Wood Specific Gravity Variations and Biomass of Central African Tree Species: The Simple Choice of the Outer Wood

Context

Wood specific gravity is a key element in tropical forest ecology. It integrates many aspects of tree mechanical properties and functioning and is an important predictor of tree biomass. Wood specific gravity varies widely among and within species and also within individual trees. Notably, contrasted patterns of radial variation of wood specific gravity have been demonstrated and related to regeneration guilds (light demanding vs. shade-bearing). However, although being repeatedly invoked as a potential source of error when estimating the biomass of trees, both intraspecific and radial variations remain little studied. In this study we characterized detailed pith-to-bark wood specific gravity profiles among contrasted species prominently contributing to the biomass of the forest, i.e., the dominant species, and we quantified the consequences of such variations on the biomass.

Methods

Radial profiles of wood density at 8% moisture content were compiled for 14 dominant species in the Democratic Republic of Congo, adapting a unique 3D X-ray scanning technique at very high spatial resolution on core samples. Mean wood density estimates were validated by water displacement measurements. Wood density profiles were converted to wood specific gravity and linear mixed models were used to decompose the radial variance. Potential errors in biomass estimation were assessed by comparing the biomass estimated from the wood specific gravity measured from pith-to-bark profiles, from global repositories, and from partial information (outer wood or inner wood).

Results

Wood specific gravity profiles from pith-to-bark presented positive, neutral and negative trends. Positive trends mainly characterized light-demanding species, increasing up to 1.8 g.cm^-3 per meter for Piptadeniastrum africanum, and negative trends characterized shade-bearing species, decreasing up to 1 g.cm^-3 per meter for Strombosia pustulata. The linear mixed model showed the greater part of wood specific gravity variance was explained by species only (45%) followed by a redundant part between species and regeneration guilds (36%). Despite substantial variation in wood specific gravity profiles among species and regeneration guilds, we found that values from the outer wood were strongly correlated to values from the whole profile, without any significant bias. In addition, we found that wood specific gravity from the DRYAD global repository may strongly differ depending on the species (up to 40% for Dialium pachyphyllum).

Main Conclusion

Therefore, when estimating forest biomass in specific sites, we recommend the systematic collection of outer wood samples on dominant species. This should prevent the main errors in biomass estimations resulting from wood specific gravity and allow for the collection of new information to explore the intraspecific variation of mechanical properties of trees.     

Direct Measurements of Drag Forces in C. elegans Crawling Locomotion

With a simple and versatile microcantilever-based force measurement technique, we have probed the drag forces involved in Caenorhabditis elegans locomotion. As a worm crawls on an agar surface, we found that substrate viscoelasticity introduces nonlinearities in the force-velocity relationships, yielding nonconstant drag coefficients that are not captured by original resistive force theory. A major contributing factor to these nonlinearities is the formation of a shallow groove on the agar surface. We measured both the adhesion forces that cause the worm’s body to settle into the agar and the resulting dynamics of groove formation. Furthermore, we quantified the locomotive forces produced by C. elegans undulatory motions on a wet viscoelastic agar surface. We show that an extension of resistive force theory is able to use the dynamics of a nematode’s body shape along with the measured drag coefficients to predict the forces generated by a crawling nematode.     

Mitotic remodeling of the replicon and chromosome structure

Animal cloning by nuclear-transfer experiments frequently fails due to the inability of transplanted nuclei to support normal embryonic development. We show here that the formation of mitotic chromosomes in the egg context is crucial for adapting differentiated nuclei for early development. Differentiated erythrocyte nuclei replicate inefficiently in Xenopus eggs but do so as rapidly as sperm nuclei if a prior single mitosis is permitted. This mitotic remodeling involves a topoisomerase II-dependent shortening of chromatin loop domains and an increased recruitment of replication initiation factors onto chromatin, leading to a short interorigin spacing characteristic of early developmental stages. It also occurs within each early embryonic cell cycle and dominantly regulates initiation of DNA replication for the subsequent S phase. These results indicate that mitotic conditioning is crucial to reset the chromatin structure of differentiated adult donor cells for embryonic DNA replication and suggest that it is an important step in nuclear cloning.     

