The influence of visual landscape on the free flight behavior of the fruit fly Drosophila melanogaster

To study the visual cues that control steering behavior in the fruit fly Drosophila melanogaster, we reconstructed three-dimensional trajectories from images taken by stereo infrared video cameras during free flight within structured visual landscapes. Flies move through their environment using a series of straight flight segments separated by rapid turns, termed saccades, during which the fly alters course by approximately 90° in less than 100 ms. Altering the amount of background visual contrast caused significant changes in the fly’s translational velocity and saccade frequency. Between saccades, asymmetries in the estimates of optic flow induce gradual turns away from the side experiencing a greater motion stimulus, a behavior opposite to that predicted by a flight control model based upon optomotor equilibrium. To determine which features of visual motion trigger saccades, we reconstructed the visual environment from the fly’s perspective for each position in the flight trajectory. From these reconstructions, we modeled the fly’s estimation of optic flow on the basis of a two-dimensional array of Hassenstein–Reichardt elementary motion detectors and, through spatial summation, the large-field motion stimuli experienced by the fly during the course of its flight. Event-triggered averages of the large-field motion preceding each saccade suggest that image expansion is the signal that triggers each saccade. The asymmetry in output of the local motion detector array prior to each saccade influences the direction (left versus right) but not the magnitude of the rapid turn. Once initiated, visual feedback does not appear to influence saccade kinematics further. The total expansion experienced before a saccade was similar for flight within both uniform and visually textured backgrounds. In summary, our data suggest that complex behavioral patterns seen during free flight emerge from interactions between the flight control system and the visual environment

Collision-avoidance and landing responses are mediated by separate pathways in the fruit fly, Drosophila melanogaster

Flies rely heavily on visual feedback for several aspects of flight control. As a fly approaches an object, the image projected across its retina expands, providing the fly with visual feedback that can be used either to trigger a collision-avoidance maneuver or a landing response. To determine how a fly makes the decision to land on or avoid a looming object, we measured the behaviors generated in response to an expanding image during tethered flight in a visual closed-loop flight arena. During these experiments, each fly varied its wing-stroke kinematics to actively control the azimuth position of a 15°×15° square within its visual field. Periodically, the square symmetrically expanded in both the horizontal and vertical directions. We measured changes in the fly's wing-stroke amplitude and frequency in response to the expanding square while optically tracking the position of its legs to monitor stereotyped landing responses. Although this stimulus could elicit both the landing responses and collision-avoidance reactions, separate pathways appear to mediate the two behaviors. For example, if the square is in the lateral portion of the fly's field of view at the onset of expansion, the fly increases stroke amplitude in one wing while decreasing amplitude in the other, indicative of a collision-avoidance maneuver. In contrast, frontal expansion elicits an increase in wing-beat frequency and leg extension, indicative of a landing response. To further characterize the sensitivity of these responses to expansion rate, we tested a range of expansion velocities from 100 to 10000° s^(-1). Differences in the latency of both the collision-avoidance reactions and the landing responses with expansion rate supported the hypothesis that the two behaviors are mediated by separate pathways. To examine the effects of visual feedback on the magnitude and time course of the two behaviors, we presented the stimulus under open-loop conditions, such that the fly's response did not alter the position of the expanding square. From our results we suggest a model that takes into account the spatial sensitivities and temporal latencies of the collision-avoidance and landing responses, and is sufficient to schematically represent how the fly uses integration of motion information in deciding whether to turn or land when confronted with an expanding object

Spatial organization of visuomotor reflexes in Drosophila

In most animals, the visual system plays a central role in locomotor guidance. Here, we examined the functional organization of visuomotor reflexes in the fruit fly, Drosophila, using an electronic flight simulator. Flies exhibit powerful avoidance responses to visual expansion centered laterally. The amplitude of these expansion responses is three times larger than those generated by image rotation. Avoidance of a laterally positioned focus of expansion emerges from an inversion of the optomotor response when motion is restricted to the rear visual hemisphere. Furthermore, motion restricted to rear quarter-fields elicits turning responses that are independent of the direction of image motion about the animal's yaw axis. The spatial heterogeneity of visuomotor responses explains a seemingly peculiar behavior in which flies robustly fixate the contracting pole of a translating flow field

