274 research outputs found
How Long Does Evolution of the Troglomorphic Form Take? Estimating Divergence Times in Astyanax Mexicanus
Features including colonization routes (stream capture) and the existence of both epigean and cave-adapted hypogean populations make Astyanax mexicanus an attractive system for investigating the subterranean evolutionary time necessary for acquisition of the troglomorphic form. Using published sequences, we have estimated divergence times for A. mexicanus using: 1) two different population-level mitochondrial datasets (cytochrome b and NADH dehydrogenase 2) with both strict and relaxed molecular clock methods, and 2) broad phylogenetic approaches combining fossil calibrations and with four nuclear (recombination activating gene, seven in absentia, forkhead, and Ī±-tropomyosin) and two mitochondrial (16S rDNA and cytochrome b) genes. Using these datasets, we have estimated divergence times for three events in the evolutionary history of troglomorphic A. mexicanus populations. First, divergence among cave haplotypes occurred in the Pleistocene, possibly correlating with fluctuating water levels allowing the colonization and subsequent isolation of new subterranean habitats. Second, in one lineage, A. mexicanus cave populations experienced introgressive hybridization events with recent surface populations (0.26-2.0 Ma), possibly also correlated with Pleistocene events. Finally, using divergence times from surface populations in the lineage without evidence of introgression as an estimate, the acquisition of the troglomorphic form in A. mexicanus is younger than 2.2 (fossil calibration estimates) ā 5.2 (cytb estimate) Ma (Pliocene)
Light Organ Photosensitivity in Deep-Sea Shrimp May Suggest a Novel Role in Counterillumination
Extraocular photoreception, the ability to detect and respond to light outside of the eye, has not been previously described in deep-sea invertebrates. Here, we investigate photosensitivity in the bioluminescent light organs (photophores) of deep-sea shrimp, an autogenic system in which the organism possesses the substrates and enzymes to produce light. Through the integration of transcriptomics, in situ hybridization and immunohistochemistry we find evidence for the expression of opsins and phototransduction genes known to play a role in light detection in most animals. Subsequent shipboard light exposure experiments showed ultrastructural changes in the photophore similar to those seen in crustacean eyes, providing further evidence that photophores are light sensitive. In many deep-sea species, it has long been documented that photophores emit light to aid in counterillumination ā a dynamic form of camouflage that requires adjusting the organās light intensity to āhideā their silhouettes from predators below. However, it remains a mystery how animals fine-tune their photophore luminescence to match the intensity of downwelling light. Photophore photosensitivity allows us to reconsider the organās role in counterillumination - not only in light emission but also light detection and regulation
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Melanization in response to wounding is ancestral in arthropods and conserved in albino cave species
Many species adapted to aphotic subterranean habitats have lost all body pigmentation. Yet, melanization is an important component of wound healing in arthropods. We amputated appendages in a variety of cave-adapted and surface-dwelling arthropods. A dark clot formed at the site of injury in most species tested, including even albino cave-adapted species. The dark coloration of the clots was due to melanin deposition. The speed of wound melanization was uncorrelated with a difference in metabolic rate between surface and cave populations of an amphipod. The chelicerate Limulus polyphemus, all isopod crustaceans tested, and the cave shrimp Troglocaris anophthalmus did not melanize wounds. The loss of wound melanization in T. anophthalmus was an apomorphy associated with adaptation to subterranean habitats, but in isopods it appeared to be a symplesiomorphy unrelated to colonization of subterranean habitats. We conclude that wound melanization i) is an important part of innate immunity because it was present in all major arthropod lineages, ii) is retained in most albino cave species, and iii) has been lost several times during arthropod evolution, indicating melanization is not an indispensable component of wound healing in arthropods
A High Spectral Resolution Study of the Soft X-ray Background with the X-ray Quantum Calorimeter
We present here a combined analysis of four high spectral resolution
observations of the Diffuse X-ray Background (DXRB), made using the University
of Wisconsin-Madison/Goddard Space Flight Center X-ray Quantum Calorimeter
(XQC) sounding rocket payload. The observed spectra support the existence of a
keV Local Hot Bubble and a keV Hot Halo, with discrepancies
between repeated observations compatible with expected contributions of
time-variable emission from Solar Wind Charge Exchange (SWCX). An additional
component of keV emission observed only at low galactic latitudes can
be consistently explained by unresolved dM stars.Comment: 21 pages, 6 figures, accepted for publication in Ap
Multiabsorber Transition-Edge Sensors for X-Ray Astronomy
We are developing arrays of position-sensitive microcalorimeters for future x-ray astronomy applications. These position-sensitive devices commonly referred to as hydras consist of multiple x-ray absorbers, each with a different thermal coupling to a single-transition-edge sensor microcalorimeter. Their development is motivated by a desire to achieve very large pixel arrays with some modest compromise in performance. We report on the design, optimization, and first results from devices with small pitch pixels (<75 m) being developed for a high-angular and energy resolution imaging spectrometer for Lynx. The Lynx x-ray space telescope is a flagship mission concept under study for the National Academy of Science 2020 decadal survey. Broadband full-width-half-maximum (FWHM) resolution measurements on a 9-pixel hydra have demonstrated E(FWHM) = 2.23 0.14 eV at Al-K, E(FWHM) = 2.44 0.29 eV at Mn-K, and E(FWHM) = 3.39 0.23 eV at Cu-K. Position discrimination is demonstrated to energies below <1 keV and the device performance is well-described by a finite-element model. Results from a prototype 20-pixel hydra with absorbers on a 50-m pitch have shown E(FWHM) = 3.38 0.20 eV at Cr-K1. We are now optimizing designs specifically for Lynx and extending the number of absorbers up to 25/hydra. Numerical simulation suggests optimized designs could achieve 3 eV while being compatible with the bandwidth requirements of the state-of-the art multiplexed readout schemes, thus making a 100,000 pixel microcalorimeter instrument a realistic goal
High-resolution Laboratory Measurements of K-shell X-ray Line Polarization and Excitation Cross Sections in Heliumlike S XV Ions
We report measurements of electron-impact excitation cross sections for the
strong K-shell n=2-1 transitions in S XV using the LLNL EBIT-I electron beam
ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The
cross sections are determined by direct normalization to the well known cross
sections of radiative electron capture, measured simultaneously. Using
contemporaneous polarization measurements with the two crystal spectrometers,
whose dispersion planes are oriented parallel and perpendicular to the electron
beam direction, the polarization of the direct excitation line emission is
determined, and in turn the isotropic total cross sections are extracted. We
further experimentally investigate various line-formation mechanisms, finding
that radiative cascades and collisional inner-shell ionization dominate the
degree of linear polarization and total line-emission cross sections of the
forbidden line .Comment: 16 Pages, 9 Figures, published in Ap
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