Chytrid fungus infection in alpine tree frogs is associated with individual heterozygosity and population isolation but not population-genetic diversity

Chytridiomycosis, a disease caused by the emerging fungus Batrachochytrium dendrobatidis (Bd), has been implicated in the decline of over 500 amphibian species. Population declines could have important genetic consequences, including reduced genetic diversity. We contrasted genetic diversity among both long-Bd-exposed and unexposed populations of the south-east Australian alpine tree frog (Litoria verreauxii alpina) across its range. At the population level, we found no significant differences in genetic diversity between Bd-exposed and unexposed populations. Encouragingly, even Bd-infected remnant populations that are now highly isolated maintain genetic diversity comparable to populations in which Bd is absent. Spatial genetic structure among populations followed an isolation-by-distance pattern, suggesting restricted movement among remnant populations. At the individual level, greater heterozygosity was associated with reduced probability of infection. Loss of genetic diversity in remnant populations that survived chytridiomycosis epidemics does not appear to be a threat to L. v. alpina. We suggest several factors underpinning maintenance of genetic diversity: (1) remnant populations have remained large enough to avoid losses of genetic diversity; (2) many individuals in the population are able to breed once before succumbing to disease; and (3) juveniles in the terrestrial environment have low exposure to Bd, providing an annual ‘reservoir’ of genetic diversity. The association between individual heterozygosity and infection status suggests that, while other work has shown all breeding adults are typically killed by Bd, males with greater heterozygosity may survive longer and obtain fitness benefits through extended breeding opportunities. Our results highlight the critical role of life history in mitigating the impacts of Bd infection for some amphibian species, but we infer that increased isolation as a result of disease-induced population extirpations will enhance population differentiation and thus biogeographic structure

Electron-Hole Recollisions in Driven Quantum Wells

Driving semiconductor quantum wells with terahertz electric fields strong enough to overcome the Coulomb attraction between bound electron-hole pairs leads to high-order sideband generation (HSG). In HSG, excitons are optically-injected into quantum wells by a weak near-infrared (NIR) laser while simultaneously being illuminated with a terahertz field from the UCSB Free Electron Laser. The phenomenon can be described by the so-called "three step model" developed in high-field atomic physics: (1) the electron and hole tunnel-ionize in the strong field, (2) the now-free particles accelerate in the field, and (3) they recollide, emitting a photon. The two lasers are continuous, so the emitted photons are sidebands on the NIR laser. Because of the large gain of kinetic energy before recollision, an HSG spectrum has a broad bandwidth with many more sidebands above the NIR frequency than below. The largest spectra span over one hundred nanometers, with over 100th order sidebands above and 20th order below.The electron and hole must remain coherent throughout their trajectories, which can last hundreds of femtoseconds and extend for more than fifty nanometers, if they are to recollide. Sidebands have been observed that result from recollisions with kinetic energies far above the threshold for optical phonon emission. These high orders persist up to room temperature. Not even quenched disorder in the quantum wells strongly attenuates the HSG signal. Because of this coherence, the electron and hole are very sensitive to the complete band structure of the material. Excitation by linear NIR polarization creates both the electron and hole in a superposition of spin-up and spin-down states with complex coefficients given by the relative orientation of the NIR polarization and the THz polarization. Interference between these the spin-up and spin-down particles, particularly in the valence band and mediated by non-Abelian Berry curvature, has large effects on both the intensity and polarization state of the sidebands. The connection between HSG and complete band structure points to the possibility of directly measuring both the band dispersion relations as well as the non-Abelian Berry curvature of the host material

Terahertz electron-hole recollisions in GaAs/AlGaAs quantum wells: robustness to scattering by optical phonons and thermal fluctuations

Electron-hole recollisions are induced by resonantly injecting excitons with a near-IR laser at frequency $f_{\text{NIR}}$ into quantum wells driven by a ~10 kV/cm field oscillating at $f_{\text{THz}} = 0.57$ THz. At $T=12$ K, up to 18 sidebands are observed at frequencies $f_{\text{sideband}}=f_{\text{NIR}}+2n f_{\text{THz}}$, with $-8 \le 2n \le 28$. Electrons and holes recollide with total kinetic energies up to 57 meV, well above the $E_{\text{LO}} = 36$ meV threshold for longitudinal optical (LO) phonon emission. Sidebands with order up to $2n=22$ persist up to room temperature. A simple model shows that LO phonon scattering suppresses but does not eliminate sidebands associated with kinetic energies above $E_{\text{LO}}$.Comment: 5 pages, 4 figure

