Showing 4551 - 4600 of 5831 Items
Retinoic acid expands the evolutionarily reduced dentition of zebrafish
Date: 2012-12-01
Creator: Pawat Seritrakul, Eric Samarut, Tenzing T.S. Lama, Yann Gibert, Vincent, Laudet, William R. Jackman
Access: Open access
- Zebrafish lost anterior teeth during evolution but retain a posterior pharyngeal dentition that requires retinoic acid (RA) cell-cell signaling for its development. The purposes of this study were to test the sufficiency of RA to induce tooth development and to assess its role in evolution. We found that exposure of embryos to exogenous RA induces a dramatic anterior expansion of the number of pharyngeal teeth that later form and shifts anteriorly the expression patterns of genes normally expressed in the posterior tooth-forming region, such as pitx2 and dlx2b. After RA exposure, we also observed a correlation between cartilage malformations and ectopic tooth induction, as well as abnormal cranial neural crest marker gene expression. Additionally, we observed that the RA-induced zebrafish anterior teeth resemble in pattern and number the dentition of fish species that retain anterior pharyngeal teeth such as medaka but that medaka do not express the aldh1a2 RA-synthesizing enzyme in tooth-forming regions. We conclude that RA is sufficient to induce anterior ectopic tooth development in zebrafish where teeth were lost in evolution, potentially by altering neural crest cell development, and that changes in the location of RA synthesis correlate with evolutionary changes in vertebrate dentitions. © FASEB.
GEM-PSO: Particle Swarm Optimization Guided by Enhanced Memory
Date: 2019-05-01
Creator: Kevin Fakai Chen
Access: Open access
- Particle Swarm Optimization (PSO) is a widely-used nature-inspired optimization technique in which a swarm of virtual particles work together with limited communication to find a global minimum or optimum. PSO has has been successfully applied to a wide variety of practical problems, such as optimization in engineering fields, hybridization with other nature-inspired algorithms, or even general optimization problems. However, PSO suffers from a phenomenon known as premature convergence, in which the algorithm's particles all converge on a local optimum instead of the global optimum, and cannot improve their solution any further. We seek to improve upon the standard Particle Swarm PSO algorithm by fixing this premature convergence behavior. We do so by storing and exploiting increased information in the form of past bests, which we deem enhanced memory. We introduce three types of modifications to each new algorithm (which we call a GEM-PSO: Particle Swarm Optimization, Guided by Enhanced Memory, because our modifications all deal with enhancing the memory of each particle). These are procedures for saving a found best, for removing a best from memory when a new one is to be added, and for selecting one (or more) bests to be used from those saved in memory. By using different combinations of these modifications, we can create many different variants of GEM-PSO that have a wide variety of behaviors and qualities. We analyze the performance of GEM-PSO, discuss the impact of PSO's parameters on the algorithms' performances, isolate different modifications in order to closely study their impact on the performance of any given GEM-PSO variant, and finally look at how multiple modifications perform. Finally, we draw conclusions about the efficacy and potential of GEM-PSO variants, and provide ideas for further exploration in this area of study. Many GEM-PSO variants are able to consistently outperform standard PSO on specific functions, and GEM-PSO variants can be shown to be promising, with both general and specific use cases.
Social Media as Technologies of Accountability: Explaining Resistance to Implementation Within Organizations
Date: 2013-04-12
Creator: Jeffrey Treem
Access: Open access
Landscape in Maine 1820-1970: A Sesquicentennial Exhibition
Date: 1970-01-01
Creator: James Morton
Access: Open access
- Catalog of an exhibition held at the Colby College Art Museum, April 4-May 10; the Bowdoin College Museum of Art, May 21-June 28; and the Carnegie Gallery, University of Maine, Orono, July 8-August 30. Sponsored by the Maine Federation of Women's Clubs.
