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Search Results: 1 - 10 of 407907 matches for " Steven M. Valles "
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Positive-Strand RNA Viruses Infecting the Red Imported Fire Ant, Solenopsis invicta
Steven M. Valles
Psyche , 2012, DOI: 10.1155/2012/821591
Abstract: The imported fire ants, Solenopsis invicta and S. richteri were introduced into the USA between 1918 and 1945. Since that time, they have expanded their USA range to include some 138 million hectares. Their introduction has had significant economic consequences with costs associated with damage and control efforts estimated at 6 billion dollars annually in the USA. The general consensus of entomologists and myrmecologists is that permanent, sustainable control of these ants in the USA will likely depend on self-sustaining biological control agents. A metagenomics approach successfully resulted in discovery of three viruses infecting S. invicta. Solenopsis invicta virus 1 (SINV-1), SINV-2, and SINV-3 are all positive, single-stranded RNA viruses and represent the first viral discoveries in any ant species. Molecular characterization, host relationships, and potential development and use of SINV-1, SINV-2, and SINV-3 as biopesticides are discussed.
Solenopsis invicta virus 3: Mapping of Structural Proteins, Ribosomal Frameshifting, and Similarities to Acyrthosiphon pisum virus and Kelp fly virus
Steven M. Valles, Susanne Bell, Andrew E. Firth
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0093497
Abstract: Solenopsis invicta virus 3 (SINV-3) is a positive-sense single-stranded RNA virus that infects the red imported fire ant, Solenopsis invicta. We show that the second open reading frame (ORF) of the dicistronic genome is expressed via a frameshifting mechanism and that the sequences encoding the structural proteins map to both ORF2 and the 3' end of ORF1, downstream of the sequence that encodes the RNA-dependent RNA polymerase. The genome organization and structural protein expression strategy resemble those of Acyrthosiphon pisum virus (APV), an aphid virus. The capsid protein that is encoded by the 3' end of ORF1 in SINV-3 and APV is predicted to have a jelly-roll fold similar to the capsid proteins of picornaviruses and caliciviruses. The capsid-extension protein that is produced by frameshifting, includes the jelly-roll fold domain encoded by ORF1 as its N-terminus, while the C-terminus encoded by the 5' half of ORF2 has no clear homology with other viral structural proteins. A third protein, encoded by the 3' half of ORF2, is associated with purified virions at sub-stoichiometric ratios. Although the structural proteins can be translated from the genomic RNA, we show that SINV-3 also produces a subgenomic RNA encoding the structural proteins. Circumstantial evidence suggests that APV may also produce such a subgenomic RNA. Both SINV-3 and APV are unclassified picorna-like viruses distantly related to members of the order Picornavirales and the family Caliciviridae. Within this grouping, features of the genome organization and capsid domain structure of SINV-3 and APV appear more similar to caliciviruses, perhaps suggesting the basis for a "Calicivirales" order.
Metatranscriptomics and Pyrosequencing Facilitate Discovery of Potential Viral Natural Enemies of the Invasive Caribbean Crazy Ant, Nylanderia pubens
Steven M. Valles, David H. Oi, Fahong Yu, Xin-Xing Tan, Eileen A. Buss
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0031828
Abstract: Background Nylanderia pubens (Forel) is an invasive ant species that in recent years has developed into a serious nuisance problem in the Caribbean and United States. A rapidly expanding range, explosive localized population growth, and control difficulties have elevated this ant to pest status. Professional entomologists and the pest control industry in the United States are urgently trying to understand its biology and develop effective control methods. Currently, no known biological-based control agents are available for use in controlling N. pubens. Methodology and Principal Findings Metagenomics and pyrosequencing techniques were employed to examine the transcriptome of field-collected N. pubens colonies in an effort to identify virus infections with potential to serve as control agents against this pest ant. Pyrosequencing (454-platform) of a non-normalized N. pubens expression library generated 1,306,177 raw sequence reads comprising 450 Mbp. Assembly resulted in generation of 59,017 non-redundant sequences, including 27,348 contigs and 31,669 singlets. BLAST analysis of these non-redundant sequences identified 51 of potential viral origin. Additional analyses winnowed this list of potential viruses to three that appear to replicate in N. pubens. Conclusions Pyrosequencing the transcriptome of field-collected samples of N. pubens has identified at least three sequences that are likely of viral origin and, in which, N. pubens serves as host. In addition, the N. pubens transcriptome provides a genetic resource for the scientific community which is especially important at this early stage of developing a knowledgebase for this new pest.
Cooper pair islanding model of insulating nanohoneycomb films
S. M. Hollen,J. M. Valles Jr
Physics , 2011, DOI: 10.1088/1742-6596/376/1/012002
Abstract: We first review evidence for the Cooper pair insulator (CPI) phase in amorphous nanohoneycomb (NHC) films. We then extend our analysis of superconducting islands induced by film thickness variations in NHC films to examine the evolution of island sizes through the magnetic field-driven SIT. Finally, using the islanding picture, we present a plausible model for the appearance and behavior of the CPI phase in amorphous NHC films.
