351 research outputs found
Retention of low-fitness genotypes over six decades of admixture between native and introduced tiger salamanders
<p>Abstract</p> <p>Background</p> <p>Introductions of non-native tiger salamanders into the range of California tiger salamanders have provided a rare opportunity to study the early stages of secondary contact and hybridization. We produced first- and second-generation hybrid salamanders in the lab and measured viability among these early-generation hybrid crosses to determine the strength of the initial barrier to gene exchange. We also created contemporary-generation hybrids in the lab and evaluated the extent to which selection has affected fitness over approximately 20 generations of admixture. Additionally, we examined the inheritance of quantitative phenotypic variation to better understand how evolution has progressed since secondary contact.</p> <p>Results</p> <p>We found significant variation in the fitness of hybrids, with non-native backcrosses experiencing the highest survival and F2 hybrids the lowest. Contemporary-generation hybrids had similar survival to that of F2 families, contrary to our expectation that 20 generations of selection in the wild would eliminate unfit genotypes and increase survival. Hybrid survival clearly exhibited effects of epistasis, whereas size and growth showed mostly additive genetic variance, and time to metamorphosis showed substantial dominance.</p> <p>Conclusions</p> <p>Based on first- and second- generation cross types, our results suggest that the initial barrier to gene flow between these two species was relatively weak, and subsequent evolution has been generally slow. The persistence of low-viability recombinant hybrid genotypes in some contemporary populations illustrates that while hybridization can provide a potent source of genetic variation upon which natural selection can act, the sorting of fit from unfit gene combinations might be inefficient in highly admixed populations. Spatio-temporal fluctuation in selection or complex genetics has perhaps stalled adaptive evolution in this system despite selection for admixed genotypes within generations.</p
Optically controlled spin-glasses in multi-qubit cavity systems
Recent advances in nanostructure fabrication and optical control, suggest
that it will soon be possible to prepare collections of interacting two-level
systems (i.e. qubits) within an optical cavity. Here we show theoretically that
such systems could exhibit novel phase transition phenomena involving
spin-glass phases. By contrast with traditional realizations using magnetic
solids, these phase transition phenomena are associated with both matter and
radiation subsystems. Moreover the various phase transitions should be tunable
simply by varying the matter-radiation coupling strength.Comment: 4 pages, 3 figure
Non-adaptive phenotypic plasticity: the effects of terrestrial and aquatic herbicides on larval salamander morphology and swim speed
Department of Biology, University of North Carolina, Chapel Hill, NC, USAPhenotypic plasticity, although ubiquitous, may not always be advantageous. Non-adaptive plasticity is likely to occur in response to novel environmental stress. Anthropogenic contaminants, such as herbicides, are novel stressors that are not present in the evolutionary history of most species. We investigated the pattern and consequences of phenotypic plasticity induced by four glyphosate-based herbicides (two terrestrial and two aquatic) in larvae of the spotted salamander, , by determining (1) whether the herbicides induced different morphologies; (2) if different morphologies translated to differences in burst swim performance; and (3) how induced individuals performed relative to non-induced controls. Different herbicide formulations led to the production of significantly different head and tail morphologies, and tail morphology correlated with fastest escape speed. However, escape speed did not vary among treatments. In addition, three out of four herbicide treatments experienced accelerated growth rates, in terms of the lateral size of tails, although the tail shapes were either similar to preliminary controls or intermediate between preliminary and final controls. These observations suggest that herbicide-induced morphology is a case of non-adaptive phenotypic plasticity, and that there is potentially a trade-off between growth and development for larvae exposed to different formulations. Understanding the functional significance of induced phenotypes is important for determining their importance in shaping an organism's ecological interactions and evolutionary trajectories. Furthermore, under different conditions, the morphological changes that we observed in response to exposure to herbicides might affect salamander fitness and influence population dynamics
