Control of the release of digestive enzymes in the caeca of the cricket Gryllus bimaculatus

Abstract In Gryllus bimaculatus, more digestive enzymes (amylase, trypsin, aminopeptidase) are secreted in the caecum of fed crickets than in unfed crickets, but the enzymes are released continuously at a basal rate in unfed animals. The rate of synthesis of the enzymes appears to parallel their rate of release. Digestive enzymes are released in response to a specific ratio of nutrients, although a high nutrient component in the food does not necessarily induce a high digestive enzyme release for that component. Rinsed flat‐sheet preparations of the caecum are incubated with specific nutrients (carbohydrates and proteins) and various concentrations of a neuropeptide (type‐A allatostatin), which affects generally the basal rates of secretion. Both maltose and glucose increase the release of amylase in vitro, but starch produces an inhibition of amylase release at lower concentrations. Bovine serum albumin (BSA), peptone and a mixture of amino acids have almost no effect on the release of aminopeptidase or carboxypeptidase, and only low concentrations of peptone increase trypsin release. High concentrations of both BSA and peptone strongly inhibit trypsin activity, perhaps by excess substrate binding to the trypsin active site. The allatostatin Grybi‐AST 5 elevates the release of amylase in vitro, but not of trypsin or aminopeptidase, in 2‐day‐old fed females. In the caeca from 1‐day‐old unfed crickets, both amylase and the trypsin release are stimulated in the presence of AST 5. The paracrine AST 5 is probably released from the gut endocrine cells and binds to the enzyme‐producing caecal cells.     

How do temperature,dissolved organic matter and nutrients influence the response of Leptodiaptomus ashlandi to UV radiation in a subalpine lake?

1. Ultraviolet radiation (UV) is an important stressor for zooplankton in alpine lake ecosystems. Multiple environmental variables such as dissolved organic matter (DOM), temperature and nutrient availability may alter how UV affects zooplankton. 2. We conducted a week‐long experiment manipulating UV, nutrients and DOM in enclosures suspended at the surface of cold and warm alpine lakes to determine the interactive effects of these variables on ovigerous Leptodiaptomus ashlandi (Marsh, 1893), a calanoid copepod. 3. UV had a negative effect on nauplii and gravid females at the colder temperature and at low, ambient DOM levels, but had no effect at the warmer temperature or when DOM was added. At the warmer temperature, fewer nauplii were produced in the +nutrient compared to ?nutrient treatment. Adult survival was not affected by UV or any other experimental variable. 4. These results demonstrate that the extent of the impact of UV radiation on zooplankton in alpine systems is altered by other environmental variables, and that these effects may not be apparent from experiments that look only at the survival of adult organisms that are better defended against UV.     

Faecal pellets of lichenivorous mites contain viable cells of the lichen-forming ascomycete Xanthoria parietina and its green algal photobiont, Trebouxia arboricola

The bright yellow wall lichen, Xanthoria parietina , is usually inhabited by oribatid mites (Acari) which do not only find shelter, but also graze on selected areas of the thallus. As X. parietina does not produce symbiotic vegetative propagules and its compatible photobiont, unicellular green algae of the genus Trebouxia , are rare outside lichen thalli, we tested the hypothesis of dispersal of viable Trebouxia cells via acarine faeces. The lichenivorous mites, Trhypochtonius tectorum and Trichoribates trimaculatus , were isolated from thalli of X. parietina and cultured in the laboratory on a lichen diet. Light microscopic investigations of faecal pellets from mites that had been feeding on X. parietina indicated gut passage of intact ascospores and photobiont cells. In a series of experiments, viable algal and fungal cells contained in such faecal pellets were cultured. The taxonomic affiliation of these isolates was identified using molecular techniques, i.e. comparative investigations of nuclear ribosomal gene data (ITS 1 and 2, 5.8S rDNA) in the algal and fungal partners, and of the species-specific hydrophobin gene sequence in the fungal partner. Our culturing experiments demonstrated that the faecal pellets of both lichenivorous mites, upon feeding on X. parietina , contain viable ascospores and photobiont cells ( Trebouxia arboricola ) and thus might be a common and successful mode of vegetative short- and long-distance dispersal of this and numerous other lichen-forming ascomycetes and their photobionts. Future studies will have to elucidate the evolutionary significance of invertebrate interactions with lichens. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 259–268.     

