University of Fribourg

About: University of Fribourg is a education organization based out in Fribourg, Freiburg, Switzerland. It is known for research contribution in the topics: Population & Glacier. The organization has 6040 authors who have published 14975 publications receiving 542500 citations. The organization is also known as: UNIFR & Universität Freiburg.

Evidence of climatic niche shift during biological invasion

Abstract: Niche-based models calibrated in the native range by relating species observations to climatic variables are commonly used to predict the potential spatial extent of species' invasion. This climate matching approach relies on the assumption that invasive species conserve their climatic niche in the invaded ranges. We test this assumption by analysing the climatic niche spaces of Spotted Knapweed in western North America and Europe. We show with robust cross-continental data that a shift of the observed climatic niche occurred between native and non-native ranges, providing the first empirical evidence that an invasive species can occupy climatically distinct niche spaces following its introduction into a new area. The models fail to predict the current invaded distribution, but correctly predict areas of introduction. Climate matching is thus a useful approach to identify areas at risk of introduction and establishment of newly or not-yet-introduced neophytes, but may not predict the full extent of invasions.

Expression Profile Matrix of Arabidopsis Transcription Factor Genes Suggests Their Putative Functions in Response to Environmental Stresses

Abstract: Numerous studies have shown that transcription factors are important in regulating plant responses to environmental stress. However, specific functions for most of the genes encoding transcription factors are unclear. In this study, we used mRNA profiles generated from microarray experiments to deduce the functions of genes encoding known and putative Arabidopsis transcription factors. The mRNA levels of 402 distinct transcription factor genes were examined at different developmental stages and under various stress conditions. Transcription factors potentially controlling downstream gene expression in stress signal transduction pathways were identified by observed activation and repression of the genes after certain stress treatments. The mRNA levels of a number of previously characterized transcription factor genes were changed significantly in connection with other regulatory pathways, suggesting their multifunctional nature. The expression of 74 transcription factor genes responsive to bacterial pathogen infection was reduced or abolished in mutants that have defects in salicylic acid, jasmonic acid, or ethylene signaling. This observation indicates that the regulation of these genes is mediated at least partly by these plant hormones and suggests that the transcription factor genes are involved in the regulation of additional downstream responses mediated by these hormones. Among the 43 transcription factor genes that are induced during senescence, 28 of them also are induced by stress treatment, suggesting extensive overlap responses to these stresses. Statistical analysis of the promoter regions of the genes responsive to cold stress indicated unambiguous enrichment of known conserved transcription factor binding sites for the responses. A highly conserved novel promoter motif was identified in genes responding to a broad set of pathogen infection treatments. This observation strongly suggests that the corresponding transcription factors play general and crucial roles in the coordinated regulation of these specific regulons. Although further validation is needed, these correlative results provide a vast amount of information that can guide hypothesis-driven research to elucidate the molecular mechanisms involved in transcriptional regulation and signaling networks in plants.

Responses of monkey dopamine neurons during learning of behavioral reactions.

Abstract: 1. Previous studies have shown that dopamine (DA) neurons respond to stimuli of behavioral significance, such as primary reward and conditioned stimuli predicting reward and eliciting behavioral reactions. The present study investigated how these responses develop and vary when the behavioral significance of stimuli changes during different stages of learning. Impulses from DA neurons were recorded with movable microelectrodes from areas A8, A9, and A10 in two awake monkeys during the successive acquisition of two behavioral tasks. Impulses of DA neurons were distinguished from other neurons by their long duration (1.8-5.0 ms) and low spontaneous frequency (0.5-7.0 imp/s). 2. In the first task, animals learned to reach in a small box in front of them when it opened visibly and audibly. Before conditioning, DA neurons were activated the first few times that the empty box opened and animals reacted with saccadic eye movements. Neuronal and behavioral responses disappeared on repeated stimulus presentation. Thus neuronal responses were related to the novelty of an unexpected stimulus eliciting orienting behavior. 3. Subsequently, the box contained a small morsel of apple in one out of six trials. Animals reacted with ocular saccades to nearly every box opening and reached out when the morsel was present. One-third of 49 neurons were phasically activated by every door opening. The response was stronger when food was present. Thus DA neurons responded simultaneously to the sight of primary food reward and to the conditioned stimulus associated with reward. 4. When the box contained a morsel of apple on every trial, animals regularly reacted with target-directed eye and arm movements, and the majority of 76 DA neurons responded to door opening. The same neurons lacked responses to a light not associated with task performance that was illuminated at the position of the food box in alternate sessions, thus demonstrating specificity for the behavioral significance of stimuli. 5. The second task employed the operant conditioning of a reaction time situation in which animals reached from a resting key toward a lever when a small light was illuminated. DA neurons lacked responses to the unconditioned light. During task acquisition lasting 2-3 days, one-half of 25 DA neurons were phasically activated when a drop of liquid reward was delivered for reinforcing the reaching movement. In contrast, neurons were not activated when reward was delivered at regular intervals (2.5-3.5 s) but a task was not performed.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine responses comply with basic assumptions of formal learning theory

Abstract: According to contemporary learning theories, the discrepancy, or error, between the actual and predicted reward determines whether learning occurs when a stimulus is paired with a reward. The role of prediction errors is directly demonstrated by the observation that learning is blocked when the stimulus is paired with a fully predicted reward. By using this blocking procedure, we show that the responses of dopamine neurons to conditioned stimuli was governed differentially by the occurrence of reward prediction errors rather than stimulus‐reward associations alone, as was the learning of behavioural reactions. Both behavioural and neuronal learning occurred predominantly when dopamine neurons registered a reward prediction error at the time of the reward. Our data indicate that the use of analytical tests derived from formal behavioural learning theory provides a powerful approach for studying the role of single neurons in learning. Classic theories assume that predictive learning occurs whenever a stimulus is paired with a reward or punishment 1,2 . However, more recent analyses of associative learning argue that simple temporal contiguity between a stimulus and a reinforcer is not sufficient for learning and that a discrepancy between the reinforcer that is predicted by a stimulus and the actual reinforcer is also required 3‐6 . This discrepancy can be characterized as a ‘prediction error’ 7 . Presentations of surprising or unpredicted reinforcers generate positive prediction errors, and thereby support learning, whereas omissions of predicted reinforcers generate negative prediction errors and lead to reduction or extinction of learned behaviour. Expected reinforcers do not generate prediction errors and therefore fail to support further learning even when the stimulus is consistently paired with the reinforcer. Modelling studies have shown that neuronal messages encoding prediction errors can act as explicit teaching signals for modifying the synaptic connections that underlie associative learning 8‐17 .

Equilibrium cluster formation in concentrated protein solutions and colloids

Abstract: Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels). The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid-polymer systems and concentrated protein solutions. However, intriguing problems like the frequent occurrence of cluster phases remain. Here we report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid-polymer mixtures. We demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions.