Spatial and temporal population genetic structure of four northeastern Pacific littorinid gastropods: the effect of mode of larval development on variation at one mitochondrial and two nuclear DNA markers

We investigated the effect of development mode on the spatial and temporal population genetic structure of four littorinid gastropod species. Snails were collected from the same three sites on the west coast of Vancouver Island, Canada in 1997 and again in 2007. DNA sequences were obtained for one mitochondrial gene, cytochrome b ( Cyt b ), and for up to two nuclear genes, heat shock cognate 70 ( HSC70 ) and aminopeptidase N intron ( APN54 ). We found that the mean level of genetic diversity and long-term effective population sizes ( N _e) were significantly greater for two species, Littorina scutulata and L. plena , that had a planktotrophic larval stage than for two species, Littorina sitkana and L. subrotundata , that laid benthic egg masses which hatched directly into crawl-away juveniles. Predictably, two poorly dispersing species, L. sitkana and L. subrotundata , showed significant spatial genetic structure at an 11- to 65-km geographical scale that was not observed in the two planktotrophic species. Conversely, the two planktotrophic species had more temporal genetic structure over a 10-year interval than did the two direct-developing species and showed highly significant temporal structure for spatially pooled samples. The greater temporal genetic variation of the two planktotrophic species may have been caused by their high fecundity, high larval dispersal, and low but spatially correlated early survivorship. The sweepstakes-like reproductive success of the planktotrophic species could allow a few related females to populate hundreds of kilometres of coastline and may explain their substantially larger temporal genetic variance but lower spatial genetic variance relative to the direct-developing species.     

Frasnian vertebrate taphonomy and sedimentology of macrofossil concentrations from the Langsēde Cliff,Latvia

Luk?evi?s, E., Ahlberg, P.E., Stinkulis, ?., Vasi?kova, J. & Zupi??, I. 2011: Frasnian vertebrate taphonomy and sedimentology of macrofossil concentrations from the Langsēde Cliff, Latvia. Lethaia, Vol. 45, pp. 356–370. The siliciclastic sequence of the Upper Devonian of Kurzeme, Western Latvia, is renowned for abundant vertebrate fossils, including the stem tetrapods Obruchevichthys gracilis and Ventastega curonica. During the first detailed taphonomic study of the vertebrate assemblage from the Ogre Formation cropping out at the Langsēde Cliff, Imula River, abundant vertebrate remains have been examined and identified as belonging to one psammosteid, two acanthodian and three sarcopterygian genera; the placoderm Bothriolepis maxima dominates the assemblage. Besides fully disarticulated placoderm and psammosteid plates, separate sarcopterygian scales and teeth, and acanthodian spines, partly articulated specimens including complete distal segments of Bothriolepis pectoral fins, Bothriolepis head shields and sarcopterygian lower jaws have been found. The size distribution of the placoderm bones demonstrates that the individuals within the assemblage are of approximately uniform age. Distinct zones have been traced within the horizontal distribution of the bones. These linear zones are almost perpendicular to the dominant dip azimuth of the cross‐beds and ripple‐laminae and most probably correspond to the depressions between subaqueous dunes. Concavity ratio varies significantly within the excavation area. The degree of fragmentation of the bones and disarticulation of the skeletons suggest that the carcasses were reworked and slightly transported before burial. Sedimentological data suggest deposition in a shallow marine environment under the influence of rapid currents. The fossiliferous bed consists of a basal bone conglomerate covered by a cross‐stratified sandstone with mud drapes, which is in turn overlain by ripple laminated sandstone, indicating the bones were buried by the gradual infilling of a tidal channel. All the Middle–Upper Devonian vertebrate bone‐beds from Latvia are associated with sandy to clayey deposits and have been formed in a sea‐coastal zone during rapid sedimentation episodes, but differ in fossil abundance and degree of preservation. □Agnathans, Devonian, facies analysis, fish, fossil assemblage, palaeoenvironment.     

Induction as well as suppression: How aphid saliva may exert opposite effects on plant defense