The Effects of Exchange Rates on Employment in Canada
Date: 2013-06-05
Creator: Yao Tang, Haifang Huang, Ke Pang
Access: Open access
- Under the flexible exchange rate regime, the Canadian economy is constantly affected by fluctuations in exchange rates. This paper focuses on employment in Canada. We find that appreciations of the Canadian dollar have significant effects on employment in manufacturing industries; such effects are mostly associated with the export-weighted exchange rate and not the import-weighted exchange rate. The export-weighted exchange rate elasticity of employment is -0.52. However, we also find that exchange rate fluctuations have little impact on Canada’s nonmanufacturing employment. Because the manufacturing sector accounts for only about 10% of the employment in Canada, the overall employment effect of exchange rates is small. In addition, we assess the potential employment impact of a boom in the global commodity market, which often leads to appreciations of the Canadian dollar. We find that a 12.21% increase in commodity prices (one standard deviation in the 1994-2007 data) reduces Canada’s manufacturing employment by 0.98%, less than 0.1% of the total industrial employment.
Reflections questionnaire response by Anonymous on March 31, 2021
Date: 2021-01-01
Creator: Anonymous
Access: Open access
- This is a response to the Documenting Bowdoin & COVID-19 Reflections Questionnaire. The questionnaire was created in March 2021 by staff of Bowdoin's George J. Mitchell Department of Special Collections & Archives. Author is class of 2023.
Emergence of dispersive shocks and rarefaction waves in power-law contact models
Date: 2017-06-16
Creator: H. Yasuda, C. Chong, J. Yang, P. G. Kevrekidis
Access: Open access
- In the present work, motivated by generalized forms of the Hertzian dynamics associated with granular crystals, we consider the possibility of such models to give rise to both dispersive shock and rarefaction waves. Depending on the value p of the nonlinearity exponent, we find that both of these possibilities are realizable. We use a quasicontinuum approximation of a generalized inviscid Burgers model in order to predict the solution profile up to times near the formation of the dispersive shock, as well as to estimate when it will occur. Beyond that time threshold, oscillations associated with the highly dispersive nature of the underlying model emerge, which cannot be captured by the quasicontinuum approximation. Our analytical characterization of the above features is complemented by systematic numerical computations.
Microblogging Practices of Virtual Organizations: Commonalities and Contrasts
Date: 2013-04-12
Creator: Jing Wang
Access: Open access
- Microblogging is becoming increasingly pervasive in computer-supported collaboration, attracting various types of users. Organizations, as one type, are willing to leverage this social media service for their operation, but lack guidance of how to effectively manage their organizational microblogs. However, research on microblogging practices at organizational level, especially in virtual organizations, is very limited. To enhance the understanding of how virtual organizations use microblogs in similar and different ways, we investigate microblogging practices of two virtual organizations by examining the content characteristics of their Twitter posts. We identify eleven categories of microblog themes of three dimensions, consisting of both common and different categories between the two organizations. We further enumerate their potential impacts on organizational context, discuss differences between the two organizations, and compare these organizational practices with personal ones.
Globalizing extraction and indigenous rights in the russian arctic: The enduring role of the state in natural resource governance
Date: 2019-12-01
Creator: Svetlana A. Tulaeva, Maria S. Tysiachniouk, Laura A. Henry, Leah S. Horowitz
Access: Open access
- The governance of extractive industries has become increasingly globalized. International conventions and multi-stakeholder institutions set out rules and standards on a range of issues, such as environmental protection, human rights, and Indigenous rights. Companies' compliance with these global rules may minimize risks for investors and shareholders, while offering people at sites of extraction more leverage. Although the Russian state retains a significant stake in the oil and gas industries, Russian oil and gas companies have globalized as well, receiving foreign investment, participating in global supply chains, and signing on to global agreements. We investigate how this global engagement has affected Nenets Indigenous communities in Yamal, an oil-and gas-rich region in the Russian Arctic, by analyzing Indigenous protests and benefit-sharing arrangements. Contrary to expectations, we find that Nenets Indigenous communities have not been empowered by international governance measures, and also struggle to use domestic laws to resolve problems. In Russia, the state continues to play a significant role in determining outcomes for Indigenous communities, in part by working with Indigenous associations that are state allies. We conclude that governance generating networks in the region are under-developed.
Meaning at the Crossroads: The Portrait in Photography
Date: 1994-01-01
Creator: Justin P. Wolff
Access: Open access
- Exhibition catalogue from the Bowdoin College Museum of Art.