Temperature dependence of the superheating field in niobium
N. R. A. Valles,M. U. Liepe
Physics , 2010,
Abstract: This study experimentally investigates the temperature dependence of superheating field, Hsh, of niobium. Accurately determining this field is important both to test theory and to understand gradient limits in superconducting cavities for particle accelerators. This paper discusses theories that have been proposed in modeling the field and discriminates between them. The experimental procedure for measuring the temperature dependence of Hsh utilizes high power pulses to drive a niobium cavity resonator, ramping up surface magnetic fields extremely quickly. The moment any part of the cavity transitions between the superconducting and normal conducting state can be determined by measuring the quality factor of the cavity as a function of time. Oscillating superleak transducers are used to demonstrate that the transition to the normal conducting state is global in nature, showing that a fundamental limit is encountered. Finally, we see that 110-120 C heat treatment of the cavity--a method commonly used to increase the quality factor at high accelerating gradients--may have the deleterious effect of reducing the superheating field of the material, which is the fundamental limiting factor in pursuing the maximal achievable accelerating gradient in superconducting niobium cavities.
Observation of Critical Amplitude Fluctuations near the Two Dimensional Superconductor-Insulator Transition
J. A. Chervenak,J. M. Valles, Jr
Physics , 1999, DOI: 10.1103/PhysRevB.59.11209
Abstract: We report results of transport measurements in the quantum critical regime of the disorder tuned, 2D superconductor-insulator transition (SIT) in homogeneously disordered films. We show that, as the superconducting transition temperature decreases, the transition width grows, appearing to diverge at the SIT. In addition, structure develops in the DC current-voltage characteristics of films closest to the SIT indicating that the 2D superconductivity is driven into a regime of extreme inhomogeneity. The data suggest a picture of the phase transition in which large amplitude fluctuations occur as the amplitude is suppressed to near zero by disorder.
Early stage morphology of quench condensed Ag, Pb and Pb/Ag hybrid films
Zhenyi Long,James M. Valles Jr
Physics , 2004, DOI: 10.1007/s10909-005-4732-3
Abstract: Scanning Tunneling Microscopy (STM) has been used to study the morphology of Ag, Pb and Pb/Ag bilayer films fabricated by quench condensation of the elements onto cold (T=77K), inert and atomically flat Highly Oriented Pyrolytic Graphite (HOPG) substrates. All films are thinner than 10 nm and show a granular structure that is consistent with earlier studies of QC films. The average lateral diameter, $\bar {2r}$, of the Ag grains, however, depends on whether the Ag is deposited directly on HOPG ($\bar {2r}$ = 13 nm) or on a Pb film consisting of a single layer of Pb grains ($\bar {2r}$ = 26.8 nm). In addition, the critical thickness for electrical conduction ($d_{G}$) of Pb/Ag films on inert glass substrates is substantially larger than for pure Ag films. These results are evidence that the structure of the underlying substrate exerts an influence on the size of the grains in QC films. We propose a qualitative explanation for this previously unencountered phenomenon.
Absence of a Zero Temperature Vortex Solid Phase in Strongly Disordered Superconducting Bi Films
J. A. Chervenak,J. M. Valles, Jr
Physics , 1999, DOI: 10.1103/PhysRevB.61.R9245
Abstract: We present low temperature measurements of the resistance in magnetic field of superconducting ultrathin amorphous Bi films with normal state sheet resistances, $R_N$, near the resistance quantum, $R_Q={\hbar\over {e^2}}$. For $R_NR_Q$ there is no vortex solid and, hence, no zero resistance state in magnetic field. We describe how disorder induced quantum and/or mesoscopic fluctuations can eliminate the vortex solid and also discuss implications for the magnetic-field-tuned superconductor-insulator transition.
Deviations from mean-field behavior in disordered nanoscale superconductor-normal-metal-superconductor arrays
Taejoon Kouh,J. M. Valles, Jr.
Physics , 2002, DOI: 10.1103/PhysRevB.67.140506
Abstract: We have fabricated quasi-two-dimensional arrays of nano-scale Pb grains coupled by an overlayer of Ag grains. Their temperature dependent resistive transitions follow predictions for an array of mesoscopic superconductor-normal-superconductor junctions. The decrease of their transition temperatures with Ag overlayer thickness systematically deviates from the Cooper limit theory of the proximity effect as the Pb grain size decreases. The deviations occur when the estimated number of Cooper pairs per grain is less than or equal to 1 and suggest the approach to a superconductor to metal transition.
"Super weakly" coupled superconductivity in ultrathin superconductor-normal-metal bilayers
Zhenyi Long,M. D. Stewart Jr,James M. Valles Jr
Physics , 2005, DOI: 10.1103/PhysRevB.73.140507
Abstract: Tunneling measurements of the temperature dependence of the electronic density of states (DOS) of ultrathin bilayers of Pb and Ag reveal that their superconducting energy gap, $\Delta (T)$, evolves similarly to BCS predictions despite the presence of a large anomalous DOS at the Fermi energy that persists as $T\to 0$. The gap ratio, ${2\Delta (0)}\over {k_BT_c}$, systematically decreases more than 20% below the BCS universal value of 3.52 in bilayers with the lowest superconducting transition temperatures, $T_c$; a behavior we deem super weakly coupled. A semi-quantitative model suggests that the reduced gap ratios result from a systematic depletion of the DOS available for pairing that occurs with the growth of the anomalous DOS at the Fermi energy.
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