DNA SYNTHESIS AND CELL TURNOVER IN RANA PIPIENS TADPOLE LIVER DURING SPONTANEOUS METAMORPHOSIS1

The relationship of DNA synthesis and cellular turnover to biochemical differentiation during metamorphosis of R. pipiens liver was investigated. Average DNA/cell was constant at 11.6 pg/ nucleus through stage XXV; but increased during juvenile growth; during metamorphosis stages, changes in total DNA content must correspond to changes in cell number. Rates of DNA synthesis were estimated by rates of ³H-thymidine incorporated into the acid-precipitable fractions, corrected for both precursor uptake into the acid-soluble pool, and for endogenous thymine pool size. DNA content increased steadily from premetamorphosis until late prometamorphosis; at preclimax stages XVIII and XX there were two successive decreases in DNA content of approximately 30%. Fluctuations in synthesis rates preceded corresponding fluctuations in content; DNA synthesis was maximal at stages XVI and XVIII, decreased nearly ten-fold at metamorphic climax, and then gradually rose again during late climax stages. The size of the endogenous thymine pool increased transitorily during spontaneous metamorphosis corresponding to a stage of maximal DNA synthesis. These results indicate that both DNA synthesis and cellular turnover play a significant role in determining net DNA synthesis rates and content during metamorphosis. Metamorphosis of the tadpole liver appears to be associated with both proliferation and cellular death, perhaps a replacement of “larval” by “adult” cells. Metamorphosis of the liver cannot be occuring in a “fixed population of cells” as is commonly assumed. An interpretation of the population dynamics of the metamorphic liver is presented.     

Physiological characterization and genetic modifiers of aberrant root thigmomorphogenesis in mutants of Arabidopsis thaliana MILDEW LOCUS O genes

Root architecture and growth patterns are plant features that are still poorly understood. When grown under in vitro conditions, seedlings with mutations in Arabidopsis thaliana genes MLO4 or MLO11 exhibit aberrant root growth patterns upon contact with hard surfaces, exemplified as tight root spirals. We used a set of physiological assays and genetic tools to characterize this thigmomorphogenic defect in detail. We observed that the mlo4/mlo11‐associated root curling phenotype is not recapitulated in a set of mutants with altered root growth patterns or architecture. We further found that mlo4/mlo11‐conditioned root curling is not dependent upon light and endogenous flavonoids, but is pH‐sensitive and affected by exogenous calcium levels. Based upon the latter two characteristics, mlo4‐associated root coiling appears to be mechanistically different from the natural strong root curvature of the Arabidopsis ecotype Landsberg erecta. Gravistimulation reversibly overrides the aberrant thigmomorphogenesis of mlo4 seedlings. Mutants with dominant negative defects in α‐tubulin modulate the extent and directionality of mlo4/mlo11‐conditioned root coils, whereas mutants defective in polar auxin transport (axr4, aux1) or gravitropism (pgm1) completely suppress the mlo4 root curling phenotype. Our data implicate a joint contribution of calcium signalling, pH regulation, microtubular function, polar auxin transport and gravitropism in root thigmomorphogenesis.     

Global decomposition experiment shows soil animal impacts on decomposition are climate‐dependent

Climate and litter quality are primary drivers of terrestrial decomposition and, based on evidence from multisite experiments at regional and global scales, are universally factored into global decomposition models. In contrast, soil animals are considered key regulators of decomposition at local scales but their role at larger scales is unresolved. Soil animals are consequently excluded from global models of organic mineralization processes. Incomplete assessment of the roles of soil animals stems from the difficulties of manipulating invertebrate animals experimentally across large geographic gradients. This is compounded by deficient or inconsistent taxonomy. We report a global decomposition experiment to assess the importance of soil animals in C mineralization, in which a common grass litter substrate was exposed to natural decomposition in either control or reduced animal treatments across 30 sites distributed from 43°S to 68°N on six continents. Animals in the mesofaunal size range were recovered from the litter by Tullgren extraction and identified to common specifications, mostly at the ordinal level. The design of the trials enabled faunal contribution to be evaluated against abiotic parameters between sites. Soil animals increase decomposition rates in temperate and wet tropical climates, but have neutral effects where temperature or moisture constrain biological activity. Our findings highlight that faunal influences on decomposition are dependent on prevailing climatic conditions. We conclude that (1) inclusion of soil animals will improve the predictive capabilities of region‐ or biome‐scale decomposition models, (2) soil animal influences on decomposition are important at the regional scale when attempting to predict global change scenarios, and (3) the statistical relationship between decomposition rates and climate, at the global scale, is robust against changes in soil faunal abundance and diversity.     