Aphids ingest from the sieve tubes and by doing so they are confronted with sieve-tube occlusion mechanisms, which are part of the plant defense system. Because aphids are able to feed over longer periods, they must be able to prevent occlusion of the sieve plates induced by stylet penetration. Occlusion probably depends upon Ca²⁺-influx into the sieve element (SE) lumen. Aphid behavior, biochemical tests and in vitro experiments demonstrated that aphid''s watery saliva, injected during initial phase of a stylet penetration into the SE lumen, contains proteins that are able to bind calcium and prevent calcium-induced SE occlusion. In this addendum, we speculate on the consequences of saliva secretion for plant resistance. (a) The release of elicitors (e.g., oligogalacturonides) due to cell wall digestion by gel saliva enzymes may increase the resistance of cortex, phloem parenchyma cells and companion cells (CC) around the puncture site. (b) Ca²⁺-binding by aphid watery saliva may suppress the local defense responses in the SEs. (c) Signaling cascades triggered in CCs may lead to systemic resistance.Key words: aphid saliva, calcium binding, elicitor, oligogalacturonides, local plant defense, systemic plant defense, phloem translocation, aphid/plant-interactionAfter having penetrated the sieve-element (SE) plasma membrane, aphids encounter unspecific wound-induced occlusion reactions to prevent sap leakage.^1–4 Occlusion mechanisms by callose, structural P-proteins and forisomes are likely induced by a sudden calcium influx into the sieve-tube lumen.⁵ Calcium possibly enters the sieve-tube lumen through the stylet wounding-site in the plasma membrane and/or stretch-activated calcium-channels.^6–8 After SE penetration, aphids secrete watery saliva that contains calcium-binding proteins presumed to sabotage sieve-plate occlusion.^9,10We demonstrated that Megoura viciae (Buckton) is most likely able to prevent or reduce sieve-tube occlusion in Vicia faba by secretion of watery saliva. By in vitro confrontation of isolated forisomes, protein bodies responsible for sieve-tube occlusion in Fabaceaen,⁵ and watery saliva concentrate, we were able to show that salivary proteins convey forisomes from a dispersed (+Ca²⁺) into a condensed (−Ca²⁺) state.¹⁰ The dispersed forisome functions in vivo as a plug, leading to stoppage of mass flow.⁵This in vitro evidence was corroborated by aphid behavior in response to leaf tip burning, which triggers an electrical potential wave (EPW) along the sieve tubes. Such an EPW induces Ca²⁺-influx and corresponding SE occlusion along the pathway.¹¹ The passage of the EPW is associated with a prolonged secretion of watery saliva of aphids. This is interpreted as an attempt to unplug the SEs by calcium binding.¹⁰ Similar behavioral changes in response to leaf-tip burning were observed in an extended set of aphid/plant species combinations, indicating that attempted sabotage of sieve-tube occlusion by aphid saliva is a widespread phenomenon (unpublished).Aphid feeding was reported to induce local (on the same leaf) and systemic (in distant leaves) reactions of the host plant. The local response led to enhanced feeding,^12–14 while the systemic response showed reduced ingestion and extended periods of watery saliva secretion in sieve tubes distant from previous feeding sites.^12–14 These contrasting observations were described to be independent of the aphid species.¹³ The question arises how aphids induce these seemingly opposite plant responses?The aphid stylet pushing forward through cortical and vascular tissue is surrounded by a sheath of gel saliva, secreted into the apoplast.^15,16 Gel saliva contains cellulase and pectinase that amongst others produce oligogalacturonides (OGs) along the stylet sheath by digestion of cell wall material.^17,18 Usually, OGs act as elicitors, triggering a variety of plant responses against pathogens and insects in which the activation of calcium channels is involved.^19,20 This seems to conflict with a suppression of resistance as observed for the impact of watery saliva in SEs.¹⁰ We will make an attempt to explain this paradoxon.OG induced defense responses may be triggered in all cell types adjacent to the salivary sheath (Fig. 1). Because watery saliva is only secreted briefly into these cells, which are punctured for orientation purposes (Hewer et al., unpublished), it seems unlikely that OG induced defense is suppressed here by saliva-mediated calcium binding.¹⁵ The diffusion range of OGs may be restricted to the close vicinity of the stylet sheath leading to an enhanced regional defense with a limited sphere of action (Fig. 1). Because the settling distance of aphids is restricted by their body size (1–10 mm),²¹ aphids feeding on the same leaf are probably hardly confronted with the regional defense induced by another aphid (Fig. 1). Otherwise, they would show an increased number of test probes before first phloem activity, as described for volatile mediated plant defense in cortex cells.¹³ Circumstantial support in favor of our hypothesis is provided by production of hydrogen peroxide in the apoplast,²² which is most likely associated with the action of OGs.²² Observations of hydrogen peroxide production during aphid (Macrosiphum euphorbiae) infestation of tomato in a limited area along the leaf veins, the preferred feeding sites of this species, indicate a locally restricted defense response (Fig. 1 and ​and22).⁴ The question arises why the cell signals are not spread via plasmodesmata to adjacent cells to induce resistance in a more extended leaf area? Dissemination of the signals may be prevented by closure of plasmodesmata (Fig. 1) through callose deposition,^23,24 which is most likely directly coupled with calcium influx induced by OGs,²⁵ by apoplastic hydrogen peroxide and to a minor extent by stylet puncture (Fig. 2).