Insights on bias and information in group-level studies

Ecological and aggregate data studies are examples of group-level studies. Even though the link between the predictors and outcomes is not preserved in these studies, inference about individual-level exposure effects is often a goal. The disconnection between the level of inference and the level of analysis expands the array of potential biases that can invalidate the inference from group-level studies. While several sources of bias, specifically due to measurement error and confounding, may be more complex in group-level studies, two sources of bias, cross-level and model specification bias, are a direct consequence of the disconnection. With the goal of aligning inference from individual versus group-level studies, I discuss the interplay between exposure and study design. I specify the additional assumptions necessary for valid inference, specifically that the between- and within-group exposure effects are equal. Then cross-level inference is possible. However, all the information in the group-level analysis comes from between-group comparisons. Models where the group-level analysis provides even a small percentage of information about the within-group exposure effect are most susceptible to model specification bias. Model specification bias can be even more serious when the group-level model isn't derived from an individual-level model.     

Alterations in cell membrane properties caused by perfluorinated compounds

The recent detection of perfluorinated compounds (PFCs) in wildlife from even remote locations has spurred interest in the environmental occurrence and effects of these chemicals. While the global distribution of PFCs is increasingly understood, there is still little information available on their effects on wildlife. The amphiphillic nature of PFCs suggests that their effects could be primarily on cell membranes. In this study we measured the effects of PFCs on membrane fluidity and mitochondrial membrane potential using flow cytometry and effects on membrane permeability using cell bioassay procedures (H4IIE, MCF-7, PLHC-1). Of the PFCs tested, only perfluorooctane sulfonic acid (PFOS) increased the permeability of cell membranes to the hydrophobic ligands used. Three PFCs were tested in the membrane fluidity assay: PFOS, perfluorohexane sulfonic acid (PFHS), and perfluorobutane sulfonic acid (PFBS). PFOS increased membrane fluidity in fish leukocytes in a dose-dependent fashion, while PFHS and PFBS had no effect in the concentration range tested. The lowest effective concentrations for the membrane fluidity effects of PFOS were 5-15 mg/l. Effects on mitochondrial membrane potential occurred in the same concentration range as effects on membrane fluidity. This suggests that PFOS effects membrane properties at concentrations below those associated with other adverse effects.     

Morphological manipulation of PC liposome by controlled bolaamphiphile doping

The morphological characterization of aqueous dispersions of PC amphiphile and bolaamphiphile AEC was observed by transmission electron microscopy, the measurement of the liposomal membrane fluidity, differential scanning calorimetry, 5(6)-CF release from liposome and zeta potential measurement. Results indicate that the bolaamphiphile AEC can be included within conventional egg-PC liposome bilayer, which leads to the decrease of liposomal membrane fluidity (P) and the release behavior of 5(6)-CF. This behavior could be due to the property of bolaamphiphile AEC and the good miscibility of bolaamphiphile AEC with PC.     

Multilocus models of sympatric speciation: Bush versus Rice versus Felsenstein

Abstract. In populations of phytophagous insects that use the host plant as a rendezvous for mating, divergence in host preference could lead to sympatric speciation. Speciation requires the elimination of "generalist" genotypes, that is, those with intermediate host preference. This could occur because such genotypes have an inherent fitness disadvantage, or because preference alleles become associated with alleles that are oppositely selected on the two hosts. Although the former mechanism has been shown to be plausible, the latter mechanism has not been studied in detail. I consider a multilocus model (the "Bush model") in which one set of biallelic loci affects host preference, and a second set affects viability on the hosts once chosen. Alleles that increase viability on one host decrease viability on the other, and all loci are assumed to be unlinked. With moderately strong selection on the viability loci, preference alleles rapidly become associated with viability alleles, and the population splits into two reproductively isolated host specialist populations. The conditions for speciation to occur in this model, as measured by the strength of selection required, are somewhat more stringent than in a model in which preference and viability are controlled by the same loci (one-trait model). In contrast, the conditions are much less stringent than in a model in which speciation requires buildup of associations between viability loci and loci controlling a host-independent assortative mating trait (canonical two-trait model). Moreover, in the one-trait model, and to a lesser extent the Bush model, the strength of selection needed to initiate speciation is only slightly greater than that needed to complete it. This indicates that documenting instances of sympatric species that are reproductively isolated only by host or habitat preference would provide evidence for the plausibility of sympatric speciation in nature.     

Divergent selection drives the adaptive radiation of crossbills

Abstract Knowledge of how phenotype influences fitness is necessary if we are to understand the basis of natural selection and how natural selection contributes to adaptive radiations. Here I quantify selection on a wild population of red crossbills ( Loxia curvirostra complex) in the South Hills, Idaho. Bill depth is the target of selection and selection on bill depth is stabilizing. I then show how fitness is related to both bill depth and performance. I use these and previously published relationships to estimate a fitness surface for five species of red crossbills that are part of an ongoing adaptive radiation in western North America. The fitness surface for crossbills has distinct peaks and valleys, with each crossbill species residing on or very near the summits. This work strongly supports a key tenet of the ecological theory of adaptive radiations; namely, divergent selection for utilizing alternative resources is the ultimate cause of adaptive radiations.     

