Many eukaryotic cells are able to sense chemical gradients by directly measuring spatial concentration differences. The precision of such gradient sensing is limited by fluctuations in the binding of diffusing particles to specific receptors on the cell surface. Here, we explore the physical limits of the spatial sensing mechanism by modeling the chemotactic cell as an Ising spin chain subject to a spatially varying field. This allows us to derive the maximum likelihood estimators of the gradient parameters as well as explicit expressions for their asymptotic uncertainties. The accuracy increases with the cell's size and our results demonstrate that this accuracy be further increased by introducing a non-zero cooperativity between neighboring receptors. Thus, consistent with recent experimental data, it is possible for small bacteria to perform spatial measurements of gradients.Cells often direct their motion under the guidance of chemical gradients. This is essential for critical biological functions including neuronal development, wound repair and cancer spreading [1,2]. To detect gradients, small organisms like bacterial cells usually employ a temporal sensing strategy by measuring and comparing concentration signals over time along their swimming tracks [3][4][5][6]. In contrast, eukaryotic cells are suffciently large to implement a spatial sensing mechanism, as they can measure the concentration differences across their cell bodies. Measurements for both strategies are accomplished by specific cellsurface receptors which diffusing chemical particles (ligands) can bind to. Spatial sensing among eukaryotes exhibits a remarkable sensitivity to gradients of merely 1-2% across the cell [7][8][9]. Given the dynamic fluctuations in ligand-receptor interaction, the receptor signal is inherently noisy, as demonstrated by single-cell imaging experiments [10,11]. This naturally raises a question concerning the reliability of spatial gradient sensing.In analyzing bacterial chemotaxis, Berg and Purcell showed that the minimal uncertainty of mean concentration measurements is set by the diffusion of ligand particles [12]. This work has been extended to include ligand-receptor binding effects and possible receptor cooperativity [13][14][15][16][17]. The spatial sensing program concerns the acquisition of information regarding the asymmetry in space (the gradient steepness and direction). The accuracy of gradient measurements should similarly be limited by physical laws governing diffusion and stochastic ligand-receptor dynamics. There have been some studies on the limits to spatial sensing, but either for idealized sensing mechanisms that ignore the receptor kinetics [16] or based on specific transduction models [18]. In this Letter, we address this problem in a general way using a statistical mechanical approach where we view the surface receptors as a (possibly coupled) spin chain and treat the chemical gradient as a perturbation field. By calculating the system's partition function, we are able to derive the ph...

The East Asian winter monsoon (EAWM)-related climate anomalies have shown large year-to-year variations in both the intensity and the meridional extent. The present study distinguishes the interannual variations of the low-latitude and mid-to high-latitude components of the EAWM to gain a better understanding of the characteristics and factors for the EAWM variability. Through composite analysis based on two indices representing the northern and southern components (modes) of the EAWM variability, the present study clearly reveals features unique to the northern and southern mode. The northern mode is associated with changes in the mid-to high-latitude circulation systems, including the Siberian high, the Aleutian low, the East Asian trough, and the East Asian westerly jet stream, whereas the southern mode is closely related to circulation changes over the global tropics, the North Atlantic, and North America. A strong northern mode is accompanied by positive, negative, and positive surface temperature anomalies in the Indochina Peninsula, midlatitude Asia, and northeast Russia, respectively. A strong southern mode features lower temperature over tropics and higher temperature over mid-to high-latitude Asia. While the southern mode is closely related to El Niño-Southern Oscillation (ENSO), the northern mode does not show an obvious relation to the tropical sea surface temperature (SST) change or to the North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) on the interannual time scale. Distinct snow cover and sea ice changes appear as responses to wind and surface temperature changes associated with the two modes and their effects on the EAWM variability need to be investigated in the future.

Late Silurian-early Devonian adakitic granodiorite, A-type and I-type granites in NW Junggar, NW China: Partial melting of mafic lower crust and implications for slab roll-back, Gondwana Research (2015), AbstractLate Silurian-early Devonian magmatism of the NW Junggar region in the Central Asian Orogenic Belt provides a critical geological record that is important for unraveling regional tectonic history and constraining geodynamic processes. In this study, we report results of U-Pb zircon ages and systematic geochemical data for late Silurian-early Devonian largely granitic rocks in NW Junggar, aiming to constrain their emplacement ages, origin and geodynamic significance. The magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks, e.g. adakitic granodiorite, K-feldspar granite, syenitic granite, gabbro and rhyrolite. U-Pb zircon ages suggest that the granitoids and gabbros were emplaced in the late Silurian-early Devonian (420-405 Ma). Adakitic granodiorites are calc-alkaline, characterized by high Sr (407-532 ppm), low Y (12.2-14.7 ppm), Yb (1.53-1.77 ppm), Cr (mostly < 8.00 ppm), Co (mostly <11.0 ppm) and Ni (mostly <4.10 ppm) and relatively high Sr/Y (31-42) ratios, analogous to those of modern adakites. K-feldspar granites and rhyolites are characterized by alkali-and Fe-enriched, with high Zr, Nb and Ga/Al ratios, geochemically similar to those of A-type granites. Syenitic granites show high alkaline (Na 2 O+K 2 O=8.39-9.34 wt.%) contents, low Fe # values (0.73-0.80) and are weakly peraluminous (A/CNK=1.00-1.07). Gabbros are characterized by low MgO (6.86-7.15 wt.%), Mg # (52-53), Cr (124-133 ppm) and Ni (84.7-86.6 ppm) contents. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 3 The geochemical characteristics of the gabbroic samples show affinity to both MORB-and arc-like sources. All granitoids have positive εNd(t) (+3.9 to +6.9) and zircon εHf(t) (+9.8 to +15.2) values and low initial 87 Sr/ 86 Sr ratios (0.7035-0.7043), with young T DM (Nd) (605-791 Ma) and T DM (Hf) (425-773 Ma) ages, suggesting significant addition of juvenile material. The adakitic granodiorites probably resulted from partial melting of mafic lower crust, leaving an amphibolite and garnet residue. The K-feldspar granites, rhyolites and syenitic granites probably formed from partial melting of the Xiemisitai mid-lower crust, while the gabbroic intrusion was probably generated by interactions between asthenospheric and metasomatized lithospheric mantle. Voluminous plutons of various types (adakites, A-type granites, I-type granites, and gabbros) formed during 420-405 Ma, and their isotopic data suggest significant additions of juvenile material. We propose that a slab roll-back model can account for the 420-405 Ma magmatic "flare up" in NW Junggar as well as an extensional setting.