Optical frequency combs from high-order sideband generation

We report on the generation of frequency combs from the recently-discovered phenomenon of high-order sideband generation (HSG). A near-band gap continuous-wave (cw) laser with frequency $f_\text{NIR}$ was transmitted through an epitaxial layer containing GaAs/AlGaAs quantum wells that were driven by quasi-cw in-plane electric fields $F_\text{THz}$ between 4 and 50 kV/cm oscillating at frequencies $f_\text{THz}$ between 240 and 640 GHz. Frequency combs with teeth at $f_\text{sideband}=f_\text{NIR}+nf_\text{THz}$ ($n$ even) were produced, with maximum reported $n>120$, corresponding to a maximum comb span $>80$ THz. Comb spectra with the identical product $f_\text{THz}\times F_\text{THz}$ were found to have similar spans and shapes in most cases, as expected from the picture of HSG as a scattering-limited electron-hole recollision phenomenon. The HSG combs were used to measure the frequency and linewidth of our THz source as a demonstration of potential applications

Dynamical birefringence: Electron-hole recollisions as probes of Berry curvature

The direct measurement of Berry phases is still a great challenge in condensed matter systems. The bottleneck has been the ability to adiabatically drive an electron coherently across a large portion of the Brillouin zone in a solid where the scattering is strong and complicated. We break through this bottleneck and show that high-order sideband generation (HSG) in semiconductors is intimately affected by Berry phases. Electron-hole recollisions and HSG occur when a near-band gap laser beam excites a semiconductor that is driven by sufficiently strong terahertz (THz)-frequency electric fields. We carried out experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum wells. The observed HSG spectra contain sidebands up to the 90th order, to our knowledge the highest-order optical nonlinearity observed in solids. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone around the \Gamma point. The principal experimental claim is a dynamical birefringence: the sidebands, when the order is high enough (> 20), are usually stronger when the exciting near-infrared (NIR) and the THz electric fields are polarized perpendicular than parallel; the sideband intensities depend on the angles between the THz field and the crystal axes in samples with sufficiently weak quenched disorder; and the sidebands exhibit significant ellipticity that increases with increasing sideband order, despite nearly linear excitation and driving fields. We explain dynamical birefringence by generalizing the three-step model for high order harmonic generation. The hole accumulates Berry phases due to variation of its internal state as the quasi-momentum changes under the THz field. Dynamical birefringence arises from quantum interference between time-reversed pairs of electron-hole recollision pathways

Cortex-wide, cellular-resolution two-photon microscopy

Functional imaging of the mouse brain in its extreme complexity involves substantial trade-offs. An optical intrinsic spectroscopy system can image the entire cortex but at the expense of spatial and temporal resolution [1]. A two-photon microscope (TPM) can image single neurons with high temporal resolution, but the field of view (FOV) is generally restricted. Advanced techniques like random-access scanning allow for imaging single neurons that are millimeters apart but only by ignoring the neurons and tissue in between [2]. By carefully considering the properties of the optical components as well as the imaging requirements, we present a TPM capable of imaging nearly the entire mouse cortex with 15 Hz frame rates and single neuron resolution. Please click Additional Files below to see the full abstract

Prion protein in the cerebrospinal fluid of healthy and naturally scrapie-affected sheep

The aim of this study was to characterize the cerebrospinal fluid (CSF) prion protein (PrP) of healthy and naturally scrapie-affected sheep. The soluble form of CSF PrPC immunoblotted with an anti-octarepeat and an anti-C terminus mAb showed two isoforms of approximately 33 and 26 kDa, corresponding to the biglycosylated and unglycosylated isoforms of brain PrPC. Neither the mean concentration nor the electrophoretic profile of CSF PrP differed between healthy and scrapieaffected sheep, whereas a slightly increased resistance of CSF PrP to mild proteolysis by proteinase K was evident in the CSF of scrapie-affected sheep. No difference in susceptibility to proteolysis was observed between the two ARR and VRQ genetic variants of the purified prokaryote recombinant PrP. It was concluded that the physicochemical properties of PrPC in the CSF could be altered during scrapie and that these changes might reflect the physiopathological process of prion disease