Mechanical Autonomous Stochastic Heat Engine
Date: 2016-06-28
Creator: Marc Serra-Garcia, André Foehr, Miguel Molerón, Joseph Lydon, Christopher, Chong, Chiara Daraio
Access: Open access
- Stochastic heat engines are devices that generate work from random thermal motion using a small number of highly fluctuating degrees of freedom. Proposals for such devices have existed for more than a century and include the Maxwell demon and the Feynman ratchet. Only recently have they been demonstrated experimentally, using, e.g., thermal cycles implemented in optical traps. However, recent experimental demonstrations of classical stochastic heat engines are nonautonomous, since they require an external control system that prescribes a heating and cooling cycle and consume more energy than they produce. We present a heat engine consisting of three coupled mechanical resonators (two ribbons and a cantilever) subject to a stochastic drive. The engine uses geometric nonlinearities in the resonating ribbons to autonomously convert a random excitation into a low-entropy, nonpassive oscillation of the cantilever. The engine presents the anomalous heat transport property of negative thermal conductivity, consisting in the ability to passively transfer energy from a cold reservoir to a hot reservoir.
All-loop-orders relation between Regge limits of N = 4 SYM and N = 8 supergravity four-point amplitudes
Date: 2021-02-01
Creator: Stephen G. Naculich
Access: Open access
- We examine in detail the structure of the Regge limit of the (nonplanar) N = 4 SYM four-point amplitude. We begin by developing a basis of color factors Cik suitable for the Regge limit of the amplitude at any loop order, and then calculate explicitly the coefficients of the amplitude in that basis through three-loop order using the Regge limit of the full amplitude previously calculated by Henn and Mistlberger. We compute these coefficients exactly at one loop, through O(ϵ 2) at two loops, and through O(ϵ) at three loops, verifying that the IR-divergent pieces are consistent with (the Regge limit of) the expected infrared divergence structure, including a contribution from the three-loop correction to the dipole formula. We also verify consistency with the IR-finite NLL and NNLL predictions of Caron-Huot et al. Finally we use these results to motivate the conjecture of an all-orders relation between one of the coefficients and the Regge limit of the N = 8 supergravity four-point amplitude.
Efficiency of incentives to jointly increase carbon sequestration and species conservation on a landscape
Date: 2008-07-15
Creator: Erik Nelson, Stephen Polasky, David J. Lewis, Andrew J. Plantinga, Eric, Lonsdorf, Denis White, David Bael, Joshua J. Lawler
Access: Open access
- We develop an integrated model to predict private land-use decisions in response to policy incentives designed to increase the provision of carbon sequestration and species conservation across heterogeneous landscapes. Using data from the Willamette Basin, Oregon, we compare the provision of carbon sequestration and species conservation under five simple policies that offer payments for conservation. We evaluate policy performance compared with the maximum feasible combinations of carbon sequestration and species conservation on the landscape for various conservation budgets. None of the conservation payment policies produce increases in carbon sequestration and species conservation that approach the maximum potential gains on the landscape. Our results show that policies aimed at increasing the provision of carbon sequestration do not necessarily increase species conservation and that highly targeted policies do not necessarily do as well as more general policies. © 2008 by The National Academy of Sciences of the USA.
Damped-driven granular chains: An ideal playground for dark breathers and multibreathers
Date: 2014-03-31
Creator: C. Chong, F. Li, J. Yang, M. O. Williams, I. G., Kevrekidis, P. G. Kevrekidis
Access: Open access
- By applying an out-of-phase actuation at the boundaries of a uniform chain of granular particles, we demonstrate experimentally that time-periodic and spatially localized structures with a nonzero background (so-called dark breathers) emerge for a wide range of parameter values and initial conditions. We demonstrate a remarkable control over the number of breathers within the multibreather pattern that can be "dialed in" by varying the frequency or amplitude of the actuation. The values of the frequency (or amplitude) where the transition between different multibreather states occurs are predicted accurately by the proposed theoretical model, which is numerically shown to support exact dark breather and multibreather solutions. Moreover, we visualize detailed temporal and spatial profiles of breathers and, especially, of multibreathers using a full-field probing technology and enable a systematic favorable comparison among theory, computation, and experiments. A detailed bifurcation analysis reveals that the dark and multibreather families are connected in a "snaking" pattern, providing a roadmap for the identification of such fundamental states and their bistability in the laboratory. © 2014 American Physical Society.
