Alkyl-methylimidazolium ionic liquids affect the growth and fermentative metabolism of Clostridium sp

In this study, the effect of ionic liquids, 1-ethyl-3-methylimidazolium acetate [EMIM][Ac], 1-ethyl-3-methylimidazolium diethylphosphate [EMIM][DEP], and 1-methyl-3-methylimidazolium dimethylphosphate [MMIM][DMP] on the growth and glucose fermentation of Clostridium sp. was investigated. Among the three ionic liquids tested, [MMIM][DMP] was found to be least toxic. Growth of Clostridium sp. was not inhibited up to 2.5, 4 and 4 g L⁻¹ of [EMIM][Ac], [EMIM][DEP] and [MMIM][DMP], respectively. [EMIM][Ac] at <2.5 g L⁻¹, showed hormetic effect and stimulated the growth and fermentation by modulating medium pH. Total organic acid production increased in the presence of 2.5 and 2 g L⁻¹ of [EMIM][Ac] and [MMIM][DMP]. Ionic liquids had no significant influence on alcohol production at <2.5 g L⁻¹. Total gas production was affected by ILs at ?2.5 g L⁻¹ and varied with type of methylimidazolium IL. Overall, the results show that the growth and fermentative metabolism of Clostridium sp. is not impacted by ILs at concentrations below 2.5 g L⁻¹.     

Torque generation by one of the motor subunits of heterotrimeric kinesin-2

Heterotrimeric kinesin-2 motors [1] and [2] transport intraflagellar transport (IFT)-particles from the base to the tip of the axoneme to assemble and maintain cilia [3], [4], [5], [6], [7], [8], [9] and [10]. These motors are distinct in containing two non-identical motor subunits together with an accessory subunit [1], [11], [12], [13], [14] and [15]. We evaluated the significance of this organization by comparing purified wild type kinesin-2 holoenzymes that support IFT in vivo, with mutant trimers containing only one type of motor domain that do not support IFT in vivo. In motility assays, wild type kinesin-2 moved microtubules (MTs) at a rate intermediate between the rates supported by the two mutants. Interestingly, one of the mutants, but not the other mutant or the wild type protein, was observed to drive a persistent counter-clock-wise rotation of the gliding MTs. Thus one of the two motor domains of heterotrimeric kinesin-2 exerts torque as well as axial force as it moves along a MT, which may allow kinesin-2 to control its circumferential position around a MT doublet within the cilium.     

Short-term feed restriction decreases the systemic and intrafollicular concentrations of leptin and increases the vascularity of the preovulatory follicle in mares

The objective of this study was to evaluate the effect of short-term feed restriction on characteristics of the preovulatory follicle and on concentrations of systemic hormones (leptin, follicle-stimulating hormone [FSH], luteinizing hormone [LH]) and follicular fluid hormones and growth factors (leptin, estradiol, inhibin-A, activin-A, free insulin-like growth factor-1 [IGF1], insulin-like growth factor binding protein 2 [IGFBP2], vascular endothelial growth factor [VEGF]). Mares were submitted to a short-term (48 h) feed restriction when the expected ovulatory follicle was ≥27 mm (Hour 0) or served as controls (n = 8/group). No effect of short-term feed restriction was detected for systemic concentrations of FSH and LH and for intrafollicular concentrations of estradiol, activin-A, free IGF1, and IGFBP2. Restricted mares had decreased systemic concentrations of leptin at Hour 24 (approached significance) and at Hours 36 and 48 (P < 0.04). Follicular fluid of restricted mares at Hour 48 had lower (P < 0.02) concentration of leptin and a tendency (P < 0.1) for greater concentrations of inhibin-A and VEGF. The percentage of wall of the preovulatory follicle with color-Doppler signals of blood flow at Hour 48 was greater (P < 0.04) in the restricted group. Intrafollicular concentration of leptin (combined groups) was positively correlated with score for body condition (r = +0.60; P < 0.002) and negatively correlated with the percentage of the follicle wall with blood-flow signals (r = −0.60; P < 0.02). Our favored interpretation is that the preovulatory follicle seems to compensate for a nutritional deficiency by increasing the blood flow in the follicle wall.     

