Latitudinal gradient of nestedness and its potential drivers in stream detritivores

Understanding what mechanisms shape the diversity and composition of biological assemblages across broad‐scale gradients is central to ecology. Litter‐consuming detritivorous invertebrates in streams show an unusual diversity gradient, with α‐diversity increasing towards high latitudes but no trend in γ‐diversity. We hypothesized this pattern to be related to shifts in nestedness and several ecological processes shaping their assemblages (dispersal, environmental filtering and competition). We tested this hypothesis, using a global dataset, by examining latitudinal trends in nestedness and several indicators of the above processes along the latitudinal gradient. Our results suggest that strong environmental filtering and low dispersal in the tropics lead to often species‐poor local detritivore assemblages, nested in richer regional assemblages. At higher latitudes, dispersal becomes stronger, disrupting the nested assemblage structure and resulting in local assemblages that are generally more species‐rich and non‐nested subsets of the regional species pools. Our results provide evidence that mechanisms underlying assemblage composition and diversity of stream litter‐consuming detritivores shift across latitudes, and provide an explanation for their unusual pattern of increasing α‐diversity with latitude. When we repeated these analyses for whole invertebrate assemblages of leaf litter and for abundant taxa showing reverse or no diversity gradients we found no latitudinal patterns, suggesting that function‐based rather than taxon‐based analyses of assemblages may help elucidate the mechanisms behind diversity gradients.     

Modeling cancer glycolysis

Most cancer cells exhibit an accelerated glycolysis rate compared to normal cells. This metabolic change is associated with the over-expression of all the pathway enzymes and transporters (as induced by HIF-1α and other oncogenes), and with the expression of hexokinase (HK) and phosphofructokinase type 1 (PFK-1) isoenzymes with different regulatory properties. Hence, a control distribution of tumor glycolysis, modified from that observed in normal cells, can be expected. To define the control distribution and to understand the underlying control mechanisms, kinetic models of glycolysis of rodent AS-30D hepatoma and human cervix HeLa cells were constructed with experimental data obtained here for each pathway step (enzyme kinetics; steady-state pathway metabolite concentrations and fluxes). The models predicted with high accuracy the fluxes and metabolite concentrations found in living cancer cells under physiological O(2) and glucose concentrations as well as under hypoxic and hypoglycemic conditions prevailing during tumor progression. The results indicated that HK≥HPI>GLUT in AS-30D whereas glycogen degradation≥GLUT>HK in HeLa were the main flux- and ATP concentration-control steps. Modeling also revealed that, in order to diminish the glycolytic flux or the ATP concentration by 50%, it was required to decrease GLUT or HK or HPI by 76% (AS-30D), and GLUT or glycogen degradation by 87-99% (HeLa), or decreasing simultaneously the mentioned steps by 47%. Thus, these proteins are proposed to be the foremost therapeutic targets because their simultaneous inhibition will have greater antagonistic effects on tumor energy metabolism than inhibition of all other glycolytic, non-controlling, enzymes.     

Stable kinesin and dynein assemblies drive the axonal transport of mammalian prion protein vesicles

Kinesin and dynein are opposite-polarity microtubule motors that drive the tightly regulated transport of a variety of cargoes. Both motors can bind to cargo, but their overall composition on axonal vesicles and whether this composition directly modulates transport activity are unknown. Here we characterize the intracellular transport and steady-state motor subunit composition of mammalian prion protein (PrP(C)) vesicles. We identify Kinesin-1 and cytoplasmic dynein as major PrP(C) vesicle motor complexes and show that their activities are tightly coupled. Regulation of normal retrograde transport by Kinesin-1 is independent of dynein-vesicle attachment and requires the vesicle association of a complete Kinesin-1 heavy and light chain holoenzyme. Furthermore, motor subunits remain stably associated with stationary as well as with moving vesicles. Our data suggest a coordination model wherein PrP(C) vesicles maintain a stable population of associated motors whose activity is modulated by regulatory factors instead of by structural changes to motor-cargo associations.     

Macroinvertebrate Assemblages of an Andean High‐Altitude Tropical Stream: The Importance of Season and Flow

We studied the Piburja stream, a high‐altitude tropical stream in Ecuador. Our main goals were to determine whether the macroinvertebrate community composition and abundance differed between seasons, reaches and velocity patches. Likewise, we aimed to examine the importance of the hydrological regime in determining these differences. Flow was significantly higher in the wet season, but the stability of flow was higher in the dry season. There was a strong increase in macroinvertebrate community metrics (richness, density and diversity) in the dry season. Seasons and velocity patches better explained the community composition. Reaches did not show differences at the community level, but some taxa showed significant differences among reaches. Our findings differed from those published in previous studies that have suggested that mountain tropical streams are non‐seasonal. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Selective oviposition of the mayfly Baetis bicaudatus

Selective oviposition can have important consequences for recruitment limitation and population dynamics of organisms with complex life cycles. Temporal and spatial variation in oviposition may be driven by environmental or behavioral constraints. The goals of this study were to: (1) develop an empirical model of the substrate characteristics that best explain observed patterns of oviposition by Baetis bicaudatus (Ephemeroptera), whose females lay eggs under rocks protruding from high-elevation streams in western Colorado; and (2) test experimentally selective oviposition of mayfly females. We surveyed the number and physical characteristics of potential oviposition sites, and counted the number and density of egg masses in different streams of one watershed throughout two consecutive flight seasons. Results of surveys showed that variability in the proportion of protruding rocks with egg masses and the density of egg masses per rock were explained primarily by seasonal and annual variation in hydrology, and variation in geomorphology among streams. Moreover, surveys and experiments showed that females preferred to oviposit under relatively large rocks located in places with high splash associated with fast current, which may provide visual, mechanical or both cues to females. Experiments also showed that high densities of egg masses under certain rocks were caused by rock characteristics rather than behavioral aggregation of ovipositing females. While aggregations of egg masses provided no survival advantage, rocks selected by females had lower probabilities of desiccating during egg incubation. Our data suggest that even when protruding rocks are abundant, not all rocks are used as oviposition sites by females, due to female selectivity and to differences in rock availability within seasons, years, or streams depending on variation in climate and hydrogeomorphology. Therefore, specialized oviposition behavior combined with variation in availability of quality oviposition substrata has the potential to limit recruitment of this species.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Entamoeba histolytica: kinetic and molecular evidence of a previously unidentified pyruvate kinase

We report the kinetic characterization of a previously unidentified pyruvate kinase (PK) activity in extracts from Entamoeba histolytica trophozoites. This activity was about 74% of the activity of pyruvate phosphate dikinase. EhPK differed from most PKs in that its pH optimum was 5.5-6.5 and was inhibited by high PEP concentrations (1-5mM); these are concentrations at which PK is usually assayed. The optimal temperature was above 40 degrees C with negligible activity below 20 degrees C. EhPK exhibited hyperbolic kinetics with respect to both PEP (K(m) = 0.018 mM) and ADP (K(m) = 1.05 mM). However, it exhibited a sigmoidal behavior with respect to PEP at sub-saturating ADP concentrations. EhPK did not require monovalent cations for activity. Fructose-1,6 bisphosphate was a potent non-essential activator; it increased the affinity for ADP without modification of the V(max) or the affinity for PEP. Phosphate, citrate, malate, and alpha-ketoglutarate significantly inhibited EhPK activity. A putative EhPK gene fragment found in EhDNA was analyzed. The data indicate that E. histolytica trophozoites contain an active PK, which might contribute to the generation of glycolytic ATP for parasite survival.