The effects of nitric oxide on the modulation of the cardiac system of the American lobster, Homarus americanus, via a peptide (GYSDRNLRFamide)
Date: 2014-08-01
Creator: Sophie Janes
Access: Open access
- The central pattern generator (CPG) is a neural network that controls the rhythmic patterned outputs, which generate locomotion as needed. The lobster provides a good model to study CPGs because it has a relatively simple CPG. The lobster CPG, or cardiac ganglion accommodates for a range of activities and changes in the environment (Cooke, 2002).The small lobster CG is made up of nine neurons that control the neurogenic heart. The lobster CG is located on the inner dorsal wall of the heart and forms long neurites that branch onto the heart muscle. The CG, through an intrinsic mechanism, generates patterned and rhythmic bursts to the heart (Cooke 2002).The H. americanus CG sends information to the heart muscle to regulate the heart beat. The patterned bursts from the CG need to be adjusted in response to changing demands, for example, activity level or blood volume. Two general mechanisms, intrinsic feedback and extrinsic neuromodulation, have been identified to facilitate this adjustment. Through an intrinsic feedback mechanism, the muscle sends information back to the CG via a positive pathway and a negative pathway. In the positive pathway, stretch-sensitive dendrites of cardiac neurons increase the frequency of heart contractions when stretched (Cooke 2002). In the negative pathway, nitric oxide (NO), produced by the cardiac muscle, slows the frequency (Mahadevan et al. 2004). The interplay between the negative and positive feedback pathways regulates the output of the CG. An extrinsic mechanism has also been identified to regulate the CG output. Chemical neuromodulators that are released either locally or as hormones signal to the heart or CG to modulate ganglion activity. The intrinsic and extrinsic mechanisms affect the contraction amplitude and frequency of the heart.Within this simple invertebrate organism, a complex layering of control exists. Studies of the effects of various extrinsic modulators suggest that these modulators may alter how the feedback pathways operate. I examined what effect the neuromodulator GYSDRNLRFamide (GYS), a peptide found in the lobster nervous system, has on the balance between the positive and negative pathways (Ma et al., 2008). Recent experiments have demonstrated that when GYS was applied at high concentrations in the whole heart, the frequency decreased. This suggests that GYS may play a role in the intrinsic feedback pathways, and likely enhances the negative pathway.I looked at if nitric oxide altered the modulation of the heartbeat frequency when enhanced by the extrinsic modulator, GYS. Based on previous experiments, I hypothesized that GYS allows the NO, or negative pathway to predominate. In order to test my hypothesis, I examined the effects of GYS when I removed nitric oxide, which allows the negative pathway to exist. I compared the characteristics of the heartbeat when saline was run through the heart to when GYS was run through the heart. I also compared the characteristics of the heartbeat when the NO inhibitor, L-NA, was applied to when GYS was applied in the presence of L-NA. I finally compared the changes in frequency between these two comparisons. I found a significant difference between the change in frequency of the heart perfused with GYS in saline as opposed to perfused with GYS in L-NA. GYS had a greater negative effect without L-NA. These results demonstrate that NO is likely the cause of the observed decrease in frequency. Final Report of research funded by the Doherty Coastal Studies Research Fellowship.
Solving which trilemma? The many interpretations of equality, Pareto, and freedom of occupational choice
Date: 2017-08-01
Creator: Kristi A. Olson
Access: Open access
- According to the trilemma claim, we cannot have all three of equality, Pareto, and freedom of occupational choice. In response to the trilemma, John Rawls famously sacrificed equality by introducing incentives. In contrast, GA Cohen and others argued that we can, in fact, have all three provided that individuals are properly motivated by an egalitarian ethos. The incentives debate, then, concerns the plausibility of the ethos solution versus the plausibility of the incentives solution. Considerable ink has been spilled on both sides of the debate. Yet, in this essay, I argue that we cannot have this debate until we clarify the terms. Once we clarify the terms, however, we might discover that there is no debate to be had. This is because, depending on how equality, Pareto, and freedom of occupational choice are interpreted, there might not be a trilemma in the first place. Specifically, I use a small but crucial distinction in how equality, the egalitarian ethos, and Pareto are assessed – what I call the internal/external distinction – to disentangle the various paths each solution – the ethos or incentives – could take. I conclude that both solutions have gained illicit plausibility by virtue of not keeping the distinction straight.
