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Joan Roughgarden 《Biology & philosophy》2009,24(4):521-529
Community ecology entered the 1970s with the belief that niche theory would supply a general theory of community structure.
The lack of wide-spread empirical support for niche theory led to a focus on models specific to classes of communities such
as lakes, intertidal communities, and forests. Today, the needs of conservation biology for metrics of “ecological health”
that can be applied across types of communities prompts a renewed interest in the possibility of general theory for community
ecology. Disputes about the existence of general patterns in community structure trace at least to the 1920s and continue
today almost unchanged in concept, although now expressed through mathematical modeling. Yet, a new framework emerged in the
1980s from findings that community composition and structure depend as much on the processes that bring species to the boundaries
of a community as by processes internal to a community, such as species interactions and co-evolution. This perspective, termed
“supply-side ecology”, argued that community ecology was to be viewed as an “organic earth science” more than as a biological
science. The absence of a general theory of the earth would then imply a corresponding absence of any general theory for the
communities on the earth, and imply that the logical structure of theoretical community ecology would consist of an atlas
of models special to place and geologic time. Nonetheless, a general theory of community ecology is possible similar in form
to the general theory for evolution if the processes that bring species to the boundary of a community are analogized to mutation,
and the processes that act on the species that arrive at a community are analogized to selection. All communities then share
some version of this common narrative, permitting general theorems to be developed pertaining to all ecological communities.
Still, the desirability of a general theory of community ecology is debatable because the existence of a general theory suppresses
diversity of thought even as it allows generalizations to be derived. The pros and cons of a general theory need further discussion. 相似文献
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Phloem injury triggers local sieve-plate occlusion including callose-mediated constriction and protein plugging of sieve pores. In intact plants, reversible sieve-plate occlusion is induced by electric potential waves (EPWs)—accompanied by Ca2+-influx—as result of distant burning. Here, we present additional results which pertain to (a) the variability of EPW-profiles in relation to forisome conformation in intact Vicia faