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Ultracold silver-atom collisions and the formation of silver dimers by photo- and magneto-association
Full text linkWe have performed three theoretical simulations relevant for describing collisions among laser-cooled silver atoms and for the formation of molecules from these colliding atoms. Firstly, we determined the relativistic electronic structure of molecules in ground and low-lying excited states. Secondly, we computed rotational and vibrational levels of the ground and excited electronic states as well as rovibrationally averaged electric transition dipole moments. Using this knowledge, we analyzed a simplified quantum-mechanical model of the one-photon photoassociation process to form electronically excited from microkelvin Ag atoms and make predictions for lineshapes and saturation effects as functions of laser frequency and intensity. Finally and thirdly, we performed coupled-channels calculations, numerical solutions of sets of coupled radial Schrödinger equations of ultracold ground-state Ag collisions in an external magnetic field. These calculations include the effects of two Born-Oppenheimer potentials as well as hyperfine Fermi-contact and Zeeman interactions. We discuss the expected range of -wave scattering lengths as well as strengths and distribution of Fano-Feshbach resonances as a function of the magnetic-field strength for the and isotopes. We highlight the periodicity of the scattering length with small changes in the depths of the Born-Oppenheimer potentials. The Fano-Feshbach resonances can be used to magneto-associate ultracold Ag atoms into weakly bound ground-state dimers
Never too Late: Gender Quotas in the Final Round of a Multistage Tournament
Full text linkAffirmative action policies have been shown to induce talented women to compete in laboratory contexts. However, evidence from actual policy changes is more ambiguous. While existing laboratory experiments have exclusively analyzed gender quotas in one-shot tournaments, we focus on a setting that models real life examples, such as quotas in corporate boards, more closely: quotas implemented at the final round of a multistage elimination contest. We find that later-stage quotas increase female participation already in the first round of the tournament, showing that women are responsive to changes in the option value of continued competition. Quotas also increase high-ability women’s representation among the final-stage competitors without significantly reducing entry among men, thereby promoting diversity without harming efficiency. We provide evidence for the importance of relative performance beliefs in determining the response to quotas among both genders
Estimating dynamic discrete-choice games of incomplete information
Full text linkWe investigate the estimation of models of dynamic discrete-choice games of incomplete information, formulating the maximum-likelihood estimation exercise as a constrained optimization problem that can be solved using state-of-the-art constrained optimization solvers. Under the assumption that only one equilibrium is played in the data, our approach avoids repeatedly solving the dynamic game or finding all equilibria for each candidate vector of the structural parameters. We conduct Monte Carlo experiments to investigate the numerical performance and finite-sample properties of the constrained optimization approach for computing the maximum-likelihood estimator, the two-step pseudo-maximum-likelihood estimator, and the nested pseudo-likelihood estimator, implemented by both the nested pseudo-likelihood algorithm and a modified nested pseudo-likelihood algorithm
Distinguishing direct interactions from global epistasis using rank statistics
Full text linkThe phenotypic effect of a mutation may depend on the genetic background in which it occurs, a phenomenon referred to as epistasis. One source of epistasis in proteins is direct interactions between residues in close physical proximity to one another. However, epistasis may also occur in the absence of specific interactions between amino acids if the genotype-to-phenotype map is nonlinear. Disentangling the contributions of these two phenomena—specific and global epistasis—from noisy, high-throughput mutagenesis experiments is highly nontrivial: The form of the nonlinearity is generally not known and model misspecification may lead to over- or underestimation of specific epistasis. In contrast to previous approaches, we do not attempt to model the fitness measurements directly. Rather, we begin with the observation that global epistasis, under the assumption of monotonicity, imposes strong constraints on the rank statistics of a combinatorial mutagenesis experiment. Namely, the rank-order of mutant phenotypes should be preserved across genetic backgrounds. We exploit this constraint to devise a simple semiparametric method to detect specific epistasis in the presence of global epistasis and measurement noise. We apply this method to three high-throughput mutagenesis experiments, uncovering known protein contacts with similar accuracy to existing, more complicated procedures. Our method immediately generalizes beyond proteins, providing a simple, yet powerful framework for interpreting the epistasis observed in combinatorial datasets.</p
Maintenance of Organ Morphology via Regulation of Tissue Material Properties in the Drosophila Retina
Full text linkTissue morphologies generated in development must be structurally suited to their physiological function. Towards this end, developing organs enact both tissue-scale deformation programs that produce complex 3-dimensional shapes and cellular morphogenesis processes that spatially pattern assemblies of specialized cell and generate fate specific cellular structures. Whether these structural changes, happening at both the tissue and cellular scale, are synergistically coordinated or happen largely independent of each other in specific biological contexts remains an important unanswered question. During the development of the Drosophila retina, which is generated from an originally simple epithelial sheet, a tissue-scale curvature is acquired that is required for resolution of the visual system. Additionally, cell-type specific optical structures are generated that require extensive cellular remodeling. How these two processes influence each other in development to produce the final retina structure is an open question. The Drosophila retina is a compound eye composed of discrete, multicellular optical units, called ommatidia, that tile across the curved surface of the retina. Ommatidia undergo a dramatic elongation process that increases the apical-basal depth of the tissue ~5-fold. This elongation process coincides with an enrichment of actomyosin stress