Photon Spectrum Produced by the Late Decay of a Cosmic Neutrino Background

We obtain the photon spectrum induced by a cosmic background of unstable neutrinos. We study the spectrum in a variety of cosmological scenarios and also we allow for the neutrinos having a momentum distribution (only a critical matter dominated universe and neutrinos at rest have been considered until now). Our results can be helpful when extracting bounds on neutrino electric and magnetic moments from cosmic photon background observations.Comment: RevTex, 14 pages, 3 figures; minor changes, references added. To appear in Phys. Rev.

Analysis of talpid3 and wild-type chicken embryos reveals roles for Hedgehog signalling in development of the limb bud vasculature

Chicken talpid mutant embryos have a wide range of Hedgehog-signalling related defects and it is now known that the talpid gene product encodes a novel protein essential for Hedgehog signalling which is required for both activator and repressor functions of Gli transcription factors (Davey, M.G., Paton, I.R., Yin, Y., Schmidt, M., Bangs, F.K., Morrice, D.R., Gordon-Smith, T., Buxton, P., Stamataki, D., Tanaka, M., Münsterberg, A.E., Briscoe, J., Tickle, C., Burt, D.W. (2006). The chicken talpid gene encodes a novel protein essential for Hedgehog signalling. Genes Dev 20 1365-77). Haemorrhaging, oedema and other severe vascular defects are a central aspect of the talpid phenotype (Ede, D.A. and Kelly, W.A (1964a). Developmental abnormalities in the head region of the talpid mutant fowl. J. Embryol. exp. Morp. 12:161-182) and, as Hedgehog (Hh) signalling has been implicated in every stage of development of the vascular system, the vascular defects seen in talpid are also likely to be attributable to abnormal Hedgehog signalling. Gene expression of members of the VEGF and Angiopoietin families of angiogenic growth factors has been linked to haemorrhaging and oedema and we find widespread expression of VEGF-D, rigf and Ang2a in the talpid limb. Furthermore, ectopic expression of these genes in talpid limbs points to regulation via Gli repression rather than activation. We monitored specification of vessel identity in talpid limb vasculature by examining expression of artery-specific genes, Np1 and EphrinB2, and the vein-specific genes, Np2a and Tie2. We show that there are supernumerary subclavian arteries in talpid limb buds and abnormal expression of an artery-specific gene in the venous submarginal sinus, despite the direction of blood flow being normal. Furthermore, we show that Shh can induce Np1 expression but has no effect on Np2a. Finally, we demonstrate that induction of VEGF and Ang2a expression by Shh in normal limb buds is accompanied by vascular remodelling. Thus Hedgehog signalling has a pivotal role in the cascade of angiogenic events in a growing embryonic organ which is similar to that proposed in tumours

Experimental Facilities Development

This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

The influence of below-ground herbivory and defoliation of a legume on nitrogen transfer to neighbouring plants

1. Both foliar and root herbivory can alter the exudation of carbon from plant roots, which in turn can affect nitrogen availability in the soil. However, few studies have investigated the effects of herbivory on N fluxes from roots, which can directly increase N availability in the soil and uptake by neighbouring plants. Moreover, the combined effects of foliar and root herbivory on N fluxes remains unexplored. 2. We subjected the legume white clover (Trifolium repens L.) to defoliation (through clipping) and root herbivory (by an obligate root-feeding nematode, Heterodera trifolii Goggart) to examine how these stresses individually, and simultaneously, affected the transfer of T. repens-derived N to neighbouring perennial ryegrass (Lolium perenne L.) plants using 15N stable-isotope techniques. We also examined the effects of defoliation and root herbivory on the size of the soil microbial community and the growth response of L. perenne. 3. Neither defoliation nor root herbivory negatively affected T. repens biomass. On the contrary, defoliation increased root biomass (34%) and total shoot production by T. repens (100%). Furthermore, defoliation resulted in a fivefold increase in T. repens-derived 15N recovered in L. perenne roots, and increased the size of the soil microbial biomass (77%). In contrast, root herbivory by H. trifolii slightly reduced 15N transfer from T. repens to L. perenne when T. repens root 15N concentration was included as a covariate, and root herbivory did not affect microbial biomass. Growth of L. perenne was not affected by any of the treatments. 4. Our findings demonstrate that defoliation of a common grassland legume can substantially increase the transfer of its N to neighbouring plants by directly affecting below-ground N fluxes. These finding require further examination under field conditions but, given the prevalence of N-limitation of plant productivity in terrestrial ecosystems, increased transfer of N from legumes to non-N-fixing species could alter competitive interactions, with implications for plant community structure