Host promiscuity in symbiont associations can influence exotic legume establishment and colonization of novel ranges

Aim Invasive Acacia species have negatively impacted natural areas in multiple regions around the globe. Almost 400 Acacia species have been introduced outside their native ranges in Australia; approximately 6% have become invasive, 12% are naturalized, and 82% have no record of naturalization or invasion. This variation in invasiveness provides a comparative framework in which to examine mechanisms that either promote or constrain establishment and colonization of species in novel regions. Here, we experimentally examine the role that the legume–rhizobia symbiosis plays in the differential invasiveness of acacias introduced outside their native Australian ranges. Location Canberra, Australia. Methods We paired 12 Acacia species ranging in invasiveness globally with 12 rhizobial strains ranging in average symbiotic effectiveness. We measured plant growth and nodulation success and abundance to assess whether invasive acacias were more promiscuous hosts, that is had positive growth and nodulation responses to a broader range of rhizobial strains than naturalized and non-invasive species. Results Invasive acacias had a higher growth response across more rhizobial strains (six of 12 strains) than naturalized and non-invasive species, but invasiveness categories differed only moderately with regard to the percentage of plants with nodules and nodulation abundance. Main conclusion With respect to plant growth, invasive acacias appear to be more promiscuous hosts than naturalized and non-invasive Australian Acacia species. Plant growth response to nodulation, however, is likely more important than nodulation alone in the successful invasion of species in novel ranges. Results from this study help elucidate an important mechanism in the invasive capacity of legumes

Differential plant invasiveness is not always driven by host promiscuity with bacterial symbionts

Identification of mechanisms that allow some species to outcompete others is a fundamental goal in ecology and invasive species management. One useful approach is to examine congeners varying in invasiveness in a comparative framework across native and invaded ranges. Acacia species have been widely introduced outside their native range of Australia, and a subset of these species have become invasive in multiple parts of the world. Within specific regions, the invasive status of these species varies. Our study examined whether a key mechanism in the life history of Acacia species, the legume-rhizobia symbiosis, influences acacia invasiveness on a regional scale. To assess the extent to which species varying in invasiveness correspondingly differ with regard to the diversity of rhizobia they associate with, we grew seven Acacia species ranging in invasiveness in California in multiple soils from both their native (Australia) and introduced (California) ranges. In particular, the aim was to determine whether more invasive species formed symbioses with a wider diversity of rhizobial strains (i.e. are more promiscuous hosts). We measured and compared plant performance, including aboveground biomass, survival, and nodulation response, as well as rhizobial community composition and richness. Host promiscuity did not differ among invasiveness categories. Acacia species that varied in invasiveness differed in aboveground biomass for only one soil and did not differ in survival or nodulation within individual soils. In addition, acacias did not differ in rhizobial richness among invasiveness categories. However, nodulation differed between regions and was generally higher in the native than introduced range. Our results suggest that all Acacia species introduced to California are promiscuous hosts and that host promiscuity per se does not explain the observed differences in invasiveness within this region. Our study also highlights the utility of assessing potential mechanisms of invasion in species’ native and introduced ranges

High-z radio starbursts host obscured X-ray AGN

We use Virtual Observatory methods to investigate the association between radio and X-ray emission at high redshifts. Fifty-five of the 92 HDF(N) sources resolved by combining MERLIN+VLA data were detected by Chandra, of which 18 are hard enough and bright enough to be obscured AGN. The high-z population of microJy radio sources is dominated by starbursts an order of magnitude more active and more extended than any found at z<1 and at least a quarter of these simultaneously host highly X-ray-luminous obscured AGN.Comment: 4 pages, 2 figures, To appear in the proceedings of 'At the Edge of the Universe' (9-13 October 2006, Sintra, Portugal

Phylogenetic signals and predictability in plant-soil feedbacks

There is strong evidence for a phylogenetic signal in the degree to which species share co-evolved biotic partners and in the outcomes of biotic interactions. This implies there should be a phylogenetic signal in the outcome of feedbacks between plants and soil microbiota they cultivate. However, attempts to identify a phylogenetic signal in plant-soil feedbacks have produced mixed results. We clarify how phylogenetic signals could arise in plant-soil feedbacks and use a recent compilation of data from feedback experiments to identify: 1) whether there is a phylogenetic signal in the outcome of plant-soil feedbacks; and 2) whether any signal arises through directional or divergent changes in feedback outcomes with evolutionary time. We find strong evidence for a divergent phylogenetic signal in feedback outcomes. Distantly related plant species show more divergent responses to each other's soil microbiota than closely related plant species. The pattern of divergence implies occasional co-evolutionary shifts in how plants interact with soil microbiota, with strongly contrasting feedback responses among some plant lineages. Our results highlight that it is difficult to predict feedback outcomes from phylogeny alone, other than to say that more closely related species tend to have more similar responses

Wildfire impact : natural experiment reveals differential short-term changes in soil microbial communities

A wildfire which overran a sensor network site provided an opportunity (a natural experiment) to monitor short-term post-fire impacts (immediate and up to three months post-fire) in remnant eucalypt woodland and managed pasture plots. The magnitude of fire-induced changes in soil properties and soil microbial communities was determined by comparing (1) variation in fire-adapted eucalypt woodland vs. pasture grassland at the burnt site; (2) variation at the burnt woodland-pasture sites with variation at two unburnt woodland-pasture sites in the same locality; and (3) temporal variation pre- and post-fire. In the eucalypt woodland, soil ammonium, pH and ROC content increased post-fire, while in the pasture soil, soil nitrate increased post-fire and became the dominant soluble N pool. However, apart from distinct changes in N pools, the magnitude of change in most soil properties was small when compared to the unburnt sites. At the burnt site, bacterial and fungal community structure showed significant temporal shifts between pre- and post-fire periods which were associated with changes in soil nutrients, especially N pools. In contrast, microbial communities at the unburnt sites showed little temporal change over the same period. Bacterial community composition at the burnt site also changed dramatically post-fire in terms of abundance and diversity, with positive impacts on abundance of phyla such as Actinobacteria, Proteobacteria and Firmicutes. Large and rapid changes in soil bacterial community composition occurred in the fire-adapted woodland plot compared to the pasture soil, which may be a reflection of differences in vegetation composition and fuel loading. Given the rapid yet differential response in contrasting land uses, identification of key soil bacterial groups may be useful in assessing recovery of fire-adapted ecosystems, especially as wildfire frequency is predicted to increase with global climate change

Network Analysis of Epidermal Growth Factor Signaling Using Integrated Genomic, Proteomic and Phosphorylation Data

To understand how integration of multiple data types can help decipher cellular responses at the systems level, we analyzed the mitogenic response of human mammary epithelial cells to epidermal growth factor (EGF) using whole genome microarrays, mass spectrometry-based proteomics and large-scale western blots with over 1000 antibodies. A time course analysis revealed significant differences in the expression of 3172 genes and 596 proteins, including protein phosphorylation changes measured by western blot. Integration of these disparate data types showed that each contributed qualitatively different components to the observed cell response to EGF and that varying degrees of concordance in gene expression and protein abundance measurements could be linked to specific biological processes. Networks inferred from individual data types were relatively limited, whereas networks derived from the integrated data recapitulated the known major cellular responses to EGF and exhibited more highly connected signaling nodes than networks derived from any individual dataset. While cell cycle regulatory pathways were altered as anticipated, we found the most robust response to mitogenic concentrations of EGF was induction of matrix metalloprotease cascades, highlighting the importance of the EGFR system as a regulator of the extracellular environment. These results demonstrate the value of integrating multiple levels of biological information to more accurately reconstruct networks of cellular response

Amelioration of bleomycin-induced lung fibrosis in hamsters by dietary supplementation with taurine and niacin: biochemical mechanisms.

Interstitial pulmonary fibrosis induced by intratracheal instillation of bleomycin (BL) involves an excess production of reactive oxygen species, unavailability of adequate levels of NAD and ATP to repair the injured pulmonary epithelium, and an overexuberant lung collagen reactivity followed by deposition of highly cross-linked mature collagen fibrils resistant to enzymatic degradation. In the present study, we have demonstrated that dietary supplementation with taurine and niacin offered almost complete protection against the lung fibrosis in a multidose BL hamster model. The mechanisms for the protective effect of taurine and niacin are multifaceted. These include the ability of taurine to scavenge HOCl and stabilize the biomembrane; niacin's ability to replenish the BL-induced depletion of NAD and ATP; and the combined effect of taurine and niacin to suppress all aspects of BL-induced increases in the lung collagen reactivity, a hallmark of interstitial pulmonary fibrosis. It was concluded from the data presented at this Conference that the combined treatment with taurine and niacin, which offers a multipronged approach, will have great therapeutic potential in the intervention of the development of chemically induced interstitial lung fibrosis in animals and humans

The radio spectra of the compact sources in Arp 220: A mixed population of supernovae and supernova remnants

We report the first detection at multiple radio wavelengths (13, 6 and 3.6 cm) of the compact sources within both nuclei of the Ultra Luminous Infra-Red Galaxy Arp 220. We present the radio spectra of the 18 detected sources. In just over half of the sources we find that these spectra and other properties are consistent with the standard model of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance of new radio sources identified with these supernova events suggests that an unusually large fraction of core collapse supernovae in Arp 220 are highly luminous; possibly implying a radically different stellar initial mass function or stellar evolution compared to galactic disks. Another possible explanation invokes very short (~3 x 10^5 year) intense (~10^3 M_Sol year^-1) star formation episodes with a duty cycle of ~10%. A second group of our detected sources, consisting of the brightest and longest monitored sources at 18 cm do not easily fit the radio supernova model. These sources show a range of spectral indexes from -0.2 to -1.9. We propose that these are young supernova remnants which have just begun interacting with a surrounding ISM with a density between 10^4 and 10^5 cm^-3. One of these sources is probably resolved at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate that the ionized component of the ISM gives rise to foreground free-free absorption with opacity at 18 cm of <0.6 along the majority of lines of sight. Other sources may be affected by absorption with opacity in the range 1 to 2. These values are consistent with previous models as fitted to the radio recombination lines and the continuum spectrum.Comment: 44 pages, 9 figures, 2 tables. Accepted for publication in Ap