Differences in breeding and motility betweenPardosa Pullata (Clerck) andPardosa Prativaga (L. Koch), (Lycosidae,Araneae) in relation to habitat

Summary Populations ofPardosa pullata (Clerck) andP. prativaga (L. Koch) were studied under experimental and field conditions. In the field attention has been paid to pure as well as to mixed populations of the species. Differences in breeding and motility were found, which are discussed in relation to the habitat of the two species.     

Counterintuitive DNA Sequence Dependence in Supercoiling-Induced DNA Melting

The metabolism of DNA in cells relies on the balance between hybridized double-stranded DNA (dsDNA) and local de-hybridized regions of ssDNA that provide access to binding proteins. Traditional melting experiments, in which short pieces of dsDNA are heated up until the point of melting into ssDNA, have determined that AT-rich sequences have a lower binding energy than GC-rich sequences. In cells, however, the double-stranded backbone of DNA is destabilized by negative supercoiling, and not by temperature. To investigate what the effect of GC content is on DNA melting induced by negative supercoiling, we studied DNA molecules with a GC content ranging from 38% to 77%, using single-molecule magnetic tweezer measurements in which the length of a single DNA molecule is measured as a function of applied stretching force and supercoiling density. At low force (<0.5pN), supercoiling results into twisting of the dsDNA backbone and loop formation (plectonemes), without inducing any DNA melting. This process was not influenced by the DNA sequence. When negative supercoiling is introduced at increasing force, local melting of DNA is introduced. We measured for the different DNA molecules a characteristic force F _char, at which negative supercoiling induces local melting of the dsDNA. Surprisingly, GC-rich sequences melt at lower forces than AT-rich sequences: F _char = 0.56pN for 77% GC but 0.73pN for 38% GC. An explanation for this counterintuitive effect is provided by the realization that supercoiling densities of a few percent only induce melting of a few percent of the base pairs. As a consequence, denaturation bubbles occur in local AT-rich regions and the sequence-dependent effect arises from an increased DNA bending/torsional energy associated with the plectonemes. This new insight indicates that an increased GC-content adjacent to AT-rich DNA regions will enhance local opening of the double-stranded DNA helix.     

Genetic diversity of SSR and ISSR markers in wild populations of Brachypodium distachyon and its close relatives B. stacei and B. hybridum (Poaceae)

The genetic diversity of seven wild populations of the grass Brachypodium distachyon (2n = 10), 4 of B. stacei (2n = 20) and 13 of B. hybridum (2n = 30) from the Mediterranean and southern areas of the Iberian Peninsula was studied via the analysis of microsatellite (SSR) and inter-microsatellite (ISSR) markers. The 11 SSR markers analysed provided a total of 156 polymorphic fragments. The B. hybridum populations returned more fragments (98) than the B. stacei populations (87), and more than twice the number recorded for the B. distachyon populations (44). Some fragments were specific to the B. distachyon (3.85 %), B. stacei (27.56 %) or B. hybridum populations (18.58 %). The analysis of 16 ISSR markers returned similar results: the B. hybridum populations returned more polymorphic fragments than the B. distachyon or B. stacei populations. The analysis of molecular variance, with distances between individuals, populations and species estimated on the basis of the presence/absence of the SSR and ISSR fragments, showed most of the variation (67 %) to occur among populations. This was followed by differences among individuals within populations (24 %), and finally among species (9 %). Grouping based on UPGMA and principal coordinate analysis showed a clear separation of three groups corresponding to the populations of the same species. Principal component analysis, involving chromosome number, low-molecular weight-glutenin subunits and the most influential climatic and geographic factors, was also performed. This revealed an obvious separation among the populations of B. distachyon and B. hybridum. The index of Mediterraneity and altitude explained 70.56 % of the total variation associated with the first axis. A trend was seen towards a greater presence of B. distachyon forms in areas of the Peninsular interior and higher altitude, with the B. hybridum forms more common in regions of greater summer rainfall and a lower index of Mediterraneity.     

CENP-A and H3 Nucleosomes Display a Similar Stability to Force-Mediated Disassembly

Centromere-specific nucleosomes are a central feature of the kinetochore complex during mitosis, in which microtubules exert pulling and pushing forces upon the centromere. CENP-A nucleosomes have been assumed to be structurally unique, thereby providing resilience under tension relative to their H3 canonical counterparts. Here, we directly test this hypothesis by subjecting CENP-A and H3 octameric nucleosomes, assembled on random or on centromeric DNA sequences, to varying amounts of applied force by using single-molecule magnetic tweezers. We monitor individual disassembly events of CENP-A and H3 nucleosomes. Regardless of the DNA sequence, the force-mediated disassembly experiments for CENP-A and H3 nucleosomes demonstrate similar rupture forces, life time residency and disassembly steps. From these experiments, we conclude that CENP-A does not, by itself, contribute unique structural features to the nucleosome that lead to a significant resistance against force-mediated disruption. The data present insights into the mechanistic basis for how CENP-A nucleosomes might contribute to the structural foundation of the centromere in vivo.     

Silk production in adult females of the Wolf spider Pardosa amentata (Lycosidae, Araneae)

The production of silk in adult females of the wolf spider Pardosa amentata (CI.) has been studied. The role of female silk in courtship behavour is discussed on teh basis of experimental data.     

NAP1-Assisted Nucleosome Assembly on DNA Measured in Real Time by Single-Molecule Magnetic Tweezers

While many proteins are involved in the assembly and (re)positioning of nucleosomes, the dynamics of protein-assisted nucleosome formation are not well understood. We study NAP1 (nucleosome assembly protein 1) assisted nucleosome formation at the single-molecule level using magnetic tweezers. This method allows to apply a well-defined stretching force and supercoiling density to a single DNA molecule, and to study in real time the change in linking number, stiffness and length of the DNA during nucleosome formation. We observe a decrease in end-to-end length when NAP1 and core histones (CH) are added to the dsDNA. We characterize the formation of complete nucleosomes by measuring the change in linking number of DNA, which is induced by the NAP1-assisted nucleosome assembly, and which does not occur for non-nucleosomal bound histones H3 and H4. By rotating the magnets, the supercoils formed upon nucleosome assembly are removed and the number of assembled nucleosomes can be counted. We find that the compaction of DNA at low force is about 56 nm per assembled nucleosome. The number of compaction steps and associated change in linking number indicate that NAP1-assisted nucleosome assembly is a two-step process.     

The significance of plant growth-forms as "shelter" for terrestrial animals

Different plant growth-forms such as tussocks and rosettes, were selected as micro ecosystems in an area in which these structures were spatially isolated from each other. These growth forms are important during winter since they serve to regulate temperatures, which is important to invertebrate animals. This was shown with intensive temperature recordings in winter, correlated with numbers of animals. Plant growth-forms are more important in young bare areas where there are fewer plants to provide shelter, than in older areas, where a denser vegetation cover affords more possibilities for shelter. Such differences can be expected to influence the abundance as well as the occurrence of animals during the development of a community.     

An experimental analysis of the significance of tuft-structures as a shelter for invertebrate fauna, with respect to wind-velocity and temperature

Adverse weather conditions (high wind velocities and low temperatures) are shown to influence the distribution pattern of invertebrate animals in tussocks (in natural tussocks in the field and in artificial tussocks in the laboratory). Tussocks may also influence the dynamics of their populations. This is to be investigated.