Evaluation of cooperativity for phase transitions in two- and three-dimensional systems

Use of the so-called "cooperative unit" (readily obtainable from the midpoint slope of phase transition curves) is discussed for the determination of cluster sizes and cooperative interaction energies. This quantity has been commonly employed in a rather empirical way since its correct interpretation is known only for some special cases (linear systems, all-or-none transitions). It is shown in the framework of a lattice model (Ising model) that the cooperative unit may be interpreted in terms of correlation functions and that it defines an average cluster corresponding to the patch size as obtained from scattering experiments. Relations between the cooperative unit and a cooperativity parameter are given for various lattices. Different types of transition curves are discussed using a simple analytical formalism, the quasichemical approximation. Some important nonideality effects are investigated which may lead to a "smearing-out" of first-order transitions.     

A combined study of aggregation, membrane affinity and pore activity of natural and modified melittin

The pore activity of melittin and several chemically modified derivatives has been investigated using conductance measurements on planar lipid bilayers and marker release from small unilamellar vesicles. The modifications included N-terminal formylation, acetylation, succinylation and modification of the tryptophan residue. All of the compounds showed bilayer permeabilizing properties, though quantitative differences were evident. These comprised changes in the voltage dependence of the conductance, in the single-pore kinetics, in the concentration of aqueous peptide required to induce a given pore activity and in the apparent 'molecularity' reflected by the power law of its concentration dependence. A strong tendency for disrupting bilayers was not always correlated with strong pore activity. For a better understanding of these results, measurements of pore activity were complemented by studying the aggregation behavior in solution and the water-membrane partition equilibrium. Modifications of charged residues gave rise to significant changes in the aggregation properties, had virtually no influence on the partition coefficient. The latter decreased strongly, however, as a result of tryptophan modification. Analysis of the isotherms was consistent with the assumption that the arginine residues in melittin do not contribute very much to charge accumulation at the immediate membrane/water interface.     

Effect of temperature on seed production in the invasive grass Glyceria maxima (Hartm.) Holmb. (Poaceae) in South Africa

Temperature is one of the main factors that determine sexual reproduction in terrestrial and emergent aquatic plant species. The effect of temperature on sexual reproduction and seed production of Glyceria maxima (Hartm.) Holmb. in the southern hemisphere is unknown. Glyceria maxima collections in February 2010 at three isolated infestations in KwaZulu-Natal failed to yield a single seed, only empty panicles. Laboratory experiments showed that vernalisation had no consistent effect on seed production. Field- and laboratory-grown plants produced seeds in the 2010/2011 season, because of having sufficient time at optimum temperatures required for seed production (1 491 and 1 585 hours, respectively), compared to a shorter period (1 352 hours) of suitable temperatures during the 2009/2010 growing season. An inadequate period of optimum temperatures (15–25°C) during seed production resulted in the lack of seeds in the field in the 2009/2010 growing season. This study showed that temperature and duration of exposure thereto during the seed-production period play vital roles in G. maxima sexual reproduction.     

The geography of divergence with gene flow facilitates multitrait adaptation and the evolution of pollinator isolation in Mimulus aurantiacus

Ecological adaptation is the driving force during divergence with gene flow and generates reproductive isolation early in speciation. Although gene flow opposes divergence, local adaptation can be facilitated by factors that prevent the breakup of favorable allelic combinations. We investigated how selection, genetic architecture, and geography have contributed to the maintenance of floral trait divergence and pollinator isolation between parapatric ecotypes of Mimulus aurantiacus. Combining greenhouse, field, and genomic studies, we show that sharp clines in floral traits are maintained by spatially varying selection. Although adaptation breaks down where the ecotypes co‐occur, leading to the formation of a hybrid zone, the largely non‐overlapping distributions of the ecotypes shield them from immigrant genes, facilitating divergence across most of the range. In contrast to the sharp genetic discontinuities observed across most hybrid zones, we observed a gradual cline in genome‐wide divergence and a pattern of isolation by distance across the landscape. Thus, contrary to a long period of allopatry followed by recent re‐contact, our data suggest that floral trait divergence in M. aurantiacus may have evolved with locally restricted, but ongoing gene flow. Therefore, our study reveals how the geographic distribution of an organism can contribute to the evolution of premating isolation in the early stages of divergence with gene flow.     

Molecular phylogenetics and complementary geographical distributions of species of the Western Australian land snail genera Plectorhagada Iredale, 1933 and Strepsitaurus Solem, 1997 (Gastropoda: Camaenidae)

The Western Australian camaenid genera Plectorhagada and Strepsitaurus have morphological similarities and mutually exclusive ranges near Cape Range. Sequences of ctyochrome c oxidase subunit I (COI) and 16S mitochondrial DNA (mtDNA) genes confirmed that the two genera are genetically close sister clades. Targeted sampling showed that Strepsitaurus, which is confined to Cape Range, lies within a hole in the distribution of the more broadly distributed Plectorhagada that occurs on the coastal strip surrounding Cape Range. Species of the two genera meet at the transition between the rocky Cape Range and the sandier coastal areas, providing a rare example of the close replacement of genera. Within each genus, mtDNA sequences confirmed the monophyly and genetic distinctness of species, with few exceptions that show the need for additional work, and with the addition of three new species in the Cape Range area. As is typical of Australian camaenids, distributions of congeneric species are mutually exclusive, but in some cases close proximity is associated with contrasting habitats, such as gorge endemics versus the species on top of Cape Range. In sympatry, Strepsitaurus rugus (Cotton, 1951) and Strepsitaurus williami Solem, 1997 are separated by microhabitat. These local associations with habitat indicate that ecological differences, and not simply allopatric divergence, contribute to the lack of sympatry between closely related Australian camaenids. © 2015 The Linnean Society of London     

Microsatellite markers isolated from a polyploid saltbush, Atriplex nummularia Lindl. (Chenopodiaceae)

Atriplex nummularia is a polyploid Australian saltbush which has been identified as a suitable species for use in the rehabilitation of agricultural land affected by salinity. We isolated 12 polymorphic loci for a preliminary assessment of genetic variability and structure within the species as a basis for a breeding programme. Preliminary screening of loci in 40 individuals from two populations revealed multibanded genotypes consisting of up to seven alleles in a single individual, with up to 29 alleles observed at a single locus. The multibanded patterns are consistent with the polyploid status of this species.     

Large-ligand adsorption to membranes. III. Cooperativity and general ligand shapes

In previous reports (Stankowski, S. (1983) Biochim. Biophys. Acta 735, 341-351 and 352-360) the ordinary Scatchard-type analysis has been shown to yield erroneous results when applied to the binding of large molecules to membranes or cells. Formulae have been given to treat the limiting cases of very thin and of very bulky ligands. These results are now extended to include ligands of any shape and cooperative interactions. As an example, data on the cooperative binding of polymyxin to charged lipid bilayers are reevaluated. Adsorption with concomitant incorporation of the large molecule into the membrane is also considered.