Immunoaffinity-based mass spectrometric characterization of the FMRFamide-related peptide family in the pericardial organ of Cancer borealis

The tetrapeptide, FMRFamide, was first discovered in 1977 in the molluscan nervous system and was found to affect the contractile force of molluscan cardiac muscle and other muscles [1]. Since then, numerous FMRFamide-related peptides (FaRPs) have been reported in both invertebrate and vertebrate species [2], [3], [4], [5], [6], [7], [8] and [9]. We have previously reported the detection and identification of numerous FaRPs in Cancer borealis pericardial organs (POs), one of the major neurosecretory structures in the crustaceans [2] and [3]. Here, we have developed two immunoaffinity-based methods, immunoprecipitation (IP) and immuno-dot blot screening assay, for the enrichment of FaRPs in C. borealis POs. A combined mass spectrometry (MS)-based approach involving both matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS) and nanoscale liquid chromatography coupled to electrospray ionization quadrupole time-of-flight tandem mass spectrometry (nanoLC-ESI-QTOF MS/MS) is used for a more comprehensive characterization of the FaRP family by utilizing high mass accuracy measurement and efficient peptide sequencing. Overall, 17 FMRFamide-related peptides were identified using these two complementary immuno-based approaches. Among them, three novel peptides were reported for the first time in this study.     

Association of VDR-gene variants with factors related to the metabolic syndrome,type 2 diabetes and vitamin D deficiency

The prevalence of metabolic syndrome (MetS) is rising alarmingly in the Saudi Arabian population. This study was conducted to assess the association between vitamin D receptor (VDR) polymorphisms and genetic susceptibility to components of the metabolic syndrome, type 2 diabetes mellitus (T2DM), and vitamin D deficiency in the Saudi Arabian population. Five-hundred-seventy Saudi individuals (285 MetS and 285 controls) were enrolled in this cross-sectional study. TaqI, BsmI, ApaI and FokI single nucleotide polymorphisms (SNPs) of the VDR gene were genotyped. The CT genotype and allele T of BsmI were associated with lower HDL-C levels [OR 0.60 (0.37, 0.96), p = 0.03] and obesity [OR 1.4 (1.0, 1.90), p = 0.04], respectively. The CT genotype and the dominant model CT + TT of BsmI were associated with increased risk of diabetes [OR 1.7 (1.2, 2.4), p = 0.007], and [OR 1.5 (1.1, 2.2), p = 0.01], respectively. On the contrary, the CT and CT + CC genotypes of FokI exhibited an association with a reduced risk of diabetes [OR 0.70 (0.49, 0.99), p = 0.05] and [OR 0.67 (0.48, 0.94), p = 0.02], respectively. The allele C of FokI was associated with lower risk of developing T2DM [OR 0.73 (0.56, 0.95), p = 0.02]. The prevalence of vitamin D deficiency was lower in subjects with the AC genotype of ApaI [OR, 0.34 (0.14, 0.80), p = 0.01]. Components of the MetS such as obesity, low HDL and T2DM were associated with the VDR gene. FokI and BsmI have protective and facilitative effects on the risk for T2DM, while the ApaI genotype was associated with reduced vitamin D deficiency.     

捕食者具有厌食性反应且食饵具有Allee效应的捕食系统

在Dubis动力系统的基础上,建立了具有Allee效应的捕食系统模型。对系统的稳定性进行了分析,受Allee效应的影响,食饵种群可能因为种群大小处于临界点以下而趋于灭绝。通过对系统进行模拟,结果表明:不受Allee效应的影响,系统的演化属于一种理想化的情形系统到达P(平衡)点的时间较不受Allee效应影响时系统到达P点的时间短,不利于生物的进化,而在Allee效应的影响下,系统的演化将达到一个平衡状态。由此,说明Allee效应为濒临灭绝物种的管理提供了重要的理论依据,对管理部门的决策有参考指导作用。     

Platinum-assisted preparation of PS ligands containing chiral centres; X-ray crystal structure of a dithiophosphinite platinum complex

The design and synthesis of some ligands containing P(III) bonded to sulfur (thiophosphinites) and chiral centres is described in this paper.Their complexes with platinum (II), [PtCl₂L], (L = bidentate dithiophosphinite) have been prepared and characterised and it has been shown that in many cases, the coordination to platinum protects these ligands from decomposition processes operated by moisture and oxygen. The first example of X-ray crystal structure of a platinum coordinated dithiophosphinite is described for complex cis-[PtCl₂L], [L = meso-2,3-bis(diphenylthiophosphinito)-dimethyl-succinate], 4a.     