The First Supernova Explosions: Energetics, Feedback, and Chemical Enrichment
We perform three-dimensional smoothed particle hydrodynamics simulations in a
realistic cosmological setting to investigate the expansion, feedback, and
chemical enrichment properties of a 200 M_sun pair-instability supernova in the
high-redshift universe. We find that the SN remnant propagates for a Hubble
time at z = 20 to a final mass-weighted mean shock radius of 2.5 kpc (proper),
roughly half the size of the HII region, and in this process sweeps up a total
gas mass of 2.5*10^5 M_sun. The morphology of the shock becomes highly
anisotropic once it leaves the host halo and encounters filaments and
neighboring minihalos, while the bulk of the shock propagates into the voids of
the intergalactic medium. The SN entirely disrupts the host halo and terminates
further star formation for at least 200 Myr, while in our specific case it
exerts positive mechanical feedback on neighboring minihalos by
shock-compressing their cores. In contrast, we do not observe secondary star
formation in the dense shell via gravitational fragmentation, due to the
previous photoheating by the progenitor star. We find that cooling by metal
lines is unimportant for the entire evolution of the SN remnant, while the
metal-enriched, interior bubble expands adiabatically into the cavities created
by the shock, and ultimately into the voids with a maximum extent similar to
the final mass-weighted mean shock radius. Finally, we conclude that dark
matter halos of at least M_vir > 10^8 M_sun must be assembled to recollect all
components of the swept-up gas.Comment: 16 pages, 14 figures, published in Ap
Local radiative feedback in the formation of the first protogalaxies
The formation of the first galaxies is influenced by the radiative feedback
from the first generations of stars. This feedback is manisfested by the
heating and ionization of the gas which lies within the H II regions
surrounding the first stars, as well as by the photodissociation of hydrogen
molecules within the larger Lyman-Werner (LW) bubbles that surround these
sources. Using a ray-tracing method in three-dimensional cosmological
simulations, we self-consistently track the formation of, and radiative
feedback from, individual stars in the course of the formation of a
protogalaxy. We compute in detail the H II regions of each of these sources, as
well as the regions affected by their molecule-dissociating radiation. We
follow the thermal, chemical, and dynamical evolution of the primordial gas, as
it becomes incorporated into the protogalaxy. While the IGM is, in general,
optically thick to LW photons only over physical distances of > 30 kpc at
redshifts z < 20, the high molecule fraction that is built up in relic H II
regions and their increasing volume-filling fraction renders even the local IGM
optically thick to LW photons over physical distances of the order of a few
kiloparsecs. We find that efficient accretion onto Population III relic black
holes may occur after ~ 60 Myr from the time of their formation, by which time
the photo-heated relic H II region gas can cool and re-collapse into the 10^6
M_solar minihalo which hosts the black hole. Also, Pop II.5 stars, postulated
to have masses of the order of 10 M_solar, can likely form from this
re-collapsing relic H II region gas. Overall, we find that the local radiative
feedback from the first generations of stars suppresses the star formation rate
by only a factor of, at most, a few.Comment: 29 pages, 7 figures; ApJ in pres
Rapid fixation of non-native alleles revealed by genome-wide SNP analysis of hybrid tiger salamanders
<p>Abstract</p> <p>Background</p> <p>Hybrid zones represent valuable opportunities to observe evolution in systems that are unusually dynamic and where the potential for the origin of novelty and rapid adaptation co-occur with the potential for dysfunction. Recently initiated hybrid zones are particularly exciting evolutionary experiments because ongoing natural selection on novel genetic combinations can be studied in ecological time. Moreover, when hybrid zones involve native and introduced species, complex genetic patterns present important challenges for conservation policy. To assess variation of admixture dynamics, we scored a large panel of markers in five wild hybrid populations formed when Barred Tiger Salamanders were introduced into the range of California Tiger Salamanders.</p> <p>Results</p> <p>At three of 64 markers, introduced alleles have largely displaced native alleles within the hybrid populations. Another marker (<it>GNAT1</it>) showed consistent heterozygote deficits in the wild, and this marker was associated with embryonic mortality in laboratory F2's. Other deviations from equilibrium expectations were idiosyncratic among breeding ponds, consistent with highly stochastic demographic effects.</p> <p>Conclusion</p> <p>While most markers retain native and introduced alleles in expected proportions, strong selection appears to be eliminating native alleles at a smaller set of loci. Such rapid fixation of alleles is detectable only in recently formed hybrid zones, though it might be representative of dynamics that frequently occur in nature. These results underscore the variable and mosaic nature of hybrid genomes and illustrate the potency of recombination and selection in promoting variable, and often unpredictable genetic outcomes. Introgression of a few, strongly selected introduced alleles should not necessarily affect the conservation status of California Tiger Salamanders, but suggests that genetically pure populations of this endangered species will be difficult to maintain.</p
Optical signatures of quantum phase transitions in a light-matter system
Information about quantum phase transitions in conventional condensed matter
systems, must be sought by probing the matter system itself. By contrast, we
show that mixed matter-light systems offer a distinct advantage in that the
photon field carries clear signatures of the associated quantum critical
phenomena. Having derived an accurate, size-consistent Hamiltonian for the
photonic field in the well-known Dicke model, we predict striking behavior of
the optical squeezing and photon statistics near the phase transition. The
corresponding dynamics resemble those of a degenerate parametric amplifier. Our
findings boost the motivation for exploring exotic quantum phase transition
phenomena in atom-cavity, nanostructure-cavity, and
nanostructure-photonic-band-gap systems.Comment: 4 pages, 4 figure
On the Anti-Correlation between Duration and Redshift in Gamma-ray Bursts
For gamma-ray bursts (GRBs) with durations greater than two seconds
(so-called long GRBs), the intrinsic prompt gamma-ray emission appears, on
average, to last longer for bursts at lower redshifts. We explore the nature of
this duration-redshift anti-correlation, describing systems and conditions in
which this cosmological evolution could arise. In particular, we explore its
dependence on metallicity of a massive star progenitor, as we can securely
count on the average stellar metallicity to increase with decreasing redshift.
Although higher metallicity/lower redshift stars lose mass and angular momentum
through line-driven winds, in some cases these stars are able to form more
extended accretion disks when they collapse, potentially leading to longer
duration GRBs. We also examine how this duration-redshift trend may show up in
interacting binary models composed of a massive star and compact object
companion, recently suggested to be the progenitors of radio bright GRBs. Under
certain conditions, mass loss and equation of state effects from higher
metallicity, lower redshift massive stars can decrease the binary separation.
This can then lead to spin-up of the massive star and allow for a longer
duration GRB upon the massive star's collapse. Finally, the duration-redshift
trend may also be supported by a relatively larger population of
small-separation binaries born in situ at low redshift.Comment: Accepted to Ap
Proof of concept: Partner-Assisted Interpersonal Psychotherapy for perinatal depression
Although poor partner support is a key risk factor for depression in pregnant and postpartum women, partners are not generally involved in treatment beyond psychoeducation. The aim of this "proof of concept" study was to test safety, acceptability, and feasibility of Partner-Assisted Interpersonal Psychotherapy (PA-IPT), an intervention that includes the partner as an active participant throughout treatment. Women more than 12 weeks estimated gestational age and less than 12 weeks postpartum were invited to participate if they fulfilled DSM-IV criteria for Major Depressive Disorder and reported moderate symptom severity (HAM-D17 ≥16). The open trial included eight acute-phase sessions and a 6-week follow-up assessment. Ten couples completed the acute phase treatment and nine presented for a 6-week follow-up assessment. There were no study-related adverse events, and no women had symptomatic worsening from intake to Session Eight. All partners attended all sessions, no couples dropped out of treatment, and all reported positive treatment satisfaction at the conclusion of the study. Nine of ten women (90 %) met the criteria for clinical response (HAM-D17=9) at the conclusion of acute phase treatment, and eight of the nine (89 %) presenting at a 6-week follow-up assessment met criteria for symptomatic recovery. Incorporating partners in the treatment of major depressive disorders during the perinatal period is safe, acceptable, and feasible, but needs further testing in a larger population to evaluate efficacy
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