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 ‘vulva equivalence group’; 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 $^6$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 $-\sum_P(\beta \cos\Theta_P + \gamma \cos2\Theta_P)$ is used with $\gamma= -0.2$ 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 $\nu_{ng}\approx 0.36$. The $A_1^{++}$ gauge-ball mass scales with the Gaussian value $\nu_{g} \approx 0.5$ in the investigated range of correlation lengths. The static potential is examined with Sommer's method. The long range part scales consistently with $\nu_{ng}$ but the short range part tends to yield smaller values of $\nu$. The $\beta$-function, having a UV stable zero, is obtained from the running coupling. These results hold for both $\gamma$ 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. Plußsee 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 Plußsee, phytoplankton biomass during summer came closer to the carrying capacity as defined by the available total N. In Plußsee 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