Biochemical indices as correlates of recent growth in juvenile green turtles (Chelonia mydas)

Nucleic acid and protein concentrations and their ratios are increasingly used as correlates of nutritional condition and growth in marine species. However, their application in studies of reptile growth has not yet been validated. The green turtle (Chelonia mydas) is an endangered marine reptile for which assessing population health requires knowledge of demographic parameters such as individual growth rates. The purpose of this study was to evaluate a number of biochemical indices ([DNA], [RNA], RNA:DNA ratio, [protein], protein:DNA ratio, and RNA:protein ratio) in liver, heart, and blood as potential predictors of recent growth rate in juvenile green turtles under controlled feeding conditions. Intake of juvenile green turtles was manipulated over twelve weeks to obtain a range of growth rates. With the exception of [RNA]_blood, [DNA]_heart, and [protein]:[DNA]_liver, all biochemical indices demonstrated significant linear relationships with growth rate during the last 1.5 weeks of the study. The best single predictors of recent growth were hepatic [RNA] and [RNA]:[protein], which explained 66% and 49%, respectively, of the variance in growth. Contrary to expectations, these two indices were negatively correlated with growth rate. To investigate the possibility that hepatic [RNA] was higher in slow-growing turtles because of elevated expression of antioxidant genes, we quantified glutathione peroxidase activity and total antioxidant potential. Both measures of antioxidant function were affected by intake and growth histories, but these effects did not explain our results for hepatic RNA and protein concentrations. We developed a model that predicted 68% of the variance in specific growth rate (SGR) with the equation SGR = − 0.913(ln[RNA]_liver) + 17.689(Condition Index) + 4.316. In addition, our findings that [DNA] and [RNA]:[DNA] for blood were significantly correlated with SGR demonstrate the potential utility of minimally invasive tissue sampling that could facilitate instantaneous population monitoring.     

Impact of Allee effect on an eco-epidemiological system

In this paper, we propose a general ratio-dependent prey-predator model with disease in predator subject to the strong Allee effect in prey. We obtain the complete dynamics of both models: (a) full model with Allee effect; (b) full model without Allee effect. Model (a) may have more than one interior equilibrium point, but model (b) has only one interior equilibrium point. Numerical results reveal that the coexistence of all the populations at the endemic state is possible for both the models. But for the model with Allee effect, the coexistence can be destroyed by an increased supply of alternative food for the predators. It can also be proved that for the full model with Allee effect, the disease can be suppressed under certain parametric conditions. Also by comparing models (a) and (b), we conclude that Allee effect can create or destroy the interior attractor. Finally, we have studied the disease free-submodel (prey and susceptible predator model) with and without Allee effect. The comparative study between these two submodels leads to the following conclusions: 1) In the presence of Allee effect, the number of interior equilibrium points can change from zero to two whereas the submodel without Allee effect has unique interior equilibrium point; 2) Both with and without Allee effect, initial conditions play an important role on the survival and extinction of prey as well as its corresponding predator; 3) In the presence of Allee effect, bi-stability occurs with stable or periodic coexistence of prey and susceptible predator and the extinction of prey and susceptible predator; 4) Allee effect can generate or destroy the interior equilibrium points.     

Allee效应对具有生境恢复的集合种群的影响

Allee效应与种群的灭绝密切相关,其研究对生态保护和管理至关重要。Allee效应对物种续存是潜在的干扰因素,濒危物种更容易受其影响,可能会增加生存于生境破碎化斑块的濒危物种的死亡风险,因此研究Allee效应对种群的动态和续存的影响是必要的。从包含由生物有机体对环境的修复产生的Allee效应的集合种群模型出发,引入由其他机制形成的Allee效应,建立了常微分动力系统模型和基于网格模型的元胞自动机模型。通过理论分析和计算机模拟表明:(1)强Allee效应不利于具有生境恢复的集合种群的续存;(2)生境恢复有利于种群续存;(3)局部扩散影响了集合种群的空间结构、动态行为和稳定性,生境斑块之间的局部作用将会减缓或消除集合种群的Allee效应,有利于集合种群的续存。     

Effect of inorganic phosphate on the force and number of myosin cross-bridges during the isometric contraction of permeabilized muscle fibers from rabbit psoas

