RNA silencing-mediated resistance to a crinivirus (Closteroviridae) in cultivated sweetpotato (Ipomoea batatas L.) and development of sweetpotato virus disease following co-infection with a potyvirus

Sweetpotato chlorotic stunt virus (SPCSV; genus Crinivirus , family Closteroviridae) is one of the most important pathogens of sweetpotato ( Ipomoea batatas L.). It can reduce yields by 50% by itself and cause various synergistic disease complexes when co-infecting with other viruses, including sweetpotato feathery mottle virus (SPFMV; genus Potyvirus , family Potyviridae). Because no sources of true resistance to SPCSV are available in sweetpotato germplasm, a pathogen-derived transgenic resistance strategy was tested as an alternative solution in this study. A Peruvian sweetpotato landrace 'Huachano' was transformed with an intron-spliced hairpin construct targeting the replicase encoding sequences of SPCSV and SPFMV using an improved genetic transformation procedure with reproducible efficiency. Twenty-eight independent transgenic events were obtained in three transformation experiments using a highly virulent Agrobacterium tumefaciens strain and regeneration through embryogenesis. Molecular analysis indicated that all regenerants were transgenic, with 1–7 transgene loci. Accumulation of transgene-specific siRNA was detected in most of them. None of the transgenic events was immune to SPCSV, but ten of the 20 tested transgenic events exhibited mild or no symptoms following infection, and accumulation of SPCSV was significantly reduced. There are few previous reports of RNA silencing-mediated transgenic resistance to viruses of Closteroviridae in cultivated plants. However, the high levels of resistance to accumulation of SPCSV could not prevent development of synergistic sweet potato virus disease in those transgenic plants also infected with SPFMV.     

Evolutionary patterns in biogeography and host plant association: 'taxonomic conservatism' in Lophopidae (Hemiptera, Fulgoromorpha)

Abstract.  The association between the Lophopidae (Hemiptera, Fulgoromorpha) and their host plants was studied within a phylogenetic framework. Host plant use was optimized on Lophopidae phylogeny and the most parsimonious hypothesis is presented. This hypothesis describes the evolution of host plant use by the Lophopidae, and postulates the ancestral plant family used. This scenario is discussed within the biogeographical evolutionary context of the fulgoromorphan families, and is corroborated by information from both insect and host plant fossils. The association of the Lophopidae and their host plants is made by comparing the angiosperms and Lophopidae phylogenies, demonstrating at this level of comparison that the insects show 'taxonomic conservatism' for their host plants.     

Variations in bird abundance in tropical arid and semi-arid habitats

Temporal variation in bird abundance was studied during a complete annual cycle in a thorn scrub, a thorn woodland and a deciduous forest in northeastern Venezuela. Abundance of site-attached and transient birds from different feeding guilds was determined by mist-netting at 2-week intervals. Diets were investigated by regurgitated samples. The overall avifauna was characterized by a low number of species but they were present all year despite showing strong seasonal fluctuations in abundance. The number of bird species and individuals peaked before and after the reproductive period. These high values probably were associated with movement of species feeding on plant food during the late dry season and the post-breeding dispersion of juveniles. Bird richness and abundance were lowest during the breeding season and in the early dry season when food abundance was low. Birds from different feeding guilds showed distinct patterns of seasonal abundance which tended to be similar at all three sites. Transient birds represented a large portion of the avifauna, particularly in nectarivores, frugivores and granivores during the dry season. We used a canonical correspondence analysis to demonstrate that bird abundance was correlated with breeding activity, rainfall seasonality and food abundance, with the influence of each parameter varying according to feeding guilds, spatial behaviour of individuals and habitats. Despite a great turnover in the occurrence of the diverse food types available, species composition remained strikingly constant during the year, with birds responding to seasonal changes primarily through a generalist feeding habit and a highly variable rate of transience.     

CELL ADHESION MOLECULES: A UNIFYING APPROACH TO TOPOGRAPHIC BIOLOGY

Cell adhesion molecules are pivotal to the development and maintenance of tissue structure in metazoan organisms. In mammals, several families of proteins are involved in cell-cell and cell-matrix adhesion. The cadherins are homophilic, primary CAMs, involved in the establishment of boundaries between cell collectives early in embryogenesis. The Ig gene superfamily have diversified widely, with homophilic and heterophilic CAMs and antigen recognition molecules amongst the members. The Integrin family play an important role in binding to extracellular matrix, as well as counter-receptors on the surface of other cells. The Selectin family and HCAM are carbohydrate-binding proteins, and play a prominent role in the circulation of lymphocytes and neoplastic cells. CAMs are fundamental to development of tissue structure in metazoan organisms. Cellular differentiation dictates adherence to a specific microenvironment, through the pattern of surface CAM expression. Conversely, CAM binding can affect gene expression within the cell itself. Cell differentiation and cell adhesion are interdependent processes. In the adult, CAM are crucial to tissue maintenance. Cells frequently change their adhesive properties in response to physiological or pathological processes. The integrity of the vascular system is maintained by circulating platelets which are capable of rapid upregulation of cell adhesion and profound changes in metabolism, on contact with subendothelial matrix. Both endothelial cells and neutrophils undergo changes in CAM expression in response to inflammatory mediators, permitting rapid and appropriate recruitment of phagocytes to damaged tissue. Tissue repair is dependent on phenotypic changes in normally static cells, allowing increased motility and replication. The immune system requires constitutive cells to undergo multiple complex adhesion and detachment events over short periods of time, and is capable of discriminating normal self from aberrant-self or non-self, through antigen specific recognition and adhesion molecules. The pathophysiology of processes such as infection and neoplasia are profoundly affected by cellular CAM expression. CAMs and related molecules are fundamental to the development, maintenance and surveillance of tissue structure.     