fibers at the basal surface of the retinal epithelium that have long been hypothesized to also generate retinal curvature. In this dissertation, I investigate the origins of retinal curvature, to temporally place this tissue morphogenesis process in the context of other cellular morphogenesis programs in the retina. I will first demonstrate that tissue-scale curvature is acquired long before ommatidial elongation, challenging current views of retinal morphogenesis. Retinal curvature is acquired earlier in development and temporally overlaps with an increase in the material rigidity of the tissue. I will demonstrate that increased tissue rigidity is required to maintain retinal curvature and discuss how this finding raises the possibility that curvature is induced via tissue extrinsic forces. Next, I will show that this increase in tissue rigidity is driven specifically by interommatidial pigment cells (IOPCs). Tissue rigidity is generated via increased IOPC junctional tension downstream of Rok-mediated actomyosin contractility, and tension is transmitted throughout a supracellular network comprised of apical actomyosin networks and mechanically coupled adherens junction complexes. Tension transmission is critical to bridge the cell and tissue scales, allowing uniform modulation of material rigidity across the expanse of the retinal epithelium. With the discovery that retinal curvature is acquired independent of tissue elongation, I then investigated how individual cellular morphogenesis events are controlled, such that larger tissue structure is maintained. In this work, it was found that mechanical interactions between discrete subpopulations of retinal cells helped maintain tissue integrity during the later cellular morphogenesis programs that elaborate specialized optical structures subsequent to curvature establishment. Taken together, these two studies highlight that the physical state of a given tissue may be employed both to leverage active forces towards generating functional morphologies and buffer forces resulting from distinct cellular morphogenesis programs to preserve overall tissue form
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Pyrogenic carbon contribution to tropical savanna soil carbon storage
No full textSavannas are fire-prone ecosystems that contribute substantially to global fire emissions, but these emissions may be partly offset by deposition of fire-derived, persistent pyrogenic carbon (PyC) in soils. Although estimates of PyC contributions to soil organic carbon (SOC) storage in savanna exist, factors driving its accumulation remain unclear due to limited measurements with consistent methods. To address this, we sampled 253 sites across tropical savannas in Kruger National Park, South Africa, spanning broad gradients in fire regimes, grass biomass, rainfall, and soil texture. Here we show, PyC measured with H2O2/HNO3 digestion contributed, on average, 14.08% (se = 0.36%, n = 253) of SOC in surface soils, with values up to 40%. While fire frequency and grass biomass influenced soil PyC stocks, savannas with higher clay content and lower rainfall – conditions favoring PyC preservation – tended to accumulate more. These results demonstrate PyC’s significant contribution to SOC storage and highlight environmental factors driving its accumulation in tropical savannas, providing an empirical basis for understanding fire’s role in the savanna carbon cycle.</p
Blank Faces: Introduction to the Special Issue
No full textThe purpose of this special issue is to explore other means and results of making faces blank. We first explored these questions with a panel on the semiotics of “Blank Faces” at the 2016 Annual Meetings of the American Anthropological Association (AAA), in Minneapolis, Minnesota, which included contributions from Silvio, Nozawa, and others. In exploring the blank face, we aim to illuminate the expectations that people have for their own faces or the ones they meet, even if those expectations so frequently fall short of the demands placed on them. This special issue includes original research articles on topics such as the editing of portraits for match-making purposes (Alpert),the range of human capacities to recognize and remember faces (Pearl), the alignment of human faces with characterological types (Occhi), and the proliferation of a face in urban landscapes to the effect that she is seemingly ubiquitous, and yet never meeting the viewer’s gaze (Manning). As such, this issue is concerned with the relationship between the face as a locus of interaction in everyday life and the face as an image that lends itself to typification, along with the layered processes of semiosis and the social relationships produced through encounters with faces
Being and Becoming Stone: Material Semiotics in Indian Religion and Spirituality
No full textThis essay employs Peircean semiotics to interpret shifts in Indian religion and spirituality in northeastern North America from the seventeenth century onwards. Patterns of material culture attest to a complex knotting of Algonquian spiritual practices and European-introduced Christianity during this period. At the center of this study are a series of contested stone features that relate both to shifting forms of spirituality in Indian country along with new agricultural practices at the time, typically associated with white farms. These two distinct histories of practice resulted in a ubiquitous set of stone features in the landscapes of New England. Archaeologists often see these diverse features as icons of the same object, while ethnohistoric records, oral histories, and contemporary indigenous and local interpretations point to the ambiguity of these features. A Peircean approach offers important insights on these contested features, both in the past and the present, demonstrating how meaning varies according to the community of human interpreters and how it shifts in a fluid and context-driven manner
Pragmatic Archaeology and Semiotic Mediation
No full textArchaeology is a semiotic enterprise engaged in the study of meaning-making practices by past actors and of archaeologists themselves. Archaeology embraced its semiotic character in the context of the processual and postprocessual debates, and in terms of various postprocessual developments. Recently some archaeologists have drawn inspiration from the material semiotics of Bruno Latour to advocate for a symmetrical archaeology. This perspective offers a novel approach to object agency and focuses on how objects and humans together form assemblages. However, it neglects a satisfying account of how objects and things transform each other. One productive way forward is a consideration of semiotic mediation offered by a pragmatic archaeology linked to the work of Charles Sanders Peirce