On the Origin of Fusiform Rust Resistance in Loblolly Pine

Studies of geographic variation in loblolly pine (Pinus taeda L.) have shown that seed sources from the western (generally west of the Mississippi River) and the northeastern part of the natural distribution are relatively resistant to fusiform rust, and those from elsewhere are susceptible. The greatest incidence of infection, on the other hand, is in the center of the distribution, exactly where the frequency of resistant genotypes is low. One might expect that the frequency of resistant genotypes would be higher where the disease is more prevalent, due to natural selection. It has been proposed that (1) fusiform rust resistance in loblolly pine in the west originates from hybridization with shortleaf pine. It is well known that shortleaf is resistant to fusiform rust, and it is also known that natural hybrids between the two species exist, and they seem to be more common in the west. (2) In the northeastern loblolly, it has been proposed that hybridization with pond pine is the source of resistance. Once again, natural hybridization between loblolly and pond pine is known to exist in the northeast, but not much is known about the relative resistance of pond pine to fusiform rust. Allozyme data was used to refute hypothesis (1) and cortical monoterpene data was used to refute hypothesis (2). A hypothesis is proposed involving selection during the Pleistocene to explain the present pattern of resistance and the development of a gene-for-gene pathosystem.Papers and abstracts from the 27th Southern Forest Tree Improvement Conference held at Oklahoma State University in Stillwater, Oklahoma on June 24-27, 2003

Evolution of Disease Response Genes in Loblolly Pine: Insights from Candidate Genes

BACKGROUND: Host-pathogen interactions that may lead to a competitive co-evolution of virulence and resistance mechanisms present an attractive system to study molecular evolution because strong, recent (or even current) selective pressure is expected at many genomic loci. However, it is unclear whether these selective forces would act to preserve existing diversity, promote novel diversity, or reduce linked neutral diversity during rapid fixation of advantageous alleles. In plants, the lack of adaptive immunity places a larger burden on genetic diversity to ensure survival of plant populations. This burden is even greater if the generation time of the plant is much longer than the generation time of the pathogen. METHODOLOGY/PRINCIPAL FINDINGS: Here, we present nucleotide polymorphism and substitution data for 41 candidate genes from the long-lived forest tree loblolly pine, selected primarily for their prospective influences on host-pathogen interactions. This dataset is analyzed together with 15 drought-tolerance and 13 wood-quality genes from previous studies. A wide range of neutrality tests were performed and tested against expectations from realistic demographic models. CONCLUSIONS/SIGNIFICANCE: Collectively, our analyses found that axr (auxin response factor), caf1 (chromatin assembly factor) and gatabp1 (gata binding protein 1) candidate genes carry patterns consistent with directional selection and erd3 (early response to drought 3) displays patterns suggestive of a selective sweep, both of which are consistent with the arm-race model of disease response evolution. Furthermore, we have identified patterns consistent with diversifying selection at erf1-like (ethylene responsive factor 1), ccoaoemt (caffeoyl-CoA-O-methyltransferase), cyp450-like (cytochrome p450-like) and pr4.3 (pathogen response 4.3), expected under the trench-warfare evolution model. Finally, a drought-tolerance candidate related to the plant cell wall, lp5, displayed patterns consistent with balancing selection. In conclusion, both arms-race and trench-warfare models seem compatible with patterns of polymorphism found in different disease-response candidate genes, indicating a mixed strategy of disease tolerance evolution for loblolly pine, a major tree crop in southeastern United States

Scanning tunneling and cathodoluminescence spectroscopy of indium nitride

Indium nitride epilayers grown by metalorganic vapor-phase epitaxy have been studied by cathodoluminescence (CL) spectroscopy, scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A broad CL emission peak centered at 0.8eV was observed at 80K. This peak was attributed to an excitonic radiative recombination mechanism as its emission intensity exhibited a super-linear dependence on beam current with a power-law exponent of m=2. A large spatial variation in the CL emission intensity was ascribed to the presence of threading dislocations, which act as non-radiative recombination centers. A surface band gap of ∼1.4eV was estimated from STS I-V curves. © 2004 Elsevier B.V. All rights reserved

Arrangement of nitrogen atoms in GaAsN alloys determined by scanning tunneling microscopy

The pair distribution function of nitrogen atoms in GaAs0.983N0.017 has been determined by scanning tunneling microscopy. Nitrogen atoms in the first and third planes relative to the cleaved (110) surface are imaged. A modest enhancement in the number of nearest-neighbor pairs particularly with [001] orientation is found, although at larger separations the distribution of N pair separations is found to be random. </em

Modular process modeling for OPC

Modular OPC modeling, describing mask, optics, resist and etch processes separately is an approach to keep efforts for OPC manageable. By exchanging single modules of a modular OPC model, a fast response to process changes during process development is possible. At the same time efforts can be reduced, since only single modular process steps have to be re-characterized as input for OPC modeling as the process is adjusted and optimized. Commercially available OPC tools for full chip processing typically make use of semi-empirical models. The goal of our work is to investigate to what extent these OPC tools can be applied to modeling of single process steps as separate modules. For an advanced gate level process we analyze the modeling accuracy over different process conditions (focus and dose) when combining models for each process step - optics, resist and etch - for differing single processes to a model describing the total process

Reconstructions of the GaAs (1 1 3) surface

We investigated GaAs (1 1 3) surfaces prepared by molecular beam epitaxy (MBE) and metal-organic vapour phase epitaxy (MOVPE) with reflectance anisotropy spectroscopy (RAS) and low energy electron diffraction (LEED) and found two different static (non-growth) reconstructions: For many conditions the well known (8×1) reconstruction is observed, while under more arsenic rich conditions a reconstruction a p(1×1) symmetry shows up. The activation energy for the transition from the p(1×1) to the (8×1) reconstruction was determined to (1.50±0.02) eV. A structure model for the arsenic rich p(1×1) reconstruction is presented in accordance with the above findings

In-situ ellipsometry: Identification of surface terminations during GaN growth

Spectroscopic ellipsometry (SE) is used to determine GaN surface termination during growth with metal-organic vapor phase epitaxy (MOVPE) by a correlation to well known results of plasma-assisted molecular beam epitaxy (PAMBE). The results manifest that in MOVPE under typical growth conditions the surface is not terminated by a Ga-bilayer as suggested for MBE. Moreover, it turns out that ellipsometry can be used to characterize the surface reconstruction in wurtzite GaN similar as reflectance anisotropy does for cubic III–V-compounds. The optical spectra for the PAMBE reveal clear differences between growth under Ga-rich and N-rich conditions, which are attributed to the presence of a Ga-bilayer and various N-rich reconstructions on the surface [1]. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</p