Negative fire feedback in a transitional forest of southeastern Amazonia

Anthropogenic understory fires affect large areas of tropical forest, particularly during severe droughts. Yet, the mechanisms that control tropical forests' susceptibility to fire remain ambiguous. We tested the widely accepted hypothesis that Amazon forest fires increase susceptibility to further burning by conducting a 150 ha fire experiment in a closed-canopy forest near the southeastern Amazon forest–savanna boundary. Forest flammability and its possible determinants were measured in adjacent 50 ha forest plots that were burned annually for 3 consecutive years (B3), once (B1), and not at all (B0). Contrary to expectation, an annual burning regime led to a decline in forest flammability during the third burn. Microclimate conditions were more favorable compared with the first burn (i.e. vapor pressure deficit increased and litter moisture decreased), yet flame heights declined and burned area halved. A slight decline in fine fuels after the second burn appears to have limited fire spread and intensity. Supporting this conclusion, fire spread rates doubled and burned area increased fivefold in B3 subplots that received fine fuel additions. Slow replacement of surface fine fuels in this forest may be explained by (i) low leaf litter production (4.3 Mg ha⁻¹ yr⁻¹), half that of other Amazon forests; and (ii) low fire-induced tree and liana mortality (5.5±0.5% yr⁻¹, SE, in B3), the lowest measured in closed-canopy Amazonian forests. In this transitional forest, where severe seasonal drought removed moisture constraints on fire propagation, a lack of fine fuels inhibited the intensity and spread of recurrent fire in a negative feedback. This reduction in flammability, however, may be short-lived if delayed tree mortality or treefall increases surface fuels in future years. This study highlights that understanding fuel input rate and timing relative to fire frequency is fundamental to predicting transitional forest flammability – which has important implications for carbon emissions and potential replacement by scrub vegetation.     

Ion-pair Movement across Bilayer Membranes

WE have used bilayer lipid membranes (BLM) as models for biological membranes to study the transport of metal halides as ion-pairs. In the presence of iodine (I₂) we find¹ that various monovalent and divalent halides can readily move across BLM along a concentration gradient. The rate of transport increases as the size of the cation increases. As the membrane interior is thought to be of a liquid hydrocarbon nature, these results can be related to those of solvent extraction studies² which showed that various metal halides could be extracted efficiently from aqueous solutions into organic solvents; the extraction efficiency increases as the cation size increases.     

Exploration and rehabilitation of hydraulic habitats in streams using principles of fluvial behaviour

• 1 The combination of elements from geomorphology, open-channel hydraulics, and hydraulic habitat requirements of stream fish forms the basis for an ecologically sound ‘soft engineering’ of river channels.
• 2 Interpreting and mapping the hydraulic geometry of streams and locally varied flow conditions can be accomplished with plane table surveys and customized field-data sheets. This information can serve to manage hydraulic habitats preferred by fish.
• 3 The use of fluvial characteristics to design preferred hydraulic habitats is illustrated in two examples: (i) a walleye (Stizostedion vitreum vitreum Mitchill) spawning rehabilitation project undertaken in a stream channelized as a lowland drainage canal, and (ii) a trout (Salvelinus fontinalis Mitchill and Oncorhynchus mykiss Walbaum) habitat-enhancement project to create additional holding and resting areas for adult fish in a stream paved with glacially deposited boulders modified by a road crossing.
• 4 In both examples the ‘soft engineering’ of the river channels enhanced the hydraulic fish habitat.

     

Species richness estimation and determinants of species detectability in butterfly monitoring programmes

Abstract 1. Species richness is the most widely used biodiversity index, but can be hard to measure. Many species remain undetected, hence raw species counts will often underestimate true species richness. In contrast, capture–recapture methods estimate true species richness and correct for imperfect and varying detectability. 2. Detectability is a crucial quantity that provides the link between a species count and true species richness. For insects, it has hardly ever been estimated, although this is required for the interpretation of species counts. 3. In the Swiss butterfly monitoring programme about 100 transect routes are surveyed seven times a year using a highly standardised protocol. In July 2003, control observers made two additional surveys on 38 transects. Data from these 38 quadrats were analysed to see whether currently available capture–recapture models can provide quadrat‐specific estimates of species richness, and to estimate species detectability in relation to transect, observer, survey, region, and abundance. 4. Species richness over the entire season cannot be estimated using current capture–recapture methods. The species pool was open, preventing use of closed population models, and detectability varied by species, preventing use of current open population models. Assuming a closed species pool during two mid‐season (July) surveys, a Jackknife capture–recapture method was used that accounts for heterogeneity to estimate mean detectability and species richness. 5. In every case, more species were present than were counted. Mean species detectability was 0.61 (SE 0.01) with significant differences between observers (range 0.37–0.83). Species‐specific detection at time t+ 1 was then modelled for those species seen at t for three mid‐season surveys. Detectability averaged 0.50 (range 0.17–0.81) for individual species and 0.65, 0.44, and 0.42 for surveys. Abundant species were detected more easily, although this relationship explained only 5% of variation in species detectability. 6. These are important, although not entirely unexpected, results for species richness estimation of short‐lived animals. Raw counts of species may be misleading species richness indicators unless many surveys are conducted. Monitoring programmes should be calibrated, i.e. the assumption of constant detectability over dimensions of interest needs to be tested. The development of capture–recapture or similar models that can cope with both open populations and heterogeneous species detectability to estimate species richness should be a research priority.