An <Emphasis Type="Italic">in situ</Emphasis> method for the investigation of vertical distributions of zooplankton in lakes: test of a two-compartment enclosure

Two-section enclosures were designed for the investigation of the effect of various physicochemical and biological factors on vertical distribution of zooplankton in situ. The framework of the enclosure was a cylindrical polyethylene column without any partitions inside, in which the isolation of animals in different sections after in situ exposure was achieved by pinching the flexible central part of the column. Enclosures were tested at the brackish stratified meromictic Lake Shira (Russia, Khakasia). The absence of fish and carnivorous zooplankton in the lake suggests that the vertical distribution of zooplankton is mainly determined by physicochemical gradients in the water column. Experiments and field observations demonstrated that all age and size groups of Arctodiaptomus salinus and Brachionus plicatilis strongly avoided surface layers during the daylight. The escape of zooplankton from the anoxic hypolimnion was less active. Statistically significant avoidance was observed only for copepodites C4–C5 and females of A. salinus. The relatively simple construction of the columns and easy handling during the experiment were the factors that favoured the use of this device to perform in situ basic tests of the effect of different factors on the vertical distribution of zooplankton.     

Antimicrobial peptides temporins B and L induce formation of tubular lipid protrusions from supported phospholipid bilayers

The binding of the antimicrobial peptides temporins B and L to supported lipid bilayer (SLB) model membranes composed of phosphatidylcholine and phosphatidylglycerol (4:1, mol/mol) caused the formation of fibrillar protrusions, visible by fluorescent microscopy of both a fluorescent lipid analog and a labeled peptide. Multicolor imaging at low peptide-to-lipid ratios (P/L < approximately 1:5) revealed an initial in-plane segregation of membrane-bound peptide and partial exclusion of lipid from the peptide-enriched areas. Subsequently, at higher P/L numerous flexible lipid fibrils were seen growing from the areas enriched in lipid. The fibrils have diameters <250 nm and lengths of up to approximately 1 mm. Fibril formation reduces the in-plane heterogeneity and results in a relatively even redistribution of bound peptide over the planar bilayer and the fibrils. Physical properties of the lipid fibrils suggest that they have a tubular structure. Our data demonstrate that the peptide-lipid interactions alone can provide a driving force for the spontaneous membrane shape transformations leading to tubule outgrowth and elongation. Further experiments revealed the importance of positive curvature strain in the tubulation process as well as the sufficient positive charge on the peptide (>/=+2). The observed membrane transformations could provide a simplified in vitro model for morphogenesis of intracellular tubular structures and intercellular connections.     

Distinct Distribution of Ectopically Expressed Histone Variants H2A.Bbd and MacroH2A in Open and Closed Chromatin Domains

Background

It becomes increasingly evident that nuclesomes are far from being identical to each other. This nucleosome diversity is due partially to the existence of histone variants encoded by separate genes. Among the known histone variants the less characterized are H2A.Bbd and different forms of macroH2A. This is especially true in the case of H2A.Bbd as there are still no commercially available antibodies specific to H2A.Bbd that can be used for chromatin immunoprecipitation (ChIP).

Methods

We have generated HeLa S3 cell lines stably expressing epitope-tagged versions of macroH2A1.1, H2A.Bbd or canonical H2A and analyzed genomic distribution of the tagged histones using ChIP-on-chip technique.

Results

The presence of histone H2A variants macroH2A1.1 and H2A.Bbd has been analyzed in the chromatin of several segments of human chromosomes 11, 16 and X that have been chosen for their different gene densities and chromatin status. Chromatin immunoprecipitation (ChIP) followed by hybridization with custom NimbleGene genomic microarrays demonstrated that in open chromatin domains containing tissue-specific along with housekeeping genes, the H2A.Bbd variant was preferentially associated with the body of a subset of transcribed genes. The macroH2A1.1 variant was virtually absent from some genes and underrepresented in others. In contrast, in closed chromatin domains which contain only tissue-specific genes inactive in HeLa S3 cells, both macroH2A1.1 and H2A.Bbd histone variants were present and often colocalized.

Conclusions

Genomic distribution of macro H2A and H2A.Bbd does not follow any simple rule and is drastically different in open and closed genomic domains.     

Heterochromatin restricts the mobility of nuclear bodies

Nuclear bodies are relatively immobile organelles. Here, we investigated the mechanisms underlying their movement using experimentally induced interphase prenucleolar bodies (iPNBs). Most iPNBs demonstrated constrained diffusion, exhibiting infrequent fusions with other iPNBs and nucleoli. Fusion events were actin-independent and appeared to be the consequence of stochastic collisions between iPNBs. Most iPNBs were surrounded by condensed chromatin, while fusing iPNBs were usually found in a single heterochromatin-delimited compartment (“cage”). The experimentally induced over-condensation of chromatin significantly decreased the frequency of iPNB fusion. Thus, the data obtained indicate that the mobility of nuclear bodies is restricted by heterochromatin.