Climate change and health costs of air emissions from biofuels and gasoline
Date: 2009-02-10
Creator: Jason Hill, Stephen Polasky, Erik Nelson, David Tilman, Hong, Huo, Lindsay Ludwig, James Neumann, Haochi Zheng, Diego Bonta
Access: Open access
- Environmental impacts of energy use can impose large costs on society. We quantify and monetize the life-cycle climate-change and health effects of greenhouse gas (GHG) and fine particulate matter (PM2.5) emissions from gasoline, corn ethanol, and cellulosic ethanol. For each billion ethanol-equivalent gallons of fuel produced and combusted in the US, the combined climate-change and health costs are $469 million for gasoline, $472-952 million for corn ethanol depending on biorefinery heat source (natural gas, corn stover, or coal) and technology, but only $123-208 million for cellulosic ethanol depending on feedstock (prairie biomass, Miscanthus, corn stover, or switchgrass). Moreover, a geographically explicit life-cycle analysis that tracks PM2.5 emissions and exposure relative to U.S. population shows regional shifts in health costs dependent on fuel production systems. Because cellulosic ethanol can offer health benefits from PM2.5 reduction that are of comparable importance to its climate-change benefits from GHG reduction, a shift from gasoline to cellulosic ethanol has greater advantages than previously recognized. These advantages are critically dependent on the source of land used to produce biomass for biofuels, on the magnitude of any indirect land use that may result, and on other as yet unmeasured environmental impacts of biofuels. © 2009 by The National Academy of Sciences of the USA.
Lie to Me: Linguistic Markers of Deception in Relation to Individual Differences in Executive Control Access to this record is restricted to members of the Bowdoin community. Log in here to view.
Date: 2014-05-01
Creator: Lauren Pashkowski
Access: Access restricted to the Bowdoin Community
Systematic ENSO-driven nutrient variability recorded by central equatorial Pacific corals
Date: 2013-08-16
Creator: Michèle LaVigne, Intan S. Nurhati, Kim M. Cobb, Helen V. McGregor, Daniel, Sinclair, Robert M. Sherrell
Access: Open access
- Variations in ocean productivity are driven largely by nutrient supply to the photic zone, but temporal records of nutrient variability are sparse. Here we show scleractinian coral P/Ca proxy records of variations in phosphate concentrations during El Niño Southern Oscillation (ENSO) cycles in the central equatorial Pacific. Covarying P/Ca records in Porites corals from Christmas and Fanning Islands show a regional ∼40% decrease during the upwelling relaxation of the 1997-1998 El Niño, consistent with less frequent nutrient measurements from this area. Similar ∼35-45% skeletal P/Ca decreases occur during the 1982-1983 and 1986-1987 El Niño events, which predate satellite color and regional nutrient measurements. After each El Niño event, nutrient increases lag temperature recovery by 4-12 months, likely reflecting uptake by massive phytoplankton blooms that followed resumption of upwelling. The results support the utility of coral P/Ca to probe the mechanisms linking ENSO, equatorial upwelling, and carbon cycling in the past. © 2013. American Geophysical Union. All Rights Reserved.
KLT-type relations for QCD and bicolor amplitudes from color-factor symmetry
Date: 2018-03-01
Creator: Robert W. Brown, Stephen G. Naculich
Access: Open access
- Color-factor symmetry is used to derive a KLT-type relation for tree-level QCD amplitudes containing gluons and an arbitrary number of massive or massless quark-antiquark pairs, generalizing the expression for Yang-Mills amplitudes originally postulated by Bern, De Freitas, and Wong. An explicit expression is given for all amplitudes with two or fewer quark-antiquark pairs in terms of the (modified) momentum kernel. We also introduce the bicolor scalar theory, the “zeroth copy” of QCD, containing massless biadjoint scalars and massive bifundamental scalars, generalizing the biadjoint scalar theory of Cachazo, He, and Yuan. We derive KLT-type relations for tree-level amplitudes of biadjoint and bicolor theories using the color-factor symmetry possessed by these theories.