The relation between the chemical and mechanical steps of the myosin-actin ATPase reaction that leads to generation of isometric force in fast skeletal muscle was investigated in demembranated fibers of rabbit psoas muscle by determining the effect of the concentration of inorganic phosphate (Pi) on the stiffness of the half-sarcomere (hs) during transient and steady-state conditions of the isometric contraction (temperature 12°C, sarcomere length 2.5 μm). Changes in the hs strain were measured by imposing length steps or small 4 kHz oscillations on the fibers in control solution (without added Pi) and in solution with 3-20 mM added Pi. At the plateau of the isometric contraction in control solution, the hs stiffness is 22.8 ± 1.1 kPa nm⁻¹. Taking the filament compliance into account, the total stiffness of the array of myosin cross-bridges in the hs (e) is 40.7 ± 3.7 kPa nm⁻¹. An increase in [Pi] decreases the stiffness of the cross-bridge array in proportion to the isometric force, indicating that the force of the cross-bridge remains constant independently of [Pi]. The rate constant of isometric force development after a period of unloaded shortening (r_F) is 23.5 ± 1.0 s⁻¹ in control solution and increases monotonically with [Pi], attaining a maximum value of 48.6 ± 0.9 s⁻¹ at 20 mM [Pi], in agreement with the idea that Pi release is a relatively fast step after force generation by the myosin cross-bridge. During isometric force development at any [Pi], e and thus the number of attached cross-bridges increase in proportion to the force, indicating that, independently of the speed of the process that leads to myosin attachment to actin, there is no significant (>1 ms) delay between generation of stiffness and generation of force by the cross-bridges.     

On the mechanistic underpinning of discrete-time population models with Allee effect

The Allee effect means reduction in individual fitness at low population densities. There are many discrete-time population models with an Allee effect in the literature, but most of them are phenomenological. Recently, Geritz and Kisdi [2004. On the mechanistic underpinning of discrete-time population models with complex dynamics. J. Theor. Biol. 228, 261-269] presented a mechanistic underpinning of various discrete-time population models without an Allee effect. Their work was based on a continuous-time resource-consumer model for the dynamics within a year, from which they derived a discrete-time model for the between-year dynamics. In this article, we obtain the Allee effect by adding different mate finding mechanisms to the within-year dynamics. Further, by adding cannibalism we obtain a higher variety of models. We thus present a generator of relatively realistic, discrete-time Allee effect models that also covers some currently used phenomenological models driven more by mathematical convenience.     

Detecting and estimating rectification of gap junction conductance based on simulations of dual-cell recordings from a pair and a network of coupled cells

Gap junctions can exhibit rectification of conductance. Some reports use inequality of coupling coefficients as the first sign of the possible existence of rectification ( [Devor and Yarom, 2002], [Fan et al., 2005], [31], [Mann-Metzer and Yarom, 1999], [Nolan et al., 1999] and [Szabadics et al., 2001]). However, mathematical modeling and simulations of electrotonic coupling between an isolated pair of neurons showed conditions where the coupling coefficients were unreliable indicators of rectification. On the other hand, the transfer resistances were found to be reliable indicators of junctional rectification. The existing mathematical model of cell coupling ( [Bennett, 1966], [Devor and Yarom, 2002] and [Verselis and Veenstra, 2000]) was extended in order to measure rectification of the junctional conductances directly between dual-recorded neurons whether isolated or surrounded by a simulated 3-dimensional network of heterogeneous cells whose gap junctions offered parallel paths for current flow between the recorded neurons. The results showed that the transfer resistances could still detect rectification of the gap junction linking the dual-recorded neurons when embedded in a coupled cell network and that a mathematical model could estimate the conductances in both directions through this gap junction using only data that would be available from real dual-intracellular penetrations which allow electrophysiological recordings and intracellular staining. Rectification of gap junctions in unrecorded cells of a biologically realistic coupled cell network had negligible effects on the voltage responses of the dual-recorded neurons because of minimal current passing through these surrounding cells.     

