Sulphur stable isotopes can distinguish trophic dependence on sediments and plankton in boreal lakes

1. Stable isotopes of carbon are useful for differentiating between freshwater food chains based on planktonic algae or benthic algae, but are reported to be of limited use for identifying food chains based on sedimentary detritus. Because data from marine systems suggest that stable isotopes of sulphur (δ³⁴S values) have potential in this regard, we tested their utility in freshwater lakes.
2. We found that sulphate in the water column of four boreal lakes was enriched in ³⁴S compared to the sulphur in bulk sediments from these lakes. Furthermore, within a given lake, insects known to feed on sediment (directly or via predation) had δ³⁴S values similar to those of sediment, whereas planktonic and benthic invertebrates known to feed on suspended particles had δ³⁴S values similar to those of sulphate in the water column.
3. Using the stable S isotope values of invertebrates that obtain their S from either the sediment or the water column as end members in a two-source mixing model, we show that two fish species obtain their food from both planktonic and sedimentary sources. Furthermore, model results suggest that, as expected, the more benthic-feeding fish species obtains more of its S from the sediment compartment than does the species that feeds in the water-column.
4. Our results suggest that measurements of stable sulphur isotopes provide a means of distinguishing between members of food chains that are based in the water column from those based on sedimentary detritus. As such, they would be a useful complement to stable C isotopes that are used to distinguish between food chains based on planktonic or benthic algae.     

Electrophysiological and biochemical responses of mouse vomeronasal receptor cells to urine-derived compounds: possible mechanism of action

Receptor cells of the vomeronasal organ (VNO) are thought to detect pheromone-like molecules important for reproductive physiology. Several compounds derived from male mouse urine have been demonstrated to affect endocrine events in female mice. In the present study, the ability of these compounds to affect VNO activity was tested. In dissociated VNO cells held under voltage clamp conditions, application of dehydro-exo-brevicomin (DHB) evoked an outward current at negative holding potentials and an inward current at positive holding potentials. Under current clamp, DHB reduced action potential firing. Since DHB application caused a decrease in membrane conductance, this compound appeared to act by reducing inward current through closing an ion channel. Biochemical experiments tested the effects of DHB and 2- (sec-butyl)-4,5-dihydrothiazole (SBT) on cAMP levels in the VNO. A mixture of DHB and SBT decreased cAMP levels in VNO sensory tissue and had no effect on VNO non-sensory tissue. The results suggest that pheromones have an inhibitory influence on action potential generation and on cAMP levels in receptor cells of the VNO.      

Rapid change of the repertoire of variant surface glycoprotein genes in trypanosomes by gene duplication and deletion

To study the evolution of the variant surface glycoprotein (VSG) repertoire of trypanosomes we have analysed the DNA region surrounding the VSG 118 gene in different trypanosome strains. We find a remarkable degree of variation in this area. Downstream from the 118 gene a 5.7 X 10(3) base-pair DNA segment containing a potential VSG gene has been quadruplicated in strain 427 of Trypanosoma brucei, but not in most other strains analysed. The VSG 1.1000 gene, located immediately upstream from the 118 gene in one trypanosome strain, has been cleanly deleted in another. Our results are most easily explained by multiple unequal cross-overs between sister chromatids and are the first indication that sister chromatid exchange occurs in trypanosomes.     

Why western spruce budworms travel so far for the winter

1. Conifer‐feeding budworms (Choristoneura) hibernate in sheltered locations on their host trees from late summer of 1 year to spring of the next. During this period, they do not feed but rely on sustenance provided in the egg. Overwinter survival is dependent on the rate of consumption of these limited reserves. 2. A process model was developed that quantifies the relationship between the rate of consumption and survival at variable temperatures and exposure times for western spruce budworm. The model supported physiological evidence that warm weather conditions early in the diapause period have a dominant influence on overwinter survival. Output compared favourably with field observations of poorer budworm survival at lower elevations where late‐summer and autumn temperatures were warmer compared to those overwintering at cooler, higher elevations. 3. Field experiments demonstrated these weather‐dependent rates of survival were modulated significantly by the degree of shelter experienced by hibernating budworms. 4. Dissection of whole trees harbouring overwintering western spruce budworms showed a significant portion of the population had travelled a considerable distance from the periphery of the tree canopy where eggs were laid to overwinter successfully on the tree bole where sheltered niches are common. 5. Thus, budworms will travel relatively long distances and risk increased mortality during this dispersal to find adequate shelter to overwinter.     

THE EFFECT OF ADDED NITROGEN ON THE RATE OF DECOMPOSITION OF ORGANIC MATTER

(1) N added to decomposing organic matter often has no effect or a negative effect on microbial activity, at least in the long term. More than 60 papers are cited in support of this statement.
(2) The negative effect of N is mainly found with recalcitrant organic matter with a high C/N ratio (straw, wood, etc.), whereas a positive effect of N is common for easily degradable organic material with low C/N ratio.
(3) The negative effect of N could be explained by: (i) N disturbs the outcome of competition between potent and less potent decomposers; (ii) through 'ammonia metabolite repression', N blocks production of certain enzymes, at least in basidiomycetes, and enhances breakdown of the most available cellulose, whereby recalcitrant lignocellulose accumulates; (iii) amino compounds condense with polyphenols and other decomposition products, forming 'browning precursors' which are toxic or inhibitory.
(4) The effect of adding N may depend on the microflora present.
(5) There are indications that some microorganisms have a 'luxury uptake' of N when it is present in sufficient amounts, thereby delaying N mineralization.
(6) The addition of N seems to increase the formation of water-soluble , brown, recalcitrant compounds, but to decrease the amount of humus formed.     

The age of major monocot groups inferred from 800+ rbcL sequences

Phylogenetic research on monocots has been extraordinarily active over the past years. With the familial interrelationships being sufficiently understood, the question of divergence times and crown node ages of major lineages comes into focus. In this study we present the first attempt to estimate crown and stem node ages for most orders and families of monocots, based on rbcL sequence data and comprehensive taxon sampling. From our analysis it is obvious that considerable monocot diversification took place during the Early Cretaceous, with most families already present at the Cretaceous–Tertiary boundary. Araceae, Arecaceae and Orchidaceae are among the oldest families with crown node ages reaching back into the Early Cretaceous. We comment on possible error sources and the necessity for methodological improvement in molecular dating.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 385–398.