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Towards a Unified Model of Language Acquisition
In this theoretical paper, we first review and rebut standard criticisms against distributional approaches to language acquisition. We then present two closely-related models that use distributional analysis. The first deals with the acquisition of vocabulary, the second with grammatical development. We show how these two models can be combined with a semantic network grown using Hebbian learning, and briefly illustrate the advantages of this combination. An important feature of this hybrid system is that it combines two different types of distributional learning, the first based on order, and the second based on co-occurrences within a context
The Next Generation Virgo Cluster Survey - Infrared (NGVS-IR): I. A new Near-UV/Optical/Near-IR Globular Cluster selection tool
The NGVS-IR project (Next Generation Virgo Survey - Infrared) is a contiguous
near-infrared imaging survey of the Virgo cluster of galaxies. It complements
the optical wide-field survey of Virgo (NGVS). The current state of NGVS-IR
consists of Ks-band imaging of 4 deg^2 centered on M87, and J and Ks-band
imaging of 16 deg^2 covering the region between M49 and M87. In this paper, we
present the observations of the central 4 deg^2 centered on Virgo's core
region. The data were acquired with WIRCam on the Canada-France-Hawaii
Telescope and the total integration time was 41 hours distributed in 34
contiguous tiles. A survey-specific strategy was designed to account for
extended galaxies while still measuring accurate sky brightness within the
survey area. The average 5\sigma limiting magnitude is Ks=24.4 AB mag and the
50% completeness limit is Ks=23.75 AB mag for point source detections, when
using only images with better than 0.7" seeing (median seeing 0.54"). Star
clusters are marginally resolved in these image stacks, and Virgo galaxies with
\mu_Ks=24.4 AB mag arcsec^-2 are detected. Combining the Ks data with optical
and ultraviolet data, we build the uiK color-color diagram which allows a very
clean color-based selection of globular clusters in Virgo. This diagnostic plot
will provide reliable globular cluster candidates for spectroscopic follow-up
campaigns needed to continue the exploration of Virgo's photometric and
kinematic sub-structures, and will help the design of future searches for
globular clusters in extragalactic systems. Equipped with this powerful new
tool, future NGVS-IR investigations based on the uiK diagram will address the
mapping and analysis of extended structures and compact stellar systems in and
around Virgo galaxies.Comment: 23 pages, 18 figures. Accepted for publication in ApJ
Radial distribution of the multiple stellar populations in omega Centauri
We present a detailed study of the radial distribution of the multiple
populations identified in the Galactic globular cluster omega Cen. We used both
space-based images (ACS/WFC and WFPC2) and ground-based images (FORS1@VLT and
[email protected] ESO telescopes) to map the cluster from the inner core to the
outskirts (~20 arcmin). These data sets have been used to extract high-accuracy
photometry for the construction of color-magnitude diagrams and astrometric
positions of ~900 000 stars. We find that in the inner ~2 core radii the blue
main sequence (bMS) stars slightly dominate the red main sequence (rMS) in
number. At greater distances from the cluster center, the relative numbers of
bMS stars with respect to rMS drop steeply, out to ~8 arcmin, and then remain
constant out to the limit of our observations. We also find that the dispersion
of the Gaussian that best fits the color distribution within the bMS is
significantly greater than the dispersion of the Gaussian that best fits the
color distribution within the rMS. In addition, the relative number of
intermediate-metallicity red-giant-branch stars which includes the progeny of
the bMS) with respect to the metal-poor component (the progeny of the rMS)
follows a trend similar to that of the main-sequence star-count ratio
N_bMS/N_rMS. The most metal-rich component of the red-giant branch follows the
same distribution as the intermediate-metallicity component. We briefly discuss
the possible implications of the observed radial distribution of the different
stellar components in omega Cen.Comment: 16 pages, 14 figures (6 in low resolution), 3 tables. Accepted for
publication in Astronomy and Astrophysics on 23 September 200
Recombination and Population Mosaic of a Multifunctional Viral Gene, Adeno-Associated Virus cap
Homologous recombination is a dominant force in evolution and results in genetic mosaics. To detect evidence of recombination events and assess the biological significance of genetic mosaics, genome sequences for various viral populations of reasonably large size are now available in the GenBank. We studied a multi-functional viral gene, the adeno-associated virus (AAV) cap gene, which codes for three capsid proteins, VP1, VP2 and VP3. VP1-3 share a common C-terminal domain corresponding to VP3, which forms the viral core structure, while the VP1 unique N-terminal part contains an enzymatic domain with phospholipase A2 activity. Our recombinant detection program (RecI) revealed five novel recombination events, four of which have their cross-over points in the N-terminal, VP1 and VP2 unique region. Comparison of phylogenetic trees for different cap gene regions confirmed discordant phylogenies for the recombinant sequences. Furthermore, differences in the phylogenetic tree structures for the VP1 unique (VP1u) region and the rest of cap highlighted the mosaic nature of cap gene in the AAV population: two dominant forms of VP1u sequences were identified and these forms are linked to diverse sequences in the rest of cap gene. This observation together with the finding of frequent recombination in the VP1 and 2 unique regions suggests that this region is a recombination hot spot. Recombination events in this region preserve protein blocks of distinctive functions and contribute to convergence in VP1u and divergence of the rest of cap. Additionally the possible biological significance of two dominant VP1u forms is inferred
Molecular logic behind the three-way stochastic choices that expand butterfly colour vision.