Complaint-making as political participation in contemporary Russia
Date: 2012-09-01
Creator: Laura A. Henry
Access: Open access
- Prior to December 2011, instances of widespread collective mobilization were relatively rare in contemporary Russia. Russian citizens are more likely to engage in a different means of airing grievances: making an official complaint to the authorities. This article considers how complaint-making, as a variety of political participation, may contribute either to authoritarian resilience or to political liberalization. The political significance of complaints made to the Commissioner for Human Rights of the Russian Federation is examined. Since it is the broader political context that shapes the significance of complaints, in the absence of meaningful elections individualized appeals to the state are unlikely to promote democratic change, although they may allow for redress of individual rights violations. © 2012 The Regents of the University of California.
Highly nonlinear wave propagation in elastic woodpile periodic structures
Date: 2015-03-17
Creator: E. Kim, F. Li, C. Chong, G. Theocharis, J., Yang, P. G. Kevrekidis
Access: Open access
- In the present work, we experimentally implement, numerically compute with, and theoretically analyze a configuration in the form of a single column woodpile periodic structure. Our main finding is that a Hertzian, locally resonant, woodpile lattice offers a test bed for the formation of genuinely traveling waves composed of a strongly localized solitary wave on top of a small amplitude oscillatory tail. This type of wave, called a nanopteron, is not only motivated theoretically and numerically, but is also visualized experimentally by means of a laser Doppler vibrometer. This system can also be useful for manipulating stress waves at will, for example, to achieve strong attenuation and modulation of high-amplitude impacts without relying on damping in the system.
"I Deny Your Authority to Try My Conscience:" Conscription and Conscientious Objectors In Britain During the Great War
Date: 2019-05-01
Creator: Albert William Wetter
Access: Open access
- During the Great War, the Military Service Act was introduced on January 27, 1916 and redefined British citizenship. Moreover, some men objected to the state’s military service mandate, adamant that compliance violated their conscience. This thesis investigates how the introduction of conscription reshaped British society, dismantled the “sacred principle” of volunteerism, and replaced it with conscription, resulting in political and popular debates, which altered the individual’s relationship with the state. British society transformed from a polity defined by the tenets of Liberalism and a free-will social contract to a society where citizenship was correlated to duty to the state. Building off Lois Bibbings’ research on conscientious objectors, this thesis nuances the analysis with the case studies of David Blelloch and Norman Gaudie. Framed by two theories—Benedict Anderson’s imagined community and Barbara Rosenwein’s emotional community—these case studies demonstrate how conscientious objectors exposed the incongruence of the British imagined and emotional community, and the redefinition of citizenship. By weaving these theories into the British Great War tapestry, this thesis contends that the British nation was imagined differently before the war than it was after the war because of the introduction of conscription. Drawing from parliamentary debate transcripts, newspaper articles, and archival material from the Imperial War Museum in London, and the Liddle Personal Collection at the University of Leeds, Blelloch’s and Gaudie’s respective case studies ultimately bait the question: “What does it mean to be British?”