Resource polymorphism in a Patagonian fish Percichthys trucha (Percichthyidae): phenotypic evidence for interlake pattern variation

Within-species differentiation in phenotypic characters related to resource use (resource polymorphism) is frequently thought to result from divergent natural selection in a heterogeneous environment with 'open niches'. In this study we found consistent resource-based polymorphism within three different populations of Percichthys trucha , a lake-dwelling fish native to the southern Andes. In each of three lakes we found two morphotypes that could be clearly distinguished by differences in gill raker length. However, the magnitude of the polymorphism, and the suite of phenotypic characteristics associated with the polymorphism, differed between lakes. Patterns of divergence were more similar between the two northern lakes which ultimately drain into a common river, than between these two lakes and a more southern, unconnected lake. The southern population, which had the largest divergence in gill raker length (32% vs. 16% and 19%), also showed substantial differences in diet. Evidence from the southern population suggests that polymorphism in P. trucha is present early during ontogeny. We conclude that while there are some strong parallels among lakes in the development of a trophic polymorphism, differences in environmental ­conditions and/or colonization history have led to substantial differences in the evolutionary history, resulting in ­different ecological roles of common morphotypes within different lakes.  © 2003 The Linnean Society of London . Biological Journal of the Linnean Society , 2003, 78 , 497–515.     

The relationship between leaf composition and morphology at elevated CO2 concentrations

The composition and morphology of leaves exposed to elevated [CO₂] usually change so that the leaf nitrogen (N) per unit dry mass decreases and the leaf dry mass per unit area increases. However, at ambient [CO₂], leaves with a high leaf dry mass per unit area usually have low leaf N per unit dry mass. Whether the changes in leaf properties induced by elevated [CO₂] follow the same overall pattern as that at ambient [CO₂] has not previously been addressed. Here we address this issue by using leaf measurements made at ambient [CO₂] to develop an empirical model of the composition and morphology of leaves. Predictions from that model are then compared with a global database of leaf measurements made at ambient [CO₂]. Those predictions are also compared with measurements showing the impact of elevated [CO₂]. In the empirical model both the leaf dry mass and liquid mass per unit area are positively correlated with leaf thickness, whereas the mass of C per unit dry mass and the mass of N per unit liquid mass are constant. Consequently, both the N:C ratio and the surface area:volume ratio of leaves are positively correlated with the liquid content. Predictions from that model were consistent with measurements of leaf properties made at ambient [CO₂] from around the world. The changes induced by elevated [CO₂] follow the same overall trajectory. It is concluded that elevated [CO₂] enhances the rate at which dry matter is accumulated but the overall trajectory of leaf development is conserved.     

Microsporidian Spore Envelope Keratins Phosphorylate and Disassemble During Spore Activation

The microsporidian spore stage of the nerve parasite, Spraguea lophii, consists of outer envelope stabilized in part by keratins, including K4 and K13. The nonepidermal K4 and K13 keratins were found only in the spore envelope and were absent in the internal microsporidian sporoplasm. At the time of spore activation, the keratin-based outer spore envelope assemblage dissociated and became phosphorylated when the spores were placed in the presence of labeled ATP. Verapamil or lanthanum, agents which block S. lophii spore activation, also blocked spore envelope keratin disassembly and phosphorylation when the spores were incubated in activation medium with labeled ATP. However, after the removal of the verapamil or lanthanum, the spores regained the capacity to activate in discharge medium and the keratin analogues appeared to dissociate and phosphorylate.