Early apoptosis-related changes triggered by HSV-1 in individual neuronlike cells

Early events of apoptosis following HSV-1 infection were investigated at the single-cell level using intensified fluorescence digital-imaging microscopy. The results provide evidence that infection of differentiated ND7 neuronlike cells by HSV-1 triggers detectable alterations indicative of physiological changes associated with the early stages of apoptosis. Less than 1 h after infection with HSV-1 (KOS strain) or K26GFP (GFP being fused to HSV-1 capsid protein VP26) we observed (i) moderate decrease in mitochondrial membrane potential (about 20%), (ii) exposure of phosphatidyl serine, (iii) morphological change in the mitochondria that became spherical instead of filamentous, and (iv) activation of caspase-8. Within 3 h changes reverted to normal, which indicated that apoptosis was counteracted very early following HSV-1 infection. Similar results were obtained with KOS-TK27GFP, lacking TK and UL24 proteins, suggesting that TK and UL24 play no role in apoptosis. In Vero cells mitochondrial changes characteristic of the apoptotic process were not observed following HSV-1 infection. The UV-inactivated K26GFP had the capacity to induce apoptosis in neuronlike cells. This real-time multiparametric analysis, in combination with relevant viral mutants, could be a useful approach for dissecting the roles of various viral genes in modulating apoptotic pathways during infection.     

Regulation of brain mitochondrial H2O2 production by membrane potential and NAD(P)H redox state

Mitochondrial production of reactive oxygen species (ROS) at Complex I of the electron transport chain is implicated in the etiology of neural cell death in acute and chronic neurodegenerative disorders. However, little is known regarding the regulation of mitochondrial ROS production by NADH-linked respiratory substrates under physiologically realistic conditions in the absence of respiratory chain inhibitors. This study used Amplex Red fluorescence measurements of H2O2 to test the hypothesis that ROS production by isolated brain mitochondria is regulated by membrane potential (DeltaPsi) and NAD(P)H redox state. DeltaPsi was monitored by following the medium concentration of the lipophilic cation tetraphenylphosphonium with a selective electrode. NAD(P)H autofluorescence was used to monitor NAD(P)H redox state. While the rate of H2O2 production was closely related to DeltaPsi and the level of NAD(P)H reduction at high values of DeltaPsi, 30% of the maximal rate of H2O2 formation was still observed in the presence of uncoupler (p-trifluoromethoxycarbonylcyanide phenylhydrazone) concentrations that provided for maximum depolarization of DeltaPsi and oxidation of NAD(P)H. Our findings indicate that ROS production by mitochondria oxidizing physiological NADH-dependent substrates is regulated by DeltaPsi and by the NAD(P)H redox state over ranges consistent with those that exist at different levels of cellular energy demand.     

A theoretical framework of ecological phase transitions for characterizing tree-grass dynamics

This paper describes a theoretical framework of ecological phase transitions for modeling tree-grass dynamics and analyzing the shifts or phase transitions from one vegetation structure to another in the southern Texas landscape. This framework implements the integration of percolation theory, fractal geometry and phase transition theory as a method for modeling the spatial patterns of tree-grass dynamics, and nonlinear Markov non-equilibrium thermodynamic stability theory as a method for characterizing temporal tree-grass dynamics and phase transition. An historical sequence of aerial photographs at a Prosopis - thornscrub savanna parkland site in southern Texas was used to determine the parameters of the models. The preliminary analytical result accords well with current understanding and field survey of vegetation dynamics in the southern Texas landscape. The potential of such approaches and other relevant theories such as self-organized criticality and synergetics to vegetation dynamics is also discussed.     

Stimulation of voltage-dependent calcium channels during capacitation and by progesterone in human sperm

To fertilize, mammalian sperm must undergo two sequential steps that require activation of calcium entry mechanisms, capacitation and acrosomal exocytosis, induced in the latter case by the egg zona pellucida glycoprotein ZP3 or by progesterone. Voltage-dependent calcium channels (VDCC) could participate in these processes. Since patch clamp recordings are extremely difficult in mature sperm, the activity of VDCC has been alternatively analyzed with optical detectors of membrane potential and intracellular calcium in sperm populations. Using this approach, we previously reported that in human sperm there is a voltage-dependent calcium influx system that strongly indicates that human sperm are endowed with functional VDCC. In this study we developed evidence indicating that calcium influx through VDCC is significantly stimulated during sperm in vitro capacitation and by progesterone action, which is present in the follicular fluid that surrounds the egg. The observed effects of capacitation and progesterone on VDCC may be physiologically significant for sperm-egg interaction.     

Activation of substrate/product couples by medium-chain acyl-CoA dehydrogenase

Natural substrate/product binding activates medium-chain acyl-CoA dehydrogenase (MCAD) to accept electrons from its substrate by inducing a positive flavin midpoint potential shift. The energy source for this activation has never been fully elucidated. If ground-state alterations of the ligand, such as polarization, are entirely responsible for enzyme activation, the ligand potential should shift equally to that of the flavin but in the opposite direction. Ligand polarization is likely responsible for only a small portion of this activation. Here, thiophenepropionoyl- and furylpropionoyl-CoA analogs were used to directly measure the redox modulations of several ligand couples upon binding to MCAD. These measurements identified the thermodynamic contribution of ligand polarization to enzyme activation. Because the ligand potential alterations are significantly smaller than modulations in the flavin potential due to binding, other phenomena such as pK(a) changes, desolvation, and charge alterations are likely responsible for the thermodynamic modulations required for MCAD's activity.