Genetic consequences of breaking migratory traditions in barnacle geese Branta leucopsis.

Cultural transmission of migratory traditions enables species to deal with their environment based on experiences from earlier generations. Also, it allows a more adequate and rapid response to rapidly changing environments. When individuals break with their migratory traditions, new population structures can emerge that may affect gene flow. Recently, the migratory traditions of the Barnacle Goose Branta leucopsis changed, and new populations differing in migratory distance emerged. Here, we investigate the population genetic structure of the Barnacle Goose to evaluate the consequences of altered migratory traditions. We used a set of 358 single nucleotide polymorphism (SNP) markers to genotype 418 individuals from breeding populations in Greenland, Spitsbergen, Russia, Sweden and the Netherlands, the latter two being newly emerged populations. We used discriminant analysis of principal components, FST , linkage disequilibrium and a comparison of geneflow models using migrate-n to show that there is significant population structure, but that relatively many pairs of SNPs are in linkage disequilibrium, suggesting recent admixture between these populations. Despite the assumed traditions of migration within populations, we also show that genetic exchange occurs between all populations. The newly established nonmigratory population in the Netherlands is characterized by high emigration into other populations, which suggests more exploratory behaviour, possibly as a result of shortened parental care. These results suggest that migratory traditions in populations are subject to change in geese and that such changes have population genetic consequences. We argue that the emergence of nonmigration probably resulted from developmental plasticity.

Discrimination of hybrid classes using cross-species amplification of microsatellite loci: methodological challenges and solutions in Daphnia.

Microsatellite markers are important tools in population, conservation and forensic studies and are frequently used for species delineation, the detection of hybridization and introgression. Therefore, marker sets that amplify variable DNA regions in two species are required; however, cross-species amplification is often difficult, as genotyping errors such as null alleles may occur. To estimate the level of potential misidentifications based on genotyping errors, we compared the occurrence of parental alleles in laboratory and natural Daphnia hybrids (Daphnia longispina group). We tested a set of 12 microsatellite loci with regard to their suitability for unambiguous species and hybrid class identification using F(1) hybrids bred in the laboratory. Further, a large set of 44 natural populations of D. cucullata, D. galeata and D. longispina (1715 individuals) as well as their interspecific hybrids were genotyped to validate the discriminatory power of different marker combinations. Species delineation using microsatellite multilocus genotypes produced reliable results for all three studied species using assignment tests. Daphnia galeata × cucullata hybrid detection was limited due to three loci exhibiting D. cucullata-specific null alleles, which most likely are caused by differences in primer-binding sites of parental species. Overall, discriminatory power in hybrid detection was improved when a subset of markers was identified that amplifies equally well in both species.

MRI with an atomic magnetometer suitable for practical imaging applications.

Conventionally implemented MRI is performed in a strong magnetic field, typically >1T. The high fields, however, can lead to many limitations. To overcome these limitations, ultra-low field (ULF) [or microtesla] MRI systems have been proposed and implemented. To-date such systems rely on low-Tc Superconducting Quantum Interference Devices (SQUIDs) leading to the requirement of cryogens. In this letter, we report ULF-MRI obtained with a non-cryogenic atomic magnetometer. This demonstration creates opportunities for developing inexpensive and widely applicable MRI scanners.

Toward direct neural current imaging by resonant mechanisms at ultra-low field.

A variety of techniques have been developed to noninvasively image human brain function that are central to research and clinical applications endeavoring to understand how the brain works and to detect pathology (e.g. epilepsy, schizophrenia, etc.). Current methods can be broadly divided into those that rely on hemodynamic responses as indicators of neural activity (e.g. fMRI, optical, and PET) and methods that measure neural activity directly (e.g. MEG and EEG). The approaches all suffer from poor temporal resolution, poor spatial localization, or indirectly measuring neural activity. It has been suggested that the proton spin population will be altered by neural activity resulting in a measurable effect on the NMR signal that can be imaged by MRI methods. We present here the physical basis and experimental evidence for the resonant interaction between magnetic fields such as those arising from neural activity, with the spin population in ultra-low field (microT) NMR experiments. We demonstrate through the use of current phantoms that, in the case of correlated zero-mean current distributions such as those one might expect to result from neural activity, resonant interactions will produce larger changes in the observed NMR signal than dephasing. The observed resonant interactions reported here might one day form the foundation of a new functional neuroimaging modality ultimately capable of simultaneous direct neural activity and brain anatomy tomography.