Psychometric Properties of a Self-Report Instrument for the Assessment of Tic Severity in Adults With Tic Disorders
Date: 2014-11-26
Creator: Amitai Abramovitch, Hannah Reese, Douglas W. Woods, Alan Peterson, Thilo, Deckersbach, John Piacentini, Lawrence Scahill, Sabine Wilhelm
Access: Open access
- The gold-standard measure of tic severity in tic disorders (TD), the Yale Global Tic Severity Scale (YGTSS), is a semistructured clinician-administered interview that can be time consuming and requires highly trained interviewers. Moreover, the YGTSS does not provide information regarding frequency and intensity of specific tics because all motor and all vocal tics are rated as a group. The aim of the present study is to describe and test the Adult Tic Questionnaire (ATQ), a measure for the assessment of tic severity in adults, and to report its preliminary psychometric properties. The ATQ is a brief self-report questionnaire that provides information regarding frequency, intensity, and severity of 27 specific tics. In addition, the ATQ produces total frequency, intensity, and severity scores for vocal and motor tics, as well as a global total tic severity score. Results showed that the ATQ demonstrated very good internal consistency and temporal stability. The total, vocal, and motor tic severity scales of the ATQ showed strong correlation with corresponding subscales of the YGTSS, indicating strong convergent validity. Weak correlations with measures of severity of obsessive-compulsive disorder and attention deficit/hyperactivity disorder, indicated strong discriminant validity. The ATQ, a promising measure for the assessment of tic severity in adults with TD, may be a valuable supplement to the current recommended assessment battery for TD. Furthermore, the ATQ enables clinicians and researchers to track changes in the frequency and intensity of specific tics, which is important given their complex and dynamic nature.
Discord between the phylogenies inferred from molecular versus functional data: Uneven rates of functional evolution or low levels of gene flow?
Date: 1996-11-20
Creator: Michael F. Palopoli, Andrew W. Davis, Chung I. Wu
Access: Open access
- According to measures of molecular divergence, the three species of the Drosophila simulans clade are closely related to and essentially equidistant from each other. We introgressed 10% of the D. sechellia X chromosome into a pure D. simulans genetic background and found that males carrying this introgressed region were consistently fertile; in contrast, males carrying the same segment from D. mauritiana are sterile and suffer from incompatibilities at a minimum of four loci. Together with other recent results, these data suggest that D. simulans and D. sechellia are much more closely related to each other than either is to D. mauritiana. How can we reconcile the phylogeny inferred from the density of hybrid sterility genes with that inferred from molecular divergence? If the molecular phylogeny is correct, the discrepancy might be explained by uneven rates of functional evolution, resulting in the uneven accumulation of substitutions with corresponding negative effects in hybrids. If the functional phylogeny is correct, then low levels of gene flow across nascent species boundaries, particularly for loci not tightly linked to a hybrid sterility gene, may have erased the original pattern of lineage splitting. We propose tests that will allow us to discriminate between the hypotheses.
Within the Atrium: A Context for Roman Daily Life
Date: 1997-01-01
Creator: Anna-Maria Cannatella
Access: Open access
- "This brochure accompanies an exhibition of the same name at the Bowdoin College Museum of Art, Brunswick, Maine, from April 3 through June 8, 1997."
Characterization of Dissolved Organic Matter in Local Marine and Terrestrial Waters
Date: 2014-08-01
Creator: Anna Bearman
Access: Open access
- There are numerous anthropogenic pollutants present in both marine and terrestrial waters.1 Though many of these chemicals do not absorb light and therefore cannot undergo photolyic degradation on their own, dissolved organic matter (DOM), found alongside pollutants in natural aquatic waters, can act as a catalyst in the attenuating process of contaminants. DOM is a complex mixture of organic compounds derived from decaying plants, animals and microorganisms. Since DOM can absorb light, it can transfer energy to contaminants, allowing them to break into smaller and often less hazardous molecules. The behavior of DOM is largely determined by its functional chemical components, and the character of DOM is constantly changing with the environment. For example, two International Humic Substances Society standards Pony Lake DOM from Antarctica and Suwannee River DOM from Georgia demonstrate very different compositions and characteristics2. More important than simply identifying varying functional groups in DOM these standards, however, may be understanding how our local water in the Androscoggin River and Gulf of Maine behaves and attenuates contaminants. The goal for this project was to first isolate DOM from the Androscoggin River and the Gulf of Maine. DOM was extracted using the Thurman and Malcolm procedure,3 beginning with collection and filtration of water from the Androscoggin River boat launch and Simpson’s Point. The water was then run through a chromatography column, through the method of absorption chromatography the dissolved organic matter sticks to the resin within the column. DOM was then eluted from the column, concentrated, and protonated with an ion exchange column. The resulting concentrated DOM solution was then freeze-dried to obtain the final powdered DOM fraction. Because the quantity of DOM isolated from the Gulf of Maine was too small for characterization, we determined that a new collection method using equipment suited for sampling larger volumes of water will be necessary for future DOM characterization. Instead we focused on collecting samples from the Androscoggin on a weekly basis. Following isolation, the Androscoggin River DOM was dissolved in Type I water to make 3mg/L DOM samples and then characterized through UV-Vis absorption and 3D fluorescence Excitation-Emission Matrix (EEM) spectroscopy techniques. The data was then processed using the parallel factor analysis (PARAFAC) method.4 PARAFAC deconvolutes the fluorescence spectra into the distinct fluorescent components present in the complex DOM mixture (Figure 1). This preliminary analysis indicates that Androscoggin River DOM is made up of at least six specific fluorophores. In the future, I will identify the types of molecules responsible for each component signature and attempt to ascertain the relative concentration of each photoactive constituent in the DOM samples. This information will have significant implications for the photochemistry of natural and anthropogenic chemicals in natural waters. Funded by the Henry L. Grace Doherty Coastal Studies Research Fellowship and James Stacy Coles Summer Research Fellowship in Chemistry.
