445 research outputs found
The Middle Cambrian fossil Pikaia and the evolution of chordate swimming
Conway Morris and Caron (2012) have recently published an account of virtually all the available information on Pikaia gracilens, a well-known Cambrian fossil and supposed basal chordate, and propose on this basis some new ideas about Pikaia’s anatomy and evolutionary significance. Chief among its chordate-like features are the putative myomeres, a regular series of vertical bands that extends the length of the body. These differ from the myomeres of living chordates in that boundaries between them (the myosepta) are gently curved, with minimal overlap, whereas amphioxus and vertebrates have strongly overlapping V- and W-shaped myomeres. The implication, on biomechanical grounds, is that myomeres in Pikaia exerted much less tension on the myosepta, so the animal would have been incapable of swimming as rapidly as living chordates operating in the fast-twitch mode used for escape and attack. Pikaia either lacked the fast-twitch fibers necessary for such speeds, having instead only slow-twitch fibers, or it had an ancestral fiber type with functional capabilities more like modern slow fibers than fast ones. The first option is supported by the sequence of development in zebrafish, where both myoseptum formation and fast fiber deployment show a dependence on slow fibers, which develop first. For Pikaia, the absence of fast fibers has both behavioral and anatomical implications, which are discussed. Among the latter is the possibility that a notochord may not have been needed as a primary stiffening device if other structures (for example, the dorsal organ) could perform that role
Phased Helical Antenna Array for CubeSat Application
CubeSats communication links are typically slow due to their power and size restrictions. However, a low power, high speed down link connection can allow for greater data collection and relaying, pushing the capability of 1U CubeSats beyond what is traditionally thought possible. Investigated here is the feasibility of a 11 Mb/s down link connection on a 1U CubeSat, accomplished via 2x2 helical antenna phased array operating on the 802.11 standard. Using a phased array also allows for beam steering to secure connection with the ground station when the on board ADCS may not provide precise pointing capability of the CubeSat, as 1U systems often do not
Community Property—Characterization of the Inflationary Increase in the Value of Separate Property Improved by Community Funds—In re Marriage of Elam, 97 Wn. 2d 811, 650 P.2d 213 (1982)
The Washington Supreme Court held that: (1) the increase in value of separate property is presumptively separate, unless the community claimant rebuts the presumption by direct and positive proof that community contributions caused the increase, and (2) the community is entitled to a share of the inflationary increase in the value of the separate property proportionate to the community contributions
A hexamer origin of the echinoderms' five rays
Of the major deuterostome groups, the echinoderms with their multiple forms
and complex development are arguably the most mysterious. Although larval
echinoderms are bilaterally symmetric, the adult body seems to abandon the
larval body plan and to develop independently a new structure with different
symmetries. The prevalent pentamer structure, the asymmetry of Loven's rule and
the variable location of the periproct and madrepore present enormous
difficulties in homologizing structures across the major clades, despite the
excellent fossil record. This irregularity in body forms seems to place
echinoderms outside the other deuterostomes. Here I propose that the
predominant five-ray structure is derived from a hexamer structure that is
grounded directly in the structure of the bilaterally symmetric larva. This
hypothesis implies that the adult echinoderm body can be derived directly from
the larval bilateral symmetry and thus firmly ranks even the adult echinoderms
among the bilaterians. In order to test the hypothesis rigorously, a model is
developed in which one ray is missing between rays IV-V (Loven's schema) or
rays C-D (Carpenter's schema). The model is used to make predictions, which are
tested and verified for the process of metamorphosis and for the morphology of
recent and fossil forms. The theory provides fundamental insight into the
M-plane and the Ubisch', Loven's and Carpenter's planes and generalizes them
for all echinoderms. The theory also makes robust predictions about the
evolution of the pentamer structure and its developmental basis. *** including
corrections (see footnotes) ***Comment: 10 pages, 6 figure
Spatial and spatiotemporal pattern formation in generalised Turing systems
Reaction-diffusion, or Turing, models have been proposed to account for a number of pattern formation phenomena in early development. However, there are a number of crucial morphogenetic phenomena that contradict the standard Turing model. Here we review three generalisations of the Turing model and show how they can be applied to two such phenomena. We discuss how these generalisations can provide insight to the processes underlying patterning in these cases
On the Origin and Evolution of Vertebrate Olfactory Receptor Genes: Comparative Genome Analysis Among 23 Chordate Species
Olfaction is a primitive sense in organisms. Both vertebrates and insects have
receptors for detecting odor molecules in the environment, but the evolutionary
origins of these genes are different. Among studied vertebrates, mammals have
∼1,000 olfactory receptor (OR) genes, whereas teleost fishes have much
smaller (∼100) numbers of OR genes. To investigate the origin and
evolution of vertebrate OR genes, I attempted to determine near-complete OR gene
repertoires by searching whole-genome sequences of 14 nonmammalian chordates,
including cephalochordates (amphioxus), urochordates (ascidian and larvacean),
and vertebrates (sea lamprey, elephant shark, five teleost fishes, frog, lizard,
and chicken), followed by a large-scale phylogenetic analysis in conjunction
with mammalian OR genes identified from nine species. This analysis showed that
the amphioxus has >30 vertebrate-type OR genes though it lacks
distinctive olfactory organs, whereas all OR genes appear to have been lost in
the urochordate lineage. Some groups of genes (θ, κ, and
λ) that are phylogenetically nested within vertebrate OR genes showed
few gene gains and losses, which is in sharp contrast to the evolutionary
pattern of OR genes, suggesting that they are actually non-OR genes. Moreover,
the analysis demonstrated a great difference in OR gene repertoires between
aquatic and terrestrial vertebrates, reflecting the necessity for the detection
of water-soluble and airborne odorants, respectively. However, a minor group
(β) of genes that are atypically present in both aquatic and
terrestrial vertebrates was also found. These findings should provide a critical
foundation for further physiological, behavioral, and evolutionary studies of
olfaction in various organisms
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