Extensive polymorphism and geographical variation at a positively selected MHC class II B gene of the lesser kestrel (Falco naumanni)

Understanding the selective forces that shape genetic variation in natural populations remains a high priority in evolutionary biology. Genes at the major histocompatibility complex (MHC) have become excellent models for the investigation of adaptive variation and natural selection because of their crucial role in fighting off pathogens. Here we present one of the first data sets examining patterns of MHC variation in wild populations of a bird of prey, the lesser kestrel, Falco naumanni. We report extensive polymorphism at the second exon of a putatively functional MHC class II gene, Fana-DAB*1. Overall, 103 alleles were isolated from 121 individuals sampled from Spain to Kazakhstan. Bayesian inference of diversifying selection suggests that several amino acid sites may have experienced strong positive selection (ω = 4.02 per codon). The analysis also suggests a prominent role of recombination in generating and maintaining MHC diversity (ρ = 4Nc = 0.389 per codon, θ = 0.017 per codon). Both the Fana-DAB*1 locus and a set of eight polymorphic microsatellite markers revealed an isolation-by-distance pattern across the Western Palaearctic (r = 0.67; P = 0.01 and r = 0.50; P = 0.04, respectively). Nonetheless, geographical variation at the MHC contrasts with relatively uniform distributions in the frequencies of microsatellite alleles. In addition, we found lower fixation rates in the MHC than those predicted by genetic drift after controlling for neutral mitochondrial sequences. Our results therefore underscore the role of balancing selection as well as spatial variations in parasitemediated selection regimes in shaping MHC diversity when gene flow is limited.Peer reviewe

Magnetotransport in a pseudomorphic GaAs/GaInAs/GaAlAs heterostructure with a Si delta-doping layer

Magnetotransport properties of a pseudomorphic GaAs/Ga0.8In0.2As/Ga0.75Al0.25As heterostructure are investigated in pulsed magnetic fields up to 50 T and at temperatures of T=1.4 K and 4.2 K. The structure studied consists of a Si delta-layer parallel to a Ga0.8In0.2As quantum well (QW). The dark electron density of the structure is n_e=1.67x 10^16 m^-2. By illumination the density can be increased up to a factor of 4; this way the second subband in the Ga0.8In0.2As QW can become populated as well as the Si delta-layer. The presence of electrons in the delta-layer results in drastic changes in the transport data, especially at magnetic fields beyond 30 T. The phenomena observed are interpreted as: 1) magnetic freeze-out of carriers in the delta-layer when a low density of electrons is present in the delta-layer, and 2) quantization of the electron motion in the two dimensional electron gases in both the Ga0.8In0.2As QW and the Si delta-layer in the case of high densities. These conclusions are corroborated by the numerical results of our theoretical model. We obtain a satisfactory agreement between model and experiment.Comment: 23 pages, RevTex, 11 Postscript figures (accepted for Phys. Rev. B

A Monte Carlo Technique to Investigate Signal Delays of Advanced Si BJT's up to High Currents

We present a new Monte Carlo technique to investigate the signal delay of advanced BJTs featuring relevant non-local effects. The method is suited to analyze base and collector signal delays in presence of significant nonequilibrium transport effects and up to high currents, and to verify the physical meaning and applicability of delay expressions for compact models.We present a new Monte Carlo technique to investigate the signal delay of advanced BJTs featuring relevant non-local effects. The method is suited to analyze base and collector signal delays in presence of significant nonequilibrium transport effects and up to high currents, and to verify the physical meaning and applicability of delay expressions for compact models

Analysis of highly non-uniform collector doping profiles for the optimization of the breakdown / speed trade off in advanced BJTs

Philips Research Nat. Laboratories Report 808/9

A figure of merit for the high-frequency noise behavior of bipolar-transistors

A new figure of merit for high-frequency noise behavior for use in the evaluation and development of bipolar silicon process technology is introduced. Basic low noise design rules for optimum transistor biasing and emitter scaling are proposed

A better insight in the performance of silicon bjt's featuring highly non-uniform collector doping profiles

This paper investigates the effects of highly nonuniform collector doping profiles on the speed and breakdown performance of silicon bipolar transistors. Monte Carlo and drift diffusion simulation results point out that a thin highly doped layer adjacent to the base collector junction can improve the device cut off frequency without deteriorating significantly the maximum oscillation frequency and the breakdown voltage, provided the voltage drop across this layer is lower than an effective threshold of approximately 1.2 V. Guidelines are given for choosing the doping, position, and thickness of this layer