Parity moderates the effect of delivery mode on maternal ratings of infant temperament
Date: 2021-08-01
Creator: Lea Takács, Samuel P. Putnam, František Bartoš, Pavel Cepický, Catherine, Monk
Access: Open access
- Objective Cesarean section (CS) rates are rising rapidly around the world but no conclusive evidence has been obtained about the possible short- and long-term effects of CS on child behavior. We evaluated prospectively the association between CS and infant temperament across the first 9 postpartum months, controlling for indications for CS and investigating parity and infant sex as moderators. Methods The sample consisted of mothers and their healthy infants. Infant temperament was measured using the Infant Characteristics Questionnaire completed by the mothers at 6 weeks (n = 452) and 9 months (n = 258) postpartum. Mode of birth was classified into spontaneous vaginal birth (n = 347 for 6 weeks sample; 197 for 9 months sample), CS planned for medical reasons (n = 55; 28) and emergency CS (n = 50; 33). Results Multiple regression analysis revealed no main effects of birth mode, but showed a significant interaction between birth mode and parity indicating that emergency CS in firstborn infants was associated with more difficult temperament at 6 weeks. There were no significant associations between indications for CS and infant temperament, although breech presentation predicted difficult temperament at 9 months. Conclusion We largely failed to support the association between CS and infant temperament. Although our results suggest that emergency CS may be associated with temperament in firstborns, further research is needed to replicate this finding, preferably using observational measures to assess child temperament.
Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket gryllus bimaculatus
Date: 2013-09-27
Creator: Alexandra Pfister, Amy Johnson, Olaf Ellers, Hadley W. Horch
Access: Open access
- Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation. © 2013 Pfister, Johnson, Ellers and Horch.
Field information links permafrost carbon to physical vulnerabilities of thawing
Date: 2012-08-16
Creator: Jennifer W. Harden, Charles D. Koven, Chien Lu Ping, Gustaf Hugelius, A., David McGuire, Phillip Camill, Torre Jorgenson, Peter Kuhry, Gary J. Michaelson, Jonathan A. O'Donnell, Edward A.G. Schuur, Charles Tarnocai, Kristopher Johnson, Guido Grosse
Access: Open access
- Deep soil profiles containing permafrost (Gelisols) were characterized for organic carbon (C) and total nitrogen (N) stocks to 3m depths. Using the Community Climate System Model (CCSM4) we calculate cumulative distributions of active layer thickness (ALT) under current and future climates. The difference in cumulative ALT distributions over time was multiplied by C and N contents of soil horizons in Gelisol suborders to calculate newly thawed C and N. Thawing ranged from 147 PgC with 10 PgN by 2050 (representative concentration pathway RCP scenario 4.5) to 436 PgC with 29 PgN by 2100 (RCP 8.5). Organic horizons that thaw are vulnerable to combustion, and all horizon types are vulnerable to shifts in hydrology and decomposition. The rates and extent of such losses are unknown and can be further constrained by linking field and modelling approaches. These changes have the potential for strong additional loading to our atmosphere, water resources, and ecosystems. © 2012. American Geophysical Union. All Rights Reserved.