9,111 research outputs found
Apoptosis and change of competence limit the size of the vulva equivalence group in Pristionchus pacificus: a genetic analysis
Background: To understand how alterations in the molecular mechanisms underlying developmental processes generate a diversity of biological forms, comparative developmental biology can be combined with genetic analysis. The formation of the nematode vulva is one tractable system for such evolutionary developmental analysis, as much is understood about its development in Caenorhabditis elegans. In Caenorhabditis, six of twelve ventral epidermal cells form the âvulva equivalence groupâ; although all six cells are competent to adopt vulval cell fates in response to an inductive signal, only three of these cells are induced to form vulval tissue.
Results: In some species of the nematode families Rhabditidae, Neodiplogastridae and Panagrolaimidae, the number of cells in the vulva equivalence group is limited by apoptosis and decreased responsiveness to inductive signals (competence). We have initiated a genetic analysis in one of these species, Pristionchus pacificus, to understand the evolution of the specification of ventral epidermal cells that are competent to generate the vulva. A ped-4 mutation restores competence to an incompetent cell. Mutation of either of two other genes of Pristionchus cause two anterior cells that die in wild-type to survive. A ped-5 mutation causes these cells to be competent to respond to inductive signals, expanding the equivalence group. A ped-6 mutation causes these cells to form ectopic, anterior vulva-like invaginations.
Conclusion: During nematode evolution, apoptosis and change of competence alter the number and potency of ventral epidermal cells. The phenotypes of Pristionchus mutants suggest that alterations in homeotic gene control of anteroposterior patterning is involved in creating this cellular diversity
Evolution of Fermion Pairing from Three to Two Dimensions
We follow the evolution of fermion pairing in the dimensional crossover from
3D to 2D as a strongly interacting Fermi gas of Li atoms becomes confined
to a stack of two-dimensional layers formed by a one-dimensional optical
lattice. Decreasing the dimensionality leads to the opening of a gap in
radio-frequency spectra, even on the BCS-side of a Feshbach resonance. The
measured binding energy of fermion pairs closely follows the theoretical
two-body binding energy and, in the 2D limit, the zero-temperature mean-field
BEC-BCS theory.Comment: 5 pages, 4 figure
Plankton ecology: The past two decades of progress
This is a selected account of recent developments
in plankton ecology. The examples have been
chosen for their degree of innovation during the
past two decades and for their general ecological
importance. They range from plankton autecology
over interactions between populations to community
ecology. The autecology of plankton is
represented by the hydromechanics of plankton
(the problem of life in a viscous environment) and
by the nutritional ecology of phyto- and zooplankton.
Population level studies are represented
by competition, herbivory (grazing), and zooplankton
responses to predation. Community
ecology is represented by the debate about bottom-
up vs. top-down control of community organization,
by the PEG model of seasonal plankton
succession, and by the recent discovery of the microbial
food web
Physical Nucleon Properties from Lattice QCD
We demonstrate that the extremely accurate lattice QCD data for the mass of
the nucleon recently obtained by CP-PACS, combined with modern chiral
extrapolation techniques, leads to a value for the mass of the physical nucleon
which has a systematic error of less than one percent.Comment: 4 pages, 2 figure
Scaling of gauge balls and static potential in the confinement phase of the pure U(1) lattice gauge theory
We investigate the scaling behaviour of gauge-ball masses and static
potential in the pure U(1) lattice gauge theory on toroidal lattices. An
extended gauge field action is used with and -0.5. Gauge-ball correlation
functions with all possible lattice quantum numbers are calculated. Most
gauge-ball masses scale with the non-Gaussian exponent .
The gauge-ball mass scales with the Gaussian value in the investigated range of correlation lengths. The static potential is
examined with Sommer's method. The long range part scales consistently with
but the short range part tends to yield smaller values of . The
-function, having a UV stable zero, is obtained from the running
coupling. These results hold for both values, supporting universality.
Consequences for the continuum limit of the theory are discussed.Comment: Contribution to the Lattice 97 proceedings, LaTeX, 3 pages, 3 figure
Disturbance-diversity relationships in two lakes of similar nutrient chemistry but contrasting disturbance regimes
Phytoplankton diversity was studied in two North German lakes of comparable nutrient chemistry but different exposure to winds. In both lakes, phytoplankton was primarily N-limited but diatoms were Si-limited. PluĂsee had a very constant mixing depth during summer, while week-to-week changes of several meters were quite common in the more exposed Behler See. In PluĂsee, phytoplankton biomass during summer came closer to the carrying capacity as defined by the available total N. In PluĂsee there was a marked decline of diversity during the summer maximum of biomass, while this decline was less pronounced in Behler See. It is concluded that disturbances which prevented phytoplankton from reaching the carrying capacity also maintained a high level of diversity. A negative response of diversity to undisturbed conditions became apparent, after phytoplankton biomass had exceeded about 5% of the carrying capacity
Distribution of the color fields around static quarks: Flux tube profiles
We report detailed calculations of the profiles of energy and action
densities in the quark-antiquark string in SU(2) lattice gauge theory.Comment: 40 pages, LSUHE 94-15
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