Elastic interaction between "hard'' or "soft" pointwise inclusions on biological membranes

We calculate the induced elastic-interaction between pointwise membrane inclusions that locally interact up to quadratic order with the membrane curvature tensor. For isotropic inclusions, we recover the usual interaction proportional to the inverse fourth power of the separation, however with a prefactor showing a non-trivial dependence on the rigidity $\Gamma$ of the quadratic potential. In the large $\Gamma$ limit, corresponding to ``hard'' inclusions, we recover the standard prefactor first obtained by Goulian et al. [Europhys. Lett. \textbf{22}, 145 (1993)]. In the small $\Gamma$ limit, corresponding to "soft" inclusions, we recover the recent result of Marchenko and Misbah [Eur. Phys. J. E \textbf{8}, 477 (2002)]. This shows that the latter result bears no fundamental discrepancy with previous works, but simply corresponds to the limit of soft inclusions. We discuss how the same inclusion can be depicted as hard or soft according to the degree of coarse-graining of the pointwise description. Finally, we argue that conical transmembrane proteins should be fundamentally considered as hard inclusions.Comment: 6 page

Guerre des sexes chez une fourmi : reproduction clonale des mâles et des reines

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Lack of inbreeding avoidance in the Argentine ant Linepithema humile

Although workers might increase their inclusive fitness by favoring closer over more distant kin, evidence suggest that nepotism generally does not occur within colonies of social insects. It has been suggested that this may be due to the cost of recognition errors. We tested whether recognition occurs in a system where a better than random ability to recognize kin should be selected for. Using DNA microsatellites, we show that sexuals of the Argentine ant Linepithema humile fail to use genetic cues to avoid sib-mating. When offspring of two queens were allowed to mate, the percentage of matings among siblings was not significantly lower than expected under the hypothesis of random mating. The finding that sexuals fail to use genetic cues to avoid sib-matings cannot be attributed to the cost of recognition errors because any recognition system that would lead to a better than random ability to avoid sib-mating should be selected for when there are costs to inbreeding. These data are thus consistent with the view that kin recognition mediated solely by genetic cues might be intrinsically error prone within colonies of social insect

Fast and accurate 3D object recognition directly from digital holograms

International audiencePattern recognition methods can be used in the context of digital holography to perform the task of object detection, classification, and position extraction directly from the hologram rather than from the reconstructed optical field. These approaches may exploit the differences between the holographic signatures of objects coming from distinct object classes and/or different depth positions. Direct matching of diffraction patterns, however, becomes computationally intractable with increasing variability of objects due to the very high dimensionality of the dictionary of all reference diffraction patterns. We show that most of the diffraction pattern variability can be captured in a lower dimensional space. Good performance for object recognition and localization is demonstrated at a reduced computational cost using a low-dimensional dictionary. The principle of the method is illustrated on a digit recognition problem and on a video of experimental holograms of particles

Sperm competition in Cataglyphis desert ants.

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Inverse problem approach for particle digital holography: accurate location

International audienceOptical holography allows to record tridimensionnal informations of a scene using only one 2D sensor. Physical optics allows to analyticaly modelise hologram formation according to objects parameters (position, size, shape...). In simple objects case (e.g. spherical particles), the model is reduced to few parameters (four per particles: x,y,z,radius). Using inverse problem approach, it is possible to determine these parameters resolving a global optimization problem. This new approach is more efficient than classical method : particle parameters estimation is far more precise and it is possible to localize particles outside of the camera field of view. The presented method achieves to detect particles in an area sixteenth times wider than the CCD field of view with equal precision on both simulated and real digital holograms. Moreover strong improvements in the precision of the localization of the particles were noticed, particularly along the depth dimension

Reconstruction of the rose of directions from a digital micro-hologram of fibers

International audienceDigital holography makes it possible to acquire quickly the interference patterns of objects spread in a volume. The digital processing of the fringes is still too slow to achieve on line analysis of the holograms. We describe a new approach to obtain information on the direction of illuminated objects. The key idea is to avoid reconstruction of the volume followed by classical three-dimensional image processing. The hologram is processed using a global analysis based on autocorrelation. A fundamental property of diffraction patterns leads to an estimate of the mean geometric-covariogram (MGC) of the objects projections. The rose of directions is connected with the MGC through an inverse problem. In the general case, only the 2D rose of the object projections can be reconstructed. The further assumption of unique-size objects gives access with the knowledge of this size to the 3D direction information. An iterative scheme is suggested to reconstruct the 3D rose in this special case. Results are provided on holograms of paper fibers

Twin-image noise reduction by phase retrieval in in-line digital holography

14 pagesInternational audienceIn-line digital holography conciles the applicative interest of a simple optical set-up with the speed, low cost and potential of digital reconstruction. We address the twin-image problem that arises in holography due to the lack of phase information in intensity measurements. This problem is of great importance in in-line holography where spatial elimination of the twin-image cannot be carried out as in off-axis holography. Applications in digital holography of particle fields greatly depend on its suppression to reach greater particle concentrations, keeping a sufficient signal to noise ratio in reconstructed images. We describe in this paper methods to improve numerically the reconstructed images by twin-image reduction. ©2005 COPYRIGHT SPI

Numerical suppression of the twin-image in in-line holography of a volume of micro-objects

This paper was published in Measurement Science and Technology and is made available as an electronic reprint with the permission of IOP. The paper can be found at the following URL on the IOP website: http://www.iop.org/EJ/journal/MSTInternational audienceWe address the twin-image problem that arises in holography due to the lack of phase information in intensity measurements. This problem is of great importance in in-line holography where spatial elimination of the twin image cannot be carried out as in off-axis holography. A unifying description of existing digital suppression methods is given in the light of deconvolution techniques. Holograms of objects spread in 3D cannot be processed through available approaches. We suggest an iterative algorithm and demonstrate its efficacy on both simulated and real data. This method is suitable to enhance the reconstructed images from a digital hologram of small objects