FoxP3 genetic variants and risk of non-small cell lung cancer in the Chinese Han population

CD4⁺CD25⁺ regulatory T cell-mediated immunosuppression is one of the crucial mechanisms that tumor cells use to evade the immune system. The forkhead box P3 (FoxP3) gene regulates regulatory T-cell development and function and may modulate the susceptibility to non-small cell lung cancer (NSCLC). Because a single nucleotide polymorphism (SNP) within the FoxP3 gene (rs3761548 in the promoter region) is associated with susceptibility to Graves' disease, this study detected rs3761548 in a hospital-based case–control study. A total of 192 NSCLC patients and 259 healthy subjects were recruited for the polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analysis of FoxP3 SNP. The data showed that the A allele of rs3761548 significantly increased NSCLC risk (P = 0.000, OR = 2.32, 95%CI = 1.736–3.102). The AC genotype, AA genotype, and the combined A variant genotype (AA + AC) were also associated with a higher risk of NSCLC (OR [95%CI] = 2.147[1.419–3.247], 4.413[2.359–8.255], and 2.563[1.746–3.761], respectively). Moreover, a significantly higher frequency of AA + AC genotype was observed in patients with stage II NSCLC (OR, 2.053; 95%CI, 1.033–4.078). In conclusion, the data from the current study demonstrated for the first time the association of the FoxP3 SNP with a risk of developing NSCLC in the Chinese Han population.     

Constant proportion harvest policies: dynamic implications in the Pacific halibut and Atlantic cod fisheries

Overfishing, pollution and other environmental factors have greatly reduced commercially valuable stocks of fish. In a 2006 Science article, a group of ecologists and economists warned that the world may run out of seafood from natural stocks if overfishing continues at current rates. In this paper, we explore the interaction between a constant proportion harvest policy and recruitment dynamics. We examine the discrete-time constant proportion harvest policy discussed in Ang et al. (2009) and then expand the framework to include stock-recruitment functions that are compensatory and overcompensatory, both with and without the Allee effect.We focus on constant proportion policies (CPPs). CPPs have the potential to stabilize complex overcompensatory stock dynamics, with or without the Allee effect, provided the rates of harvest stay below a threshold. If that threshold is exceeded, CPPs are known to result in the sudden collapse of a fish stock when stock recruitment exhibits the Allee effect. In case studies, we analyze CPPs as they might be applied to Gulf of Alaska Pacific halibut fishery and the Georges Bank Atlantic cod fishery based on harvest rates from 1975 to 2007. The best fit models suggest that, under high fishing mortalities, the halibut fishery is vulnerable to sudden population collapse while the cod fishery is vulnerable to steady decline to zero. The models also suggest that CPP with mean harvesting levels from the last 30 years can be effective at preventing collapse in the halibut fishery, but these same policies would lead to steady decline to zero in the Atlantic cod fishery. We observe that the likelihood of collapse in both fisheries increases with increased stochasticity (for example, weather variability) as predicted by models of global climate change.     

Dangerously few liaisons: a review of mate-finding Allee effects

In this paper, we review mate-finding Allee effects from ecological and evolutionary points of view. We define ‘mate-finding’ as mate searching in mobile animals, and also as the meeting of gametes for sessile animals and plants (pollination). We consider related issues such as mate quality and choice, sperm limitation and physiological stimulation of reproduction by conspecifics, as well as discussing the role of demographic stochasticity in generating mate-finding Allee effects. We consider the role of component Allee effects due to mate-finding in generating demographic Allee effects (at the population level). Compelling evidence for demographic Allee effects due to mate-finding (as well as via other mechanisms) is still limited, due to difficulties in censusing rare populations or a failure to identify underlying mechanisms, but also because of fitness trade-offs, population spatial structure and metapopulation dynamics, and because the strength of component Allee effects may vary in time and space. Mate-finding Allee effects act on individual fitness and are thus susceptible to change via natural selection. We believe it is useful to distinguish two routes by which evolution can act to mitigate mate-finding Allee effects. The first is evolution of characteristics such as calls, pheromones, hermaphroditism, etc. which make mate-finding more efficient at low density, thus eliminating the Allee effect. Such adaptations are very abundant in the natural world, and may have arisen to avoid Allee effects, although other hypotheses are also possible. The second route is to avoid low density via adaptations such as permanent or periodic aggregation. In this case, the Allee effect is still present, but its effects are avoided. These two strategies may have different consequences in a world where many populations are being artificially reduced to low density: in the first case, population growth rate can be maintained, while in the second case, the mechanism to avoid Allee effects has been destroyed. It is therefore in these latter populations that we predict the greatest evidence for mate-finding Allee effects and associated demographic consequences. This idea is supported by the existing empirical evidence for demographic Allee effects. Given a strong effect that mate-finding appears to have on individual fitness, we support the continuing quest to find connections between component mate-finding Allee effects (individual reproductive fitness) and the demographic consequences. There are many reasons why such studies are difficult, but it is important, particularly given the increasing number of populations and species of conservation concern, that the ecological community understands more about how widespread demographic Allee effects really are, and why.     