Ultra-low field NMR measurements of liquids and gases with short relaxation times.

Interest in nuclear magnetic resonance measurements at ultra-low magnetic fields (ULF, approximately microT fields) has been motivated by various benefits and novel applications including narrow NMR peak-width, negligible susceptibility artifacts, imaging of samples inside metal containers, and possibility of directly imaging neuronal currents. ULF NMR/MRI is also compatible with simultaneous measurements of biomagnetic signals. However the most widely used technique in ULF NMR-prepolarization at high field and measurement at lower field-results in large transient signals which distort the free induction decay signal. This is especially severe for the measurement of signals from samples and materials with short T1 time. We have devised an approach that largely cancels the transient signals. The technique was successfully used to measure NMR signals from liquids and gases with T1 in the range 1-4 ms.

On concomitant gradients in low-field MRI.

Growing interest in magnetic resonance imaging (MRI) at ultra-low magnetic fields (ULF, approximately muT fields) has been motivated by several advantages over its counterparts at higher magnetic fields. These include narrow line widths, the possibility of novel imaging schemes, reduced imaging artifacts from susceptibility variations within a sample, and reduced system cost and complexity. In addition, ULF NMR/MRI with superconducting quantum interference devices is compatible with simultaneous measurements of biomagnetic signals, a capability conventional systems cannot offer. Acquisition of MRI at ULF must, however, account for concomitant gradients that would otherwise result in severe image distortions. In this paper, we introduce the general theoretical framework that describes concomitant gradients, explain why such gradients are more problematic at low field, and present possible approaches to correct for these unavoidable gradients in the context of a non-slice-selective MRI protocol.

Noise-free magnetoencephalography recordings of brain function.

Perhaps the greatest impediment to acquiring high-quality magnetoencephalography (MEG) recordings is the ubiquitous ambient magnetic field noise. We have designed and built a whole-head MEG system using a helmet-like superconducting imaging surface (SIS) surrounding the array of superconducting quantum interference device (SQUID) magnetometers used to measure the MEG signal. We previously demonstrated that the SIS passively shields the SQUID array from ambient magnetic field noise, independent of frequency, by 25-60 dB depending on sensor location. SQUID 'reference sensors' located on the outside of the SIS helmet measure ambient magnetic fields in very close proximity to the MEG magnetometers while being nearly perfectly shielded from all sources in the brain. The fact that the reference sensors measure no brain signal yet are located in close proximity to the MEG sensors enables very accurate estimation and subtraction of the ambient field noise contribution to the MEG sensors using an adaptive algorithm. We have demonstrated total ambient noise reduction factors in excess of 10(6) (> 120 dB). The residual noise for most MEG SQUID channels is at or near the intrinsic SQUID noise floor, typically 2-3 fT Hz-1/2. We are recording MEG signals with greater signal-to-noise than equivalent EEG measurements.

Forward model theoretical basis for a superconducting imaging surface magnetoencephalography system.

A novel magnetoencephalography (MEG) system was designed at Los Alamos National Laboratory (LANL) that incorporates a helmet-shaped superconductor in order to increase the signal to noise ratio. The magnetic field perturbations caused by the superconducting surface must be included in the forward physics for accurate source localization. In this paper, the theoretical basis for the forward model that calculates the field of any magnetic source in the presence of an arbitrarily shaped superconducting surface is presented. Appropriate magnetic field integral equations are derived that provide a description of the physics of the forward model. These equations are derived starting from Maxwell's equations in the presence of inhomogeneous media, with the appropriate boundary conditions for a superconductor. A discretized version of this equation is then compared with known analytic solutions for simple superconducting surface geometries.

Simultaneously detected biomagnetic signals and NMR.

We have obtained 1H NMR spectra simultaneously with high temporal resolution biomagnetic signals such as the magnetocardiogram (MCG) and magnetomyogram (MMG). The NMR spectra are acquired at measurement fields of 2-50 microT, with corresponding proton Larmor frequencies of 80-2000 Hz. Our measurements demonstrate a method suitable for MR imaging with concurrent measurement of biomagnetic signals that can provide sub-millisecond temporal resolution. The narrow line widths, reduction in susceptibility noise and enhanced spectral resolution at ultra low fields provide a new and extremely sensitive measurement method that may enable direct imaging of biological currents by detecting the phase or frequency shifts produced by magnetic fields arising from those currents. The results of our simultaneous measurements of NMR with MCG and MMG are compared to results from a current phantom to investigate the exciting potential of direct MRI of bioelectric currents.