Recursive Construction of Higgs-Plus-Multiparton Loop Amplitudes: The Last of the Phi-nite Loop Amplitudes

We consider a scalar field, such as the Higgs boson H, coupled to gluons via the effective operator H tr G_{mu nu} G^{mu nu} induced by a heavy-quark loop. We treat H as the real part of a complex field phi which couples to the self-dual part of the gluon field-strength, via the operator phi tr G_{SD mu nu} G_{SD}^{mu nu}, whereas the conjugate field phi^dagger couples to the anti-self-dual part. There are three infinite sequences of amplitudes coupling phi to quarks and gluons that vanish at tree level, and hence are finite at one loop, in the QCD coupling. Using on-shell recursion relations, we find compact expressions for these three sequences of amplitudes and discuss their analytic properties.Comment: 63 pages, 7 figures; v2 references added; v3 minor typos corrected and note added; v4 fixed error in eq. (7.11) (lower limit of sum should be l=2, not l=3), also affecting eqs. (7.14), (8.20), (8.21), (8.27) and (8.28

Regulation of eukaryotic Mcm2-7 activity

The transfer of genetic material from one cell generation to the next requires precise genome duplication. Aberrant DNA replication can lead to genomic instability and contribute to diseases arising from an unregulated cell cycle, such as cancer. Replicative DNA polymerases require a single-stranded (ssDNA) template from which to produce newly synthesized DNA. In eukaryotes, ssDNA is generated by the heterohexameric minichromosome maintenance 2 through 7 (Mcm2-7) replicative helicase that unwinds duplex DNA. Strict temporal separation of helicase loading and activation at multiple replication origins ensures once per cell cycle replication. The processes involved in activating Mcm2-7 to unwind DNA during S phase are poorly understood. Through in vivo and in vitro analyses, the current study examines the factors involved in modulating S. cerevisiae Mcm2-7 activity. Mec1, a member of the PIKK (phosphoinositide three-kinase-related kinase) family of proteins, is involved in the response to replicative stress and DNA damage. It also plays a role during an unperturbed cell cycle and is required to phosphorylate Mcm2-7 prior to helicase activation. We characterized alleles of S. cerevisiae mec1 that alter the conserved FATC domain. Mutants of Mec1 resulted in temperature sensitive growth, sensitivity to hydroxyurea and reduced kinase activity in vitro. These mutants were also less stable than wild-type Mec1 and demonstrated reduced nuclear localization. We also identified rpn3-L140P, which encodes a component of the 19S proteasomal regulatory particle of the 26S proteasome, as a suppressor of the temperature-sensitive growth caused by mec1-W2368A. As Cdt1 is required for the nuclear import and origin loading of Mcm2-7, we also sought to investigate the interaction between these two components in more detail. Using reconstituted Mcm2-7·Cdt1 complexes from bacterial-expressed proteins, we demonstrated that these complexes exhibit lower ATPase and helicase activity than Mcm2-7. We also showed that Mcm2-7 dissociates into subcomplexes, and that Mcm3, 5 and 7 bound origins in the absence of Cdt1. We propose that the reduced ATPase activity of Mcm2-7 by Cdt1 binding is induced by structural changes in the Mcm2-7 ring. We also suggest that Cdt1 helps to stabilize the Mcm2-7 hexamer. To investigate the role of phosphorylation on Mcm2-7, we utilized a phosphomimetic mutant of Mcm4 that when incorporated into Mcm2-7 can bypass the requirement for DDK. While phosphomimetic Mcm4 demonstrated slightly lower ATPase activity than the wildtype protein, phosphomimetic Mcm2-7 complexes exhibited wildtype ATPase, helicase and DNA binding activity. Taken together, our work identifies the functional role of the C-terminal residues of Mec1 and the protein’s turnover by the proteosome. Our studies also provide new insights into the factors and processes involved in the activation of Mcm2-7 to unwind DNA

The Hybrid Nature of the Property Clause: Implications For Judicial Review of National Monument Reductions

In the recurring and contentious debates regarding the President’s authority to declare (and perhaps rescind) National Monuments, both those who argue for an expansive authority and those who favor restricted authority treat the Antiquities Act as a delegation of legislative power; they only disagree on whether the delegation is appropriate or, in the case of rescission, whether a delegation exists at all. However, this framework is wrong. The Property Clause is not strictly a legislative power. Rather, it is a hybrid; rulemaking power is interspersed with an administrative one—the power to manage property. The Supreme Court has recognized this distinction in the past, including in a case decided the same year Congress passed the Antiquities Act. The distinction makes a difference because when Congress enlists the Executive in the management of public lands, as it did with the Antiquities Act, it is not delegating legislative power; rather, it is sharing its plenary powers of proprietorship. The principle that underlies the non-delegation doctrine—separation of powers—is not applicable when Congress delegates a non-legislative power. This has implications for the proper method of recourse for inappropriate presidential action made pursuant to delegated Property Clause power. Whereas recourse for an improper delegation of legislative authority lies with the courts, recourse for alleged inappropriate executive action under a delegated proprietor power lies with the entity that enlisted the Executive as its property manager in the first place—Congress. Indeed, Congress has often stepped in to correct presidential mismanagement of public lands, including the management of National Monuments. At other times, Congress has acquiesced in presidential action, a practice the Court has previously accepted as evidence of the constitutionality and legality of presidential action. Courts have systematically refused to second-guess presidential actions pursued under the Property Clause, including the creation of National Monuments, even when those monuments arguably exceed the “smallest area compatible” with the protection and care of the objects to be protected as delineated in the statute. Courts should also refuse to second-guess the President’s decision to reduce National Monuments, leaving to Congress the important work of correcting the President’s missteps, as it has done in the past. Let Congress guard its own authority over land management

Bootstrapping One-Loop QCD Amplitudes with General Helicities

The recently developed on-shell bootstrap for computing one-loop amplitudes in non-supersymmetric theories such as QCD combines the unitarity method with loop-level on-shell recursion. For generic helicity configurations, the recursion relations may involve undetermined contributions from non-standard complex singularities or from large values of the shift parameter. Here we develop a strategy for sidestepping difficulties through use of pairs of recursion relations. To illustrate the strategy, we present sets of recursion relations needed for obtaining n-gluon amplitudes in QCD. We give a recursive solution for the one-loop n-gluon QCD amplitudes with three or four color-adjacent gluons of negative helicity and the remaining ones of positive helicity. We provide an explicit analytic formula for the QCD amplitude A_{6;1}(1^-,2^-,3^-,4^+,5^+,6^+), as well as numerical results for A_{7;1}(1^-,2^-,3^-,4^+,5^+,6^+,7^+), A_{8;1}(1^-,2^-,3^-,4^+,5^+,6^+,7^+,8^+), and A_{8;1}(1^-,2^-,3^-,4^-,5^+,6^+,7^+,8^+). We expect the on-shell bootstrap approach to have widespread applications to phenomenological studies at colliders.Comment: 77 pages, 17 figures; v2, corrected minor typos in text and small equation

Weight optimization of an aerobrake structural concept for a lunar transfer vehicle

An aerobrake structural concept for a lunar transfer vehicle was weight optimized through the use of the Taguchi design method, finite element analyses, and element sizing routines. Six design parameters were chosen to represent the aerobrake structural configuration. The design parameters included honeycomb core thickness, diameter-depth ratio, shape, material, number of concentric ring frames, and number of radial frames. Each parameter was assigned three levels. The aerobrake structural configuration with the minimum weight was 44 percent less than the average weight of all the remaining satisfactory experimental configurations. In addition, the results of this study have served to bolster the advocacy of the Taguchi method for aerospace vehicle design. Both reduced analysis time and an optimized design demonstrated the applicability of the Taguchi method to aerospace vehicle design