Deceptively simple but simply deceptive - Caenorhabditis elegans lifespan studies: Considerations for aging and antioxidant effects

The nematode worm Caenorhabditis elegans (C. elegans) is increasingly popular as a model organism for aging studies as well as for testing antioxidants and other compounds for effects on longevity. However, results in the literature are sometimes confusing and contradictory [1], [2], [3] and [4]. This review introduces C. elegans as a model organism, discusses aspects that make it attractive for aging and antioxidant research, and addresses some problems and potential artifacts.     

In vivo differential effects of fasting, re-feeding, insulin and insulin stimulation time course on insulin signaling pathway components in peripheral tissues

Background: Components of the insulin receptor signaling pathway are probably some of the best studied ones. Even though methods for studying these components are well established, the in vivo effects of different fasting regimens, and the time course of insulin receptor phosphorylation and that of its downstream components in insulin-sensitive peripheral tissues have not been analyzed in detail. Rationale: When assessing insulin signaling, it may be beneficial to drive insulin levels as low as possible by performing an overnight fast before injecting a supra-physiological dose of insulin. Recent studies have shown however that 5 or 6 h fast in mice is sufficient to assess physiological responses to insulin and/or glucose in glucose tolerance tests [1], insulin tolerance tests [2], [3] and [4] and euglycemic hyperinsulinemic clamp studies [5] and [6]. Moreover, mice are nocturnal feeders, with ∼70% of their daily caloric intake occurring during the dark cycle [5], and their metabolic rate is much higher than humans. Therefore, an overnight fast in mice is closer to starvation than just food withdrawal. Thus our aim was to assess insulin signaling components from the insulin receptor to downstream targets IRS1, Akt/PKB, GSK3, Erk1/2 and ribosomal protein S6 in muscle, liver and adipose tissue in 5 h versus 16 h (overnight) fasted mice, and the time course (0-30 min) of these phosphorylation events. We also assessed whether re-feeding under 5 h and 16 h fasting conditions was a more robust stimulus than insulin alone. Conclusions: Our study determines that a short food withdrawal from mice, for a period of 5 h, results in a similar insulin-stimulated response in phosphorylation events as the long overnight fast, presenting a more physiological experimental set up. We also demonstrate that in vivo, insulin-stimulated phosphorylation of its signaling components is different between different peripheral tissues, and depending on the tissue(s) and protein(s) of interest, an appropriate time course should be chosen.     

Modeling non-homologous end joining

Non-homologous end joining (NHEJ) is an important DNA repair pathway for DNA double-strand breaks. Several proteins, including Ku, DNA-PKcs, Artemis, XRCC4/Ligase IV and XLF, are involved in the NHEJ for the DNA damage detection, DNA free end processing and ligation. The classical model of NHEJ is a sequential model in which DNA-PKcs is first recruited by the Ku bound DNA prior to any other repair proteins. Recent experimental study ( [McElhinny et al., 2000], [Costantini et al., 2007], [17] and [Yano and Chen, 2008]) suggested that the recruitment ordering is not crucial. In this work, by proposing a mathematical model in terms of biochemical reaction network and performing stability and related analysis, we demonstrate theoretically that if DSB repair pathway independent of DNA-PKcs exists, then the classical sequential model and new two-phase model are essentially indistinguishable in the sense that DSB can be repaired thoroughly in both models when the repair proteins are sufficient.     

Backward bifurcations and strong Allee effects in matrix models for the dynamics of structured populations

In nonlinear matrix models, strong Allee effects typically arise when the fundamental bifurcation of positive equilibria from the extinction equilibrium at r=1 (or R₀=1) is backward. This occurs when positive feedback (component Allee) effects are dominant at low densities and negative feedback effects are dominant at high densities. This scenario allows population survival when r (or equivalently R₀) is less than 1, provided population densities are sufficiently high. For r>1 (or equivalently R₀>1) the extinction equilibrium is unstable and a strong Allee effect cannot occur. We give criteria sufficient for a strong Allee effect to occur in a general nonlinear matrix model. A juvenile–adult example model illustrates the criteria as well as some other possible phenomena concerning strong Allee effects (such as positive cycles instead of equilibria).