Illus. 1962.—The karyotypes of 3 Asiatic and 3 American taxa of Aster are reported here for the first time. These are: A. ageratoides subsp. sugimotoi (Kitamura) Kitamura 2n = 36, A. ageratoides subsp. (Taxon AIII) 2n = 72, A. himalaicus C. B. Clarke 2n = 18, A. ericoides L. 2n = 32, A. meritus A. Nels. 2n = 27, A. umbellatus Mill. 2n = 18. Two American taxa, namely, A. meritus and A. umbellatus, are considered to be ancestral taxa which have retained primitive karyotypes similar to those of Asiatic species of Aster.     

Molecular Phylogeny of an Indian Population of Kleinstyla dorsicirrata (Foissner, 1982) Foissner et al., 2002. comb. nov. (Hypotrichia,Oxytrichidae): an Oxytrichid with Incomplete Dorsal Kinety Fragmentation

Kleinstyla dorsicirrata (Foissner, 1982) Foissner et al., 2002. comb. nov. (basionym: Gastrostyla dorsicirrata) is a slightly flexible oxytrichid, measuring about 88–115 × 27–46 μm in life and possesses cortical granules. Kleinstyla dorsicirrata is the only oxytrichid known so far with incompletely fragmented dorsal kinety. Morphological and morphogenetic data recognise K. dorsicirrata as nonstylonychine oxytrichid. Molecular phylogeny of an Indian population was inferred using 18S rRNA gene sequences and was examined with respect to oxytrichids exhibiting variation in dorsal kinety fragmentation. Kleinstyla dorsicirrata clusters with Oxytricha lanceolata; this proximity is quite significant as both show deviation from typical oxytrichid fragmentation of dorsal kinety. Molecular phylogeny of Indian population confirms its nonstylonychine oxytrichid status.     

Investigation of the effect of amylose/iodine complexation on the conformation of amylose in aqueous solution

Using a potato amylose fraction of 8 × 10⁵, molecular-weight viscosity studies were carried out at 25°C on solutions containing 0.176–0.042% polymer, 8.67 mM KI, 1% ethanol, and different concentrations of iodine. By a novel extrapolation method, the intrinsic viscosities of the amylose/iodine complex were determined under various conditions of iodine binding (0–0.133 g I₂/g amylose). Contrary to the view long held in this research area, it was found that the intrinsic viscosity of amylose solutions decreases significantly upon complex formation with iodine. Taking into account the results of our previous kinetic studies, the present findings are interpreted in terms of an amylose model characterized by loose, extended helical regions which are interrupted by short disordered regions. It is proposed that the intrinsic viscosity decrease observed is due to a shortening of the linear dimension of the polymer chain. This conformation change is apparently caused by the contraction of loose helical regions of the amylose macromolecule due to the entrapment of iodine (and perhaps other) atoms inside the helical cavities.     

Histochemical distribution of digestive enzymes in intestine of goldline, Sarpa salpa L. 1758

Histochemical localization of non‐specific esterase, alkaline and acid phosphatase in the intestine of free‐living goldline (Sarpa salpa L. 1758) was investigated. Fish were caught in the vicinity of the town of Zadar (Adriatic Sea, Croatia), and samples of three parts of the intestine proper (anterior, middle and posterior) as well as the rectum were used for presentation of non‐specific esterases, alkaline phosphatase and acid phosphatase. Non‐specific esterase activity was found in the cytoplasm and brush border of enterocytes in all investigated intestinal segments and the rectum. The activity was stronger in the middle and posterior part of the intestine but weaker in the anterior segment of the intestine as well as in the rectum. Intestinal alkaline phosphatase was detected in the brush border and supranuclear cytoplasm of enterocytes of all investigated intestinal segments. Enzymatic activity gradually decreased in a posterior direction. Acid phosphatase activity was observed as a fine granular reaction product in the supranuclear region of enterocytes and was almost equal in all investigated intestinal segments as well as in the rectum. The possible role of enzymes in intracellular digestion and transport is discussed.     

Population biology of the clonal moss Hylocomium splendens in Norwegian boreal spruce forests. 5. Vertical dynamics of individual shoot segments

Demographic information was obtained for 12 528 mature segments (for which dry weight was estimated and vertical position in the bryophyte carpet recorded) and 3109 regenerated growing points for the perennial clonal moss Hylocomium splendens, recorded in Norwegian boreal spruce forests during a 6‐year period. Branching frequency varied with vertical position in the bryophyte carpet. Termination risk (probability of producing no offspring) was highest (44%) for buried segments, lowest (12%) for segments at intermediate vertical positions, and also high (26%) for emergent segments (due to increasing exposure to external mortality agents). Segment size increased from low levels in the bryophyte carpet to a maximum ca 2–10 mm below the top of the bryophyte carpet. This intermediate level was interpreted as the optimal compromise between incoming radiation (attenuating downwards) and microclimatic moisture conditions (improving downwards). Size‐corrected fitness, the number of offspring emerging from a mature segment within one year after maturation after allowance for differences in size, was lower for buried and emergent segments than for segments at intermediate positions. Small emergent segments were apparently liable to suffer from vitality reductions due to desiccation. The vertical position of a daughter segment depended on that of its parent segment, but also showed considerable stochastic variation. Burial acted as a strong sink for small segments regardless of vertical position. No evidence was found for species‐specific differences in the way pleurocarpous bryophytes interact, but reduced vertical mobility of H. splendens when growing among acrocarps indicated that growth‐form is an important determinant of bryophyte interactions. Evidence was found for vertical layering of the bryophyte carpet according to dominant type of interactions among individuals: none (environmental stress) above and at top, facilitation [a (+, +) interaction] at intermediate levels because of favourable water relationships in closed stands, and amensalism [a (0, ?) interaction] from higher‐situated segments that deprive lower‐situated segments access to light at lower relative levels. The intensity of amensalism increased downwards in the bryophyte carpet as indicated by a reinforced size hierarchy. The tendency for small H. splendens segments to become buried and lost from the population by amensalism is likely to represent a general mechanism for interactions between bryophyte species and succession in bryophyte‐dominated stands. Population effects of climatic and local environmental factors (favourability vs stress), disturbance and apparently random events are discussed with reference to their impact on the relative sizes of subpopulations acting as sources (due to facilitation) and sinks (due to amensalism).     

Sorbus cibagouensis sp. nov. (Rosaceae) from Zayü County,southeastern Xizang,China

Sorbus cibagouensis sp. nov. (Rosaceae subfam. Rosaceae), a new taxon from Cibagou National Nature Reserve, Zayü County, southeastern Xizang (Tibet), China, is described and illustrated. It is related to S. monbeigii (Cardot) Balakr., but primarily differs in the number of styles (S. cibagouensis = 5; S. monbeigii = 4) and the shape of stipules and leaves (S. cibagouensis: stipules caducous, small, with entire margin, leaflets in 9–11 pairs; S. monbeigii: stipules persistent, large, serrate, leaflets in 6–8 (–10) pairs).