^7,26Open in a separate windowFigure 1Hypothetical model on how stylet penetration induces and suppresses plant defense. Sheath saliva (light blue) that envelopes the stylet during propagation through the apoplast contains cellulase and pectinase,^17,18 enzymes producing elicitors (e.g., oligogalacturonides (oGs)) by local cell wall digestion.¹⁹ Parenchyma cells adjacent to the sheath may develop a defense response owing to signaling cascades triggered by oG-mediated Ca²⁺-influx.¹⁹ Together with a Ca²⁺-dependent transient closure of plasmodesmata by callose (black crosses),^23,24 the focused production of oGs may cause a defense response with a limited sphere of action (red—strong, brown—light, green—none). This restricted domain of defense may not be perceived by other aphids, since the settling distance is limited by the aphid body size. Nearby aphids do not show any sign of defense perception in their probing and feeding behavior.¹⁴ Signaling cascade compounds may be channeled from parenchyma cells to CCs (dashed yellow arrows), where they are subsequently released into the SEs. There they may act as long-distance systemic defense components (grey arrows). In contrast to the parenchyma domain (where only minor amounts of watery saliva are secreted), Ca²⁺-mediated reactions such as defense cascades and sieve-plate (SP) occlusion are suppressed in SEs by large amounts of watery saliva. The left aphid penetrates an SE and injects watery saliva (red cloud; ws) that inhibits local sieve-plate occlusion and,¹⁰ most likely, is transported by mass flow (black arrow) to adjacent SEs,²⁷ where occlusion is impeded as well. A short-distance systemic spread over a few centimeters may explain local suppression of plant defense resulting in a higher rate of colonization. Salivary proteins or their degradation products may serve as systemic defense signals as well (grey arrows), but may also diffuse via the PPUs into CCs where additional systemic signals are induced (yellow arrows).Open in a separate windowFigure 2Hypothetical involvement of Ca²⁺-channels in aphid-induced cell defense (detail). During probing with its stylet the aphid secretes gel saliva as a lubrication substance (light blue) into the apoplast.¹⁵ on the way to the sieve tubes, aphids briefly puncture most non-phloem cells (red) after which the puncturing sites are sealed with gel saliva.^7,16 Gel saliva also most likely prevents the influx of apoplastic calcium into pierced sieve elements (green) by sealing the penetration site.⁷ Watery saliva (red cloud), injected into the SE lumen,⁹ contains proteins which bind calcium ions (marked by X) that enter the SE via e.g., mechano sensitive Ca²⁺-channels activated by stylet penetration (blue tons).¹⁰ In this way, aphids suppress SE occlusion and activation of local defense cascades. In the parenchyma cells around the gel saliva sheath, a small cylindrical zone of defense may be induced by oligogalacturonides (oGs; brown triangles) produced by cell wall (grey) digestion.^17–19 Perceived by unknown receptor proteins (R; e.g., a receptor like protein kinase)³⁴ and kinase mediation (black dotted and dashed arrows), oGs lead to a Ca²⁺-influx through kinase activated calcium channels (orange tons).²⁵ Around the probing site, aphids apparently induce the production of superoxide by Ca²⁺-induced activation of the NADPH oxidase (violet box) and its following conversion to hydrogen peroxide (red spots) is mediated by superoxide dismutase (SoD).⁴ Hydrogen peroxide activates Ca²⁺-channels (violet tons) and diffuses through plasma membrane (curled arrows) therefore potentially acting as a intracellular signal.²⁶By contrast, Ca²⁺-influx into SEs, induced by presence of OGs or stylet insertion (Fig. 2), is not expected to trigger local defense given the abundant excretion of Ca²⁺-binding watery saliva.^7,10,25 Watery saliva may spread to down-stream and adjacent SEs through transverse and lateral sieve plates (Fig. 1).^7,27 Aphids puncturing nearby SEs may therefore encounter less severe sieve-plate occlusion which results in facilitated settling and thus in increased population growth. Aggregation of feeding aphids would self-amplify population growth until a certain density is attained. Farther from the colonization site, this effect may be lost due to dilution. Stimulation of aphid feeding by aphid infestation was observed locally on potato by Myzus persicae and M. euphorbiae, respectively, 96 h after infestation.¹³ However, a similar effect was not observed for M. persicae on Arabidopsis thaliana where aphids induced premature leaf senescence and resistance 12 h after infestation,²⁸ possibly induced by OGs.¹⁹As a speculation, OG induced Ca²⁺-influx into parenchyma cells adjacent to the salivary sheath activate Ca²⁺-induced signaling cascades via CaM,^26,29 CDPKs,^30,31 MAPKinases and reactive oxygen species (Fig. 2).³² Systemic resistance, induced by aphid infestation,^12–14 is mediated by unknown compounds such as, e.g., salivary proteins, their degradation products, signal cascade products or volatiles.¹³ Compounds produced in CCs first have to pass the PPUs, while SE signaling elements can be directly transported via mass flow (Fig. 1).The question arises if aphids profit from induced resistance on local (cortex and parenchyma cells) and systemic (distant plant organs) levels as holds for suppression of defense in SEs. Possibly settling and subsequent spread of competing pathogens/herbivores (e.g., fungi or other piercing-sucking insects) are suppressed by induced defense. In this context it is intriguing to understand how aphids cope with the self-induced systemic resistance, which probably lasts over weeks.³³     

