Fish can infer social rank by observation alone

Transitive inference (TI) involves using known relationships to deduce unknown ones (for example, using A > B and B > C to infer A > C), and is thus essential to logical reasoning. First described as a developmental milestone in children, TI has since been reported in nonhuman primates, rats and birds. Still, how animals acquire and represent transitive relationships and why such abilities might have evolved remain open problems. Here we show that male fish (Astatotilapia burtoni) can successfully make inferences on a hierarchy implied by pairwise fights between rival males. These fish learned the implied hierarchy vicariously (as 'bystanders'), by watching fights between rivals arranged around them in separate tank units. Our findings show that fish use TI when trained on socially relevant stimuli, and that they can make such inferences by using indirect information alone. Further, these bystanders seem to have both spatial and featural representations related to rival abilities, which they can use to make correct inferences depending on what kind of information is available to them. Beyond extending TI to fish and experimentally demonstrating indirect TI learning in animals, these results indicate that a universal mechanism underlying TI is unlikely. Rather, animals probably use multiple domain-specific representations adapted to different social and ecological pressures that they encounter during the course of their natural lives.     

An unexpected cooling effect in Saturn's upper atmosphere

The upper atmospheres of the four Solar System giant planets exhibit high temperatures that cannot be explained by the absorption of sunlight. In the case of Saturn the temperatures predicted by models of solar heating are approximately 200 K, compared to temperatures of approximately 400 K observed independently in the polar regions and at 30 degrees latitude. This unexplained 'energy crisis' represents a major gap in our understanding of these planets' atmospheres. An important candidate for the source of the missing energy is the magnetosphere, which injects energy mostly in the polar regions of the planet. This polar energy input is believed to be sufficient to explain the observed temperatures, provided that it is efficiently redistributed globally by winds, a process that is not well understood. Here we show, using a numerical model, that the net effect of the winds driven by the polar energy inputs is not to heat but to cool the low-latitude thermosphere. This surprising result allows us to rule out known polar energy inputs as the solution to the energy crisis at Saturn. There is either an unknown--and large--source of polar energy, or, more probably, some other process heats low latitudes directly.     

Aggregation and vesiculation of membrane proteins by curvature-mediated interactions

Membrane remodelling plays an important role in cellular tasks such as endocytosis, vesiculation and protein sorting, and in the biogenesis of organelles such as the endoplasmic reticulum or the Golgi apparatus. It is well established that the remodelling process is aided by specialized proteins that can sense as well as create membrane curvature, and trigger tubulation when added to synthetic liposomes. Because the energy needed for such large-scale changes in membrane geometry significantly exceeds the binding energy between individual proteins and between protein and membrane, cooperative action is essential. It has recently been suggested that curvature-mediated attractive interactions could aid cooperation and complement the effects of specific binding events on membrane remodelling. But it is difficult to experimentally isolate curvature-mediated interactions from direct attractions between proteins. Moreover, approximate theories predict repulsion between isotropically curving proteins. Here we use coarse-grained membrane simulations to show that curvature-inducing model proteins adsorbed on lipid bilayer membranes can experience attractive interactions that arise purely as a result of membrane curvature. We find that once a minimal local bending is realized, the effect robustly drives protein cluster formation and subsequent transformation into vesicles with radii that correlate with the local curvature imprint. Owing to its universal nature, curvature-mediated attraction can operate even between proteins lacking any specific interactions, such as newly synthesized and still immature membrane proteins in the endoplasmic reticulum.     

Evolution of cooperation in a finite homogeneous graph

Recent theoretical studies of selection in finite structured populations have worked with one of two measures of selective advantage of an allele: fixation probability and inclusive fitness. Each approach has its own analytical strengths, but given certain assumptions they provide equivalent results. In most instances the structure of the population can be specified by a network of nodes connected by edges (that is, a graph), and much of the work here has focused on a continuous-time model of evolution, first described by ref. 11. Working in this context, we provide an inclusive fitness analysis to derive a surprisingly simple analytical condition for the selective advantage of a cooperative allele in any graph for which the structure satisfies a general symmetry condition ('bi-transitivity'). Our results hold for a broad class of population structures, including most of those analysed previously, as well as some for which a direct calculation of fixation probability has appeared intractable. Notably, under some forms of population regulation, the ability of a cooperative allele to invade is seen to be independent of the nature of population structure (and in particular of how game partnerships are specified) and is identical to that for an unstructured population. For other types of population regulation our results reveal that cooperation can invade if players choose partners along relatively 'high-weight' edges.     

Ecology: global warming and amphibian losses

Is global warming contributing to amphibian declines and extinctions by promoting outbreaks of the chytrid fungus Batrachochytrium dendrobatidis? Analysing patterns from the American tropics, Pounds et al. envisage a process in which a single warm year triggers die-offs in a particular area (for instance, 1987 in the case of Monteverde, Costa Rica). However, we show here that populations of two frog species in the Australian tropics experienced increasing developmental instability, which is evidence of stress, at least two years before they showed chytrid-related declines. Because the working model of Pounds et al. is incomplete, their test of the climate-linked epidemic hypothesis could be inconclusive.     

The structural basis of yeast prion strain variants

Among the many surprises to arise from studies of prion biology, perhaps the most unexpected is the strain phenomenon whereby a single protein can misfold into structurally distinct, infectious states that cause distinguishable phenotypes. Similarly, proteins can adopt a spectrum of conformations in non-infectious diseases of protein folding; some are toxic and others are well tolerated. However, our understanding of the structural differences underlying prion strains and how these differences alter their physiological impact remains limited. Here we use a combination of solution NMR, amide hydrogen/deuterium (H/D) exchange and mutagenesis to study the structural differences between two strain conformations, termed Sc4 and Sc37 (ref. 5), of the yeast Sup35 prion. We find that these two strains have an overlapping amyloid core spanning most of the Gln/Asn-rich first 40 amino acids that is highly protected from H/D exchange and very sensitive to mutation. These features indicate that the cores are composed of tightly packed beta-sheets possibly resembling 'steric zipper' structures revealed by X-ray crystallography of Sup35-derived peptides. The stable structure is greatly expanded in the Sc37 conformation to encompass the first 70 amino acids, revealing why this strain shows increased fibre stability and decreased ability to undergo chaperone-mediated replication. Our findings establish that prion strains involve large-scale conformational differences and provide a structural basis for understanding a broad range of functional studies, including how conformational changes alter the physiological impact of prion strains.     

'Rejuvenation' protects neurons in mouse models of Parkinson's disease

Why dopamine-containing neurons of the brain's substantia nigra pars compacta die in Parkinson's disease has been an enduring mystery. Our studies suggest that the unusual reliance of these neurons on L-type Ca(v)1.3 Ca2+ channels to drive their maintained, rhythmic pacemaking renders them vulnerable to stressors thought to contribute to disease progression. The reliance on these channels increases with age, as juvenile dopamine-containing neurons in the substantia nigra pars compacta use pacemaking mechanisms common to neurons not affected in Parkinson's disease. These mechanisms remain latent in adulthood, and blocking Ca(v)1.3 Ca2+ channels in adult neurons induces a reversion to the juvenile form of pacemaking. Such blocking ('rejuvenation') protects these neurons in both in vitro and in vivo models of Parkinson's disease, pointing to a new strategy that could slow or stop the progression of the disease.