Butterflies rely extensively on colour vision to adapt to the natural world. Most species express a broad range of colour-sensitive Rhodopsin proteins in three types of ommatidia (unit eyes), which are distributed stochastically across the retina. The retinas of Drosophila melanogaster use just two main types, in which fate is controlled by the binary stochastic decision to express the transcription factor Spineless in R7 photoreceptors. We investigated how butterflies instead generate three stochastically distributed ommatidial types, resulting in a more diverse retinal mosaic that provides the basis for additional colour comparisons and an expanded range of colour vision. We show that the Japanese yellow swallowtail (Papilio xuthus, Papilionidae) and the painted lady (Vanessa cardui, Nymphalidae) butterflies have a second R7-like photoreceptor in each ommatidium. Independent stochastic expression of Spineless in each R7-like cell results in expression of a blue-sensitive (Spineless(ON)) or an ultraviolet (UV)-sensitive (Spineless(OFF)) Rhodopsin. In P. xuthus these choices of blue/blue, blue/UV or UV/UV sensitivity in the two R7 cells are coordinated with expression of additional Rhodopsin proteins in the remaining photoreceptors, and together define the three types of ommatidia. Knocking out spineless using CRISPR/Cas9 (refs 5, 6) leads to the loss of the blue-sensitive fate in R7-like cells and transforms retinas into homogeneous fields of UV/UV-type ommatidia, with corresponding changes in other coordinated features of ommatidial type. Hence, the three possible outcomes of Spineless expression define the three ommatidial types in butterflies. This developmental strategy allowed the deployment of an additional red-sensitive Rhodopsin in P. xuthus, allowing for the evolution of expanded colour vision with a greater variety of receptors. This surprisingly simple mechanism that makes use of two binary stochastic decisions coupled with local coordination may prove to be a general means of generating an increased diversity of developmental outcomes
MegaPipe: the MegaCam image stacking pipeline at the Canadian Astronomical Data Centre
This paper describes the MegaPipe image processing pipeline at the Canadian
Astronomical Data Centre. The pipeline combines multiple images from the
MegaCam mosaic camera on CFHT and combines them into a single output image.
MegaPipe takes as input detrended MegaCam images and does a careful astrometric
and photometric calibration on them. The calibrated images are then resampled
and combined into image stacks. The astrometric calibration of the output
images is accurate to within 0.15 arcseconds relative to external reference
frames and 0.04 arcseconds internally. The photometric calibration is good to
within 0.03 magnitudes. The stacked images and catalogues derived from these
images are available through the CADC website:Comment: Data available at
http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/megapipe/index.htm
Natural recovery of genetic diversity by gene flow in reforested areas of the endemic Canary Island pine, Pinus canariensis
The endemic pine, Pinus canariensis, forms one of the main forest ecosystems
in the Canary Islands. In this archipelago, pine forest is a mosaic of natural
stands (remnants of past forest overexploitation) and artificial stands planted
from the 1940's. The genetic makeup of the artificially regenerated forest is
of some concern. The use of reproductive material with uncontrolled origin or
from a reduced number of parental trees may produce stands ill adapted to local
conditions or unable to adapt in response to environmental change. The genetic
diversity within a transect of reforested stands connecting two natural forest
fragments has been studied with nuclear and chloroplast microsatellites. Little
genetic differentiation and similar levels of genetic diversity to the
surrounding natural stands were found for nuclear markers. However, chloroplast
microsatellites presented lower haplotype diversity in reforested stands, and
this may be a consequence of the lower effective population size of the
chloroplast genome, meaning chloroplast markers have a higher sensitivity to
bottlenecks. Understory natural regeneration within the reforestation was also
analysed to study gene flow from natural forest into artificial stands.
Estimates of immigration rate into artificially regenerated forest were high
(0.68-0.75), producing a significant increase of genetic diversity (both in
chloroplast and nuclear microsatellites), which indicates the capacity for
genetic recovery for P. canariensis reforestations surrounded by larger natural
stands
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