Phylogenetic alpha and beta diversity in tropical tree assemblages along regional-scale environmental gradients in northwest South America

Aims Environmental gradients are drivers of species diversity; however, we know relatively little about the evolutionary processes underlying these relationships. A potentially powerful approach to studying diversity gradients is to quantify the phylogenetic structure within and between assemblages arrayed along broad spatial and environmental gradients. Here, we evaluate the phylogenetic structure of plant assemblages along an environmental gradient with the expectation that the habitat specialization of entire lineages is an important evolutionary pattern influencing the structure of tree communities along environmental gradients.Methods We evaluated the effect of several environmental variables on the phylogenetic structure of plant assemblages in 145 plots distributed in northwestern South America that cover a broad environmental gradient. The phylogenetic alpha diversity was quantified for each plot and the phylogenetic beta diversity between each pair of plots was also quantified. Both the alpha and beta diversity measures were then related to spatial and environmental gradients in the study system.Important findings We found that gradients in temperature and potential evapotranspiration have a strong relationship with the phylogenetic alpha diversity in our study system, with phylogenetic overdispersion in low temperatures and phylogenetic clustering at higher temperatures. Further, the phylogenetic beta diversity between two plots increases with an increasing difference in temperature, whereas annual precipitation was not a significant predictor of community phylogenetic turnover. We also found that the phylogenetic structure of the plots in our study system was related to the degree of seasonal flooding and seasonality in precipitation. In particular, more stressful environments such as dry forests and flooded forests showed phylogenetic clustering. Finally, in contrast with previous studies, we find that phylogenetic beta diversity was not strongly related to the spatial distance separating two forest plots, which may be the result of the importance of the three independent mountain ranges in our study system, which generate a high degree of environmental variation over very short distances. In conclusion, we found that environmental gradients are important drivers of both phylogenetic alpha and phylogenetic beta diversities in these forests over spatial distance.     

Mechanistic basis for differential inhibition of the F1Fo‐ATPase by aurovertin

The mitochondrial F₁F_o‐ATPase performs the terminal step of oxidative phosphorylation. Small molecules that modulate this enzyme have been invaluable in helping decipher F₁F_o‐ATPase structure, function, and mechanism. Aurovertin is an antibiotic that binds to the β subunits in the F₁ domain and inhibits F₁F_o‐ATPase‐catalyzed ATP synthesis in preference to ATP hydrolysis. Despite extensive study and the existence of crystallographic data, the molecular basis of the differential inhibition and kinetic mechanism of inhibition of ATP synthesis by aurovertin has not been resolved. To address these questions, we conducted a series of experiments in both bovine heart mitochondria and E. coli membrane F₁F_o‐ATPase. Aurovertin is a mixed, noncompetitive inhibitor of both ATP hydrolysis and synthesis with lower K_i values for synthesis. At low substrate concentrations, inhibition is cooperative suggesting a stoichiometry of two aurovertin per F₁F_o‐ATPase. Furthermore, aurovertin does not completely inhibit the ATP hydrolytic activity at saturating concentrations. Single‐molecule experiments provide evidence that the residual rate of ATP hydrolysis seen in the presence of saturating concentrations of aurovertin results from a decrease in the binding change mechanism by hindering catalytic site interactions. The results from these studies should further the understanding of how the F₁F_o‐ATPase catalyzes ATP synthesis and hydrolysis. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 830–840, 2009. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com     

Reactions of nitrite with hemoglobin measured by membrane inlet mass spectrometry

Membrane inlet mass spectrometry was used to observe nitric oxide in the well-studied reaction of nitrite with hemoglobin. The membrane inlet was submerged in the reaction solutions and measured NO in solution via its flux across a semipermeable membrane leading to the mass spectrometer detecting the mass-to-charge ratio m/z 30. This method measures NO directly in solution and is an alternate approach compared with methods that purge solutions to measure NO. Addition to deoxy-Hb(Fe(II)) (near 38 microM heme concentration) of nitrite in a range of 80 microM to 16 mM showed no accumulation of either NO or N(2)O(3) on a physiologically relevant time scale with a sensitivity near 1 nM. The addition of nitrite to oxy-Hb(Fe(II)) and met-Hb(Fe(III)) did not accumulate free NO to appreciable extents. These observations show that for several minutes after mixing nitrite with hemoglogin, free NO does not accumulate to levels exceeding the equilibrium level of NO. The presence of cyanide ions did not alter the appearance of the data; however, the presence of 2 mM mercuric ions at the beginning of the experiment with deoxy-Hb(Fe(II)) shortened the initial phase of NO accumulation and increased the maximal level of free, unbound NO by about twofold. These experiments appear consistent with no role of met-Hb(Fe(III)) in the generation of NO and an increase in nitrite reductase activity caused by the presumed binding of mercuric to cysteine residues. These results raise questions about the ability of reduction of nitrite mediated by deoxy-Hb(Fe(II)) to play a role in vasodilation.     

