Affichage des résultats 421 à 440 sur 1371 au total
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Postcopulatory sexual selection is thought to be an important force driving the evolution of sperm design. Yet, despite numerous theoretical and empirical studies of the relationship between sperm design and sperm competition (as one postcopulatory sexual selection mechanism) is still poorly understood. A series of comparative studies in several taxonomic groups have found inconclusive and sometimes even contradictory results suggesting that in different taxonomic groups the evolution of sperm design in the context of sperm competition show markedly different patterns. We conducted the largest to date comparative study including over 250 species of passerine birds to investigate the relationship between sperm design and sperm competition and to determine the evolutionary rate of sperm design as a sexually selected trait. We did so in three ways: (I) we investigated the relationship between sperm design and sperm competition at different phylogenetic levels its association with the underlying phylogeny; (II) we tested the interspecific, the intraspecific and the intra-male variation of sperm design in the context of sperm competition; (III) we performed experiments to improve our understanding of the behavioural mechanisms influencing sperm design and function. Our results provide new insights into the evolution of sperm design as a sexually selected trait.
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Genetic robustness, the preservation of an optimal phenotype in the face of mutations, is critical to the understanding of evolution as phenotypically expressed genetic variation is the fuel of natural selection. The origin of genetic robustness, whether it evolves directly by natural selection or it is a correlated byproduct of other phenotypic traits, is, however, unresolved. Examining micro-RNA (miRNA) genes of several eukaryotic species, Borenstein and Ruppin (Borenstein E, Ruppin E. 2006. Direct evolution of genetic robustness in microRNA. Proc Natl Acad Sci USA. 103: 6593) showed that the structure of miRNA precursor stem loops exhibits significantly increased mutational robustness in comparison with a sample of random RNA sequences with the same stem-loop structure. The observed robustness was found to be uncorrelated with traditional measures of environmental robustness--implying that miRNA sequences show evidence of the direct evolution of genetic robustness. These findings are surprising as theoretical results indicate that the direct evolution of robustness requires high mutation rates and/or large effective population sizes only found among RNA viruses, not multicellular eukaryotes. We demonstrate that the sampling method used by Borenstein and Ruppin introduced significant bias that lead to an overestimation of robustness. Introducing a novel measure of environmental robustness based on the equilibrium thermodynamic ensemble of secondary structures of the miRNA precursor sequences, we demonstrate that the biophysics of RNA folding induces a high level of correlation between genetic (mutational) and environmental (thermodynamic) robustness, as expected from the theory of plastogenetic congruence introduced by Ancel and Fontana (Ancel LW, Fontana W. 2000. Plasticity, evolvability, and modularity in RNA. J Exp Zool. 288: 242-283). In light of theoretical considerations, we believe that this correlation strongly suggests that genetic robustness observed in miRNA sequences is the byproduct of selection for environmental robustness.
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The seminar will focus on application of open source tools for analysis of biodiversity data. The focus is put on R statistical computing environment and its various Environometrics and Spatial packages. There are three main opportunities for using R: (1) vitality and high speed of development of R, (2) academic openness of developers and their willingness to collaborate, and (3) increasing sympathy for spatial data analysis and visualization. Spatial statisticians generally believe that geostatistical techniques are suited only for modeling of features that are inherently continuous (spatial fields); discrete objects (points, lines, polygons) should be analyzed using point pattern analysis and similar methods. A method is suggested that tries to bridge this gap by combining the strong points of various packages in R with the geographical analysis capabilities of the open source GIS. The presenter will demonstrates how to combine geostatistical techniques with conceptually different techniques --- point pattern analysis and Niche analysis --- to allow prediction of species' distributions using regression-kriging.References: 1. Hengl, T., Sierdsema, H., Radovic, A., Dilo, A., 2009. Spatial prediction of species' distributions from occurrence-only records: combining point pattern analysis, ENFA and regression-kriging. Ecological Modelling, in press. 2. Hengl, T., 2007. A Practical Guide to Geostatistical Mapping of Environmental Variables. EUR 22904 EN Scientific and Technical Research series, Office for Official Publications of the European Communities, Luxemburg, 143 pp. ISBN: 978-92-79-06904-8 3. Hengl T., Heuvelink G.B.M., Rossiter D.G., 2007. About regression-kriging: from equations to case studies. Computers and Geosciences, 33(10): 1301-1315
Biological networks of large dimensions, with their diagram of interactions, are often well represented by a Boolean model with a family of logical rules. An advantage of Boolean and discrete modelling is the possibility of fully characterizing all qualitative dynamical trajectories of a particular network, based simply on the structure of links and interactions between nodes. A biological network may have different qualitative behaviours in response to different conditions. For instance, in response to different inputs, the system may have a single steady state, or multiple steady states, or exhibit oscillatory behaviour. In this context, using the asynchronous transition graph of the Boolean network, we have developed a method for identifying the groups of active or operational interactions that are responsible for a given dynamic behaviour.As an example, a model of an apoptosis network will be analysed. Two core groups of elements and interactions are identified: they correspond to two different mechanisms that may be used by the cell for the decision between apoptosis or cell survival.
The distributions of the Trans-Himalayan large herbivores are fragmented, engendering a spatial heterogeneity in their species-richness. We capitalised on this natural-experiment situation to understand the niche dynamics of herbivores in relation to the number of sympatric species. We used the blue sheep Pseudois nayaur, a relatively widely distributed mountain ungulate, as a model species to address the issue. We selected three discrete valleys in three protected areas with almost similar environmental features but varying wild ungulate species richness, and studied the species' diet and habitat utilization in them. Habitat variables were observed in the field and microhistological faecal analysis was carried out to determine the habitat and diet widths of the animal in the three areas with different ungulate species richness. The habitat- and diet-niche widths were determined using the Shannon's H' Index. The results showed that habitat width of blue sheep has a negative relationship with the number of sympatric species. However, contrary to our expectation, there was a hump-shaped relationship between blue sheep's diet width and the sympatric species richness, with the diet width being narrower in areas of allopatry as well as in areas with greater number of sympatric species, and the widest diet spectrum in areas with moderate species richness. We suspect that the narrow diet width in allopatry is out of choice, while it is out of necessity in areas with greater number of sympatric species due to resource partitioning. We suggest that interactions with sympatric species lead to niche adjustment of mountain ungulates, and underscore the importance of including biotic interactions in species distribution models, which have often been neglected.