Genetic islands of <Emphasis Type="Italic">Streptococcus agalactiae</Emphasis> strains NEM316 and 2603VR and their presence in other Group B Streptococcal strains

Background  

Streptococcus agalactiae (Group B Streptococcus; GBS) is a major contributor to obstetric and neonatal bacterial sepsis. Serotype III strains cause the majority of late-onset sepsis and meningitis in babies, and thus appear to have an enhanced invasive capacity compared with the other serotypes that cause disease predominantly in immunocompromised pregnant women. We compared the serotype III and V whole genome sequences, strains NEM316 and 2603VR respectively, in an attempt to identify genetic attributes of strain NEM316 that might explain the propensity of strain NEM316 to cause late-onset disease in babies. Fourteen putative pathogenicity islands were described in the strain NEM316 whole genome sequence. Using PCR- and targeted microarray- strategies, the presence of these islands were assessed in a diverse strain collection including 18 colonizing isolates from healthy pregnant women, and 13 and 8 invasive isolates from infants with early- and late-onset sepsis, respectively.     

Assembly and disassembly of mammalian chromosome pellicle

By means of indirect double immunofluorescent staining,the coordination of PI antigen and perichromonucleolin(PCN),the constituent of nuclear periphery and nucleolus respectively,in the assembly and disassembly of chromosome pellicle during mitosis was studied.It was found that in 3T3 cells,during mitosis PI antigen began to coat the condensing chromosome surface earlier than PCN did.However,both of them completed their coating on chromosome at approximately the same stage of mitosis,prometaphase metaphase,The dissociation of mitosis,Prometaphase metaphase.The dissociation of PI antigen from chromosome pellicle to participate the formation of nuclear periphery took place also ahead of that of PCN,At early telophase PI antigen had been extensively involved in the formation of nuclear periphery,while PCN remained in association with the surface of decondensing chromosomes.At late telophase,when PI antigen was localized in an fairly well formed nuclear periphery,PCN was in a stage of forming prenucleolar bodies.     

Field evaluation of the effects of the insect growth regulator tebufenozide on entomophagous arthropods and pests of apples

Organophosphate insecticides are very widely used in commercial apple production to control fruit-attacking pests but their broad-spectrum activity constrains biological control of other pests. Compounds with narrow-spectrum activity are therefore desirable. The insect growth regulator (IGR) tebufenozide was compared with another IGR, fenoxycarb, and the organophosphate, azinphos-methyl, in a replicated field trial in the 1994/1995 apple-production season. Vacuum sampling of the tree foliage on five occasions during the growing season showed significantly lower populations of various natural enemies (spiders, lacewings and the specialist mite predator Stethorus spp. adults and larvae) in the azinphos-methyl treatment than in either of the two IGR treatments. The two-spotted mite ( Tetranychus urticae Koch) was most numerous in the azinphos-methyl treatment. In 1995/1996, the entire trial area was placed under a tebufenozide treatment program to determine the extent to which natural enemy populations would recover when broad-spectrum insecticide (azinphos-methyl) use was halted. Populations of polyphagous natural enemies assumed levels broadly equivalent to those observed under IGR treatments the previous year. Numbers of Stethorus spp. were lower than in the 1994/1995 season, possibly because T. urticae (prey) populations were much reduced from the previous season's densities. All three insecticide treatments were equally effective in controlling the lepidopteran pests, codling moth ( Cydia pomonella (L.)), lightbrown apple moth ( Epiphyas postvittana (Walker)) and early season caterpillars (predominantly Helicoverpa punctigera (Wallengren)). Results indicate that tebufenozide provides good control of lepidopteran pests, while allowing the rapid build-up of natural enemies which contribute to control of other pests.