Rho kinase differentially regulates phosphorylation of nonmuscle myosin II isoforms A and B during cell rounding and migration

The actin-myosin cytoskeleton is generally accepted to produce the contractile forces necessary for cellular processes such as cell rounding and migration. All vertebrates examined to date are known to express at least two isoforms of non-muscle myosin II, referred to as myosin IIA and myosin IIB. Studies of myosin IIA and IIB in cultured cells and null mice suggest that these isoforms perform distinct functions. However, how each myosin II isoform contributes individually to all the cellular functions attributed to "myosin II" has yet to be fully characterized. Using isoform-specific small-interfering RNAs, we found that depletion of either isoform resulted in opposing migration phenotypes, with myosin IIA- and IIB-depleted cells exhibiting higher and lower wound healing migration rates, respectively. In addition, myosin IIA-depleted cells demonstrated impaired thrombin-induced cell rounding and undertook a more motile morphology, exhibiting decreased amounts of stress fibers and focal adhesions, with concomitant increases in cellular protrusions. Cells depleted of myosin IIB, however, were efficient in thrombin-induced cell rounding, displayed a more retractile phenotype, and maintained focal adhesions but only in the periphery. Last, we present evidence that Rho kinase preferentially regulates phosphorylation of the regulatory light chain associated with myosin IIA. Our data suggest that the myosin IIA and IIB isoforms are regulated by different signaling pathways to perform distinct cellular activities and that myosin IIA is preferentially required for Rho-mediated contractile functions.     

Acute in vitro hypoxia and high-altitude (4,559 m) exposure decreases leukocyte oxygen consumption

Hypoxia impairs metabolic functions by decreasing activity and expression of ATP-consuming processes. To separate hypoxia from systemic effects, we tested whether hypoxia at high altitude affects basal and PMA-stimulated leukocyte metabolism and how this compares to acute (15 min) and 24 h of in vitro hypoxia. Leukocytes were prepared at low altitude and ～24 h after arrival at 4559 m. Mitochondrial oxygen consumption (JO?) was measured by respirometry, oxygen radicals by electron spin resonance spectroscopy, both at a Po? = 100 mmHg (JO?,???) and 20 mmHg (JO?,??). Acute hypoxia of leukocytes decreased JO? at low altitude. Exposure to high altitude decreased JO?,???, whereas JO?,?? was not affected. Acute hypoxia of low-altitude samples decreased the activity of complexes I, II, and III. At high altitude, activity of complexes I and III were decreased when measured in normoxia. Stimulation of leukocytes with PMA increased JO?,??? at low (twofold) and high altitude (five-fold). At both locations, PMA-stimulated JO? was decreased by acute hypoxia. Basal and PMA-stimulated reactive oxygen species (ROS) production were unchanged at high altitude. Separate in vitro experiments performed at low altitude show that ～75% of PMA-induced increase in JO? was due to increased extra-mitochondrial JO? (JO?(,res); in the presence of rotenone and antimycin A). JO?(,res) was doubled by PMA. Acute hypoxia decreased basal JO?(,res) by ～70% and PMA-stimulated JO?(,res) by about 50% in cells cultured in normoxia and hypoxia (1.5% O?; 24 h). Conversely, 24 h in vitro hypoxia decreased mitochondrial JO?,??? and JO?,??, extra-mitochondrial, basal, and PMA-stimulated JO? were not affected. These results show that 24 h of high altitude but not 24 h in vitro hypoxia decreased basal leukocyte metabolism, whereas PMA-induced JO? and ROS formation were not affected, indicating that prolonged high-altitude hypoxia impairs mitochondrial metabolism but does not impair respiratory burst. In contrast, acute hypoxia impairs respiratory burst at either altitude.     

Litter loss triggers estrus in a nonsocial seasonal breeder

Sexually selected infanticide (SSI) is often presumed to be rare among seasonal breeders, because it would require a near immediate return to estrus after the loss of an entire litter during the mating season. We evaluated changes in reproductive strategies and the reproductive fate of females that experienced litter loss during the mating season in a seasonal breeder with strong evidence for SSI, the brown bear. First, we used a long‐term demographic dataset (1986–2011) to document that a large majority of females (>91%) that lose their entire litter during the mating season in fact do enter estrus, mate, and give birth during the subsequent birthing season. Second, we used high‐resolution movement data (2005–2011) to evaluate how females changed reproductive strategies after losing their entire litter during the mating season. We hypothesized that females would shift from the sedentary lifestyle typical for females with cubs‐of‐the‐year to a roam‐to‐mate behavior typical for receptive females in no more than a few (~3) days after litter loss. We found that females with cubs‐of‐the‐year moved at about 1/3 of the rate and in a less bimodal diurnal pattern than receptive females during the mating season. The probability of litter loss was positively related with movement rate, suggesting that being elusive and sedentary is a strategy to enhance cub survival rather than a relic of cub mobility itself. The movement patterns of receptive females and females after litter loss were indistinguishable within 1–2 days after the litter loss, and we illustrate that SSI can significantly reduce the female interbirth interval (50–85%). Our results suggest that SSI can also be advantageous for males in seasonally breeding mammals. We propose that infanticide as a male reproductive strategy is more prevalent among mammals with reproductive seasonality than observed or reported.