The Fringe Detection Laser Metrology for the GRAVITY Interferometer at the VLTI

Interferometric measurements of optical path length differences of stars over large baselines can deliver extremely accurate astrometric data. The interferometer GRAVITY will simultaneously measure two objects in the field of view of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) and determine their angular separation to a precision of 10 micro arcseconds in only 5 minutes. To perform the astrometric measurement with such a high accuracy, the differential path length through the VLTI and the instrument has to be measured (and tracked since Earth's rotation will permanently change it) by a laser metrology to an even higher level of accuracy (corresponding to 1 nm in 3 minutes). Usually, heterodyne differential path techniques are used for nanometer precision measurements, but with these methods it is difficult to track the full beam size and to follow the light path up to the primary mirror of the telescope. Here, we present the preliminary design of a differential path metrology system, developed within the GRAVITY project. It measures the instrumental differential path over the full pupil size and up to the entrance pupil location. The differential phase is measured by detecting the laser fringe pattern both on the telescopes' secondary mirrors as well as after reflection at the primary mirror. Based on our proposed design we evaluate the phase measurement accuracy based on a full budget of possible statistical and systematic errors. We show that this metrology design fulfills the high precision requirement of GRAVITY.Comment: Proc. SPIE in pres

Uncovering the genomic basis of an extraordinary plant invasion

Invasive species are a key driver of the global biodiversity crisis, but the drivers of invasiveness, including the role of pathogens, remain debated. We investigated the genomic basis of invasiveness in Ambrosia artemisiifolia (common ragweed), introduced to Europe in the late 19th century, by resequencing 655 ragweed genomes, including 308 herbarium specimens collected up to 190 years ago. In invasive European populations, we found selection signatures in defense genes and lower prevalence of disease-inducing plant pathogens. Together with temporal changes in population structure associated with introgression from closely related Ambrosia species, escape from specific microbial enemies likely favored the plant's remarkable success as an invasive species.Peer reviewe

Spatiotemporal dynamics of nicotinic acetylcholine receptors and lipid platforms

Abstract: The relationships between neurotransmitter receptors and their membrane environment are complex, mutual (bidirectional) and physiologically important. Some of these relationships are established with subsets of the membrane lipid population, in the form of lipid platforms, lateral heterogeneities of the bilayer lipid having a dynamic chemical composition distinct from that of the bulk membrane. In addition to the equilibrium between the biosynthetic production, exocytic delivery and recycling of receptors on the one hand, and the endocytic internalization on the other, lateral diffusion, clustering and anchorage of receptors at the lipid platforms play key roles in determining the amount of active receptors at the synapse. Mobile receptors traffic between reservoir non-synaptic membranes and the synapse predominantly by thermally driven Brownian motion, and become immobilized at the perisynaptic region or the synapse proper by various mechanisms. These comprise: (a) clustering mediated by homotropic inter-molecular receptor-receptor associations; (b) heterotropic associations with non-receptor scaffolding proteins or the subjacent cytoskeletal meshwork, leading to diffusional “trapping”, and (c) protein-lipid interactions, particularly with the neutral lipid cholesterol. Preceded by a brief introduction on the currently used methods to study protein lateral mobility in membranes, this review assesses the contribution of some of these mechanisms to the supramolecular organization and dynamics of the paradigm neurotransmitter receptor of muscle and neuronal cells—the nicotinic acetylcholine receptor (nAChR). The translational mobility of nAChRs at these two cell surfaces differs in terms of diffusion coefficients and residence intervals at the synapse, which cover an ample range of time regimes. Neuronal α7 nAChRs exhibit diffusion coefficients similar to those of other neurotransmitter receptors and spend part of their lifetime confined to the perisynaptic region of glutamatergic (excitatory) and GABAergic (inhibitory) synapses; they may also be involved in the regulation of the dynamic equilibrium between excitation and inhibition in brain