Maternal Expression Relaxes Constraint on Innovation of the Anterior Determinant, bicoid

The origin of evolutionary novelty is believed to involve both positive selection and relaxed developmental constraint. In flies, the redesign of anterior patterning during embryogenesis is a major developmental innovation and the rapidly evolving Hox gene, bicoid (bcd), plays a critical role. We report evidence for relaxation of selective constraint acting on bicoid as a result of its maternal pattern of gene expression. Evolutionary theory predicts 2-fold greater sequence diversity for maternal effect genes than for zygotically expressed genes, because natural selection is only half as effective acting on autosomal genes expressed in one sex as it is on genes expressed in both sexes. We sample an individual from ten populations of Drosophila melanogaster and nine populations of D. simulans for polymorphism in the tandem gene duplicates bcd, which is maternally expressed, and zerknüllt (zen), which is zygotically expressed. In both species, we find the ratio of bcd to zen nucleotide diversity to be two or more in the coding regions but one in the noncoding regions, providing the first quantitative support for the theoretical prediction of relaxed selective constraint on maternal-effect genes resulting from sex-limited expression. Our results suggest that the accelerated rate of evolution observed for bcd is owing, at least partly, to variation generated by relaxed selective constraint

IDSC Hecla Response to CDAT Appeal

Hecla Limited\u27s Consolidated Response to the Coeur d\u27Alene Tribe\u27s Appea

IDSC Hecla Response to CDAT Appeal

Hecla Limited\u27s Consolidated Response to the Coeur d\u27Alene Tribe\u27s Appea

IDSC Hecla Response to USA Appeal

Hecla Limited\u27s Consolidated Response to the United States\u27 Appea

IDSC Hecla Response to USA Appeal

Hecla Limited\u27s Consolidated Response to the United States\u27 Appea

Constrained Monte Carlo Method and Calculation of the Temperature Dependence of Magnetic Anisotropy

We introduce a constrained Monte Carlo method which allows us to traverse the phase space of a classical spin system while fixing the magnetization direction. Subsequently we show the method's capability to model the temperature dependence of magnetic anisotropy, and for bulk uniaxial and cubic anisotropies we recover the low-temperature Callen-Callen power laws in M. We also calculate the temperature scaling of the 2-ion anisotropy in L10 FePt, and recover the experimentally observed M^2.1 scaling. The method is newly applied to evaluate the temperature dependent effective anisotropy in the presence of the N'eel surface anisotropy in thin films with different easy axis configurations. In systems having different surface and bulk easy axes, we show the capability to model the temperature-induced reorientation transition. The intrinsic surface anisotropy is found to follow a linear temperature behavior in a large range of temperatures

Communication, information and responsibility distribution strategies for effective real-time transit service management

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2002.Includes bibliographical references (p. 163-164).Resolving disruptions is a continual challenge to providing quality, cost-effective transit service. While a number of recovery techniques exist to recover from disruptions, detecting a disruption, choosing a response and implementing it in a timely manner is a difficult task. Different agencies use different combinations of field supervision, centralized control, and traditional and advanced communication technology. While these different service management strategies have different results, there is no consensus on what makes a good strategy, nor a systematic method for evaluating a proposed strategy and predicting its strengths and weaknesses. The purpose of this thesis is to create a framework for studying bus service management strategies and draw general lessons from an application of that framework. This thesis categorizes 15 distinct disruptions in bus service, the most common responses to each, and the information and resources necessary both to reach a decision on the most appropriate response and to implement it. It introduces a spreadsheet model for starting with the number of disruptions an agency faces and its chain of command for dealing with them and calculating the number of conversations that take place and the demand those conversations put on communications channels. Values gathered from studying Chicago Transit Authority (CTA) supervisor radio recordings allow this model to show the unused capacity of communications channels, if any, so that the feasibility of a prospective strategy can be determined. This method of studying strategy is applied to CTA. It is found that CTA bus operations suffer from two bottlenecks. The control center relays delay reports too slowly for them to be useful, and the communications channels allotted to supervisors are less than they would be required to air all messages related to service restoration. As a result, street supervisors have few service restoration options available to respond to delays, and they lack the information needed to choose an option effectively. The net result is that minor delays typically go unaddressed until they deteriorate into major ones, and major delays impose greater cost on passengers than they should. The impact of adding handheld computers with real-time location information is studied, and it is found that this would let supervisors use a wider range of restoration techniques, allow them to choose the best technique more accurately, let them address minor delays before they become more serious and free the supervisory radio channels for more effective management of breakdowns, accidents and disturbances. It is concluded that there are inherent advantages in managing schedule adherence from the field and managing incidents from a control center, regardless of an agency's level of communication investment. It is further concluded that digital messaging has a natural strength in dealing with routine and well-understood instructions, while voice communication is essential for tasks that are less predictable or require collaboration. Digital messaging can play a substantial role in a good service management strategy but can never replace voice radio.by David P. Barker.S.M

Analysis of Clumps in Molecular Cloud Models: Mass Spectrum, Shapes, Alignment and Rotation

Observations reveal concentrations of molecular line emission on the sky, called ``clumps,'' in dense, star-forming molecular clouds. These clumps are believed to be the eventual sites of star formation. We study the three-dimensional analogs of clumps using a set of self-consistent, time-dependent numerical models of molecular clouds. The models follow the decay of initially supersonic turbulence in an isothermal, self-gravitating, magnetized fluid. We find the following. (1) Clumps are intrinsically triaxial. This explains the observed deficit of clumps with a projected axis ratio near unity, and the apparent prolateness of clumps. (2) Simulated clump axes are not strongly aligned with the mean magnetic field within clumps, nor with the large-scale mean fields. This is in agreement with observations. (3) The clump mass spectrum has a high-mass slope that is consistent with the Salpeter value. There is a low-mass break in the slope at \sim 0.5 \msun, although this may depend on model parameters including numerical resolution. (4) The typical specific spin angular momentum of clumps is $4 \times 10^{22} {\rm cm^2 s^{-1}}$. This is larger than the median specific angular momentum of binary stars. Scaling arguments suggest that higher resolution simulations may soon be able to resolve the scales at which the angular momentum of binary stars is determined.Comment: 14 pages, 13 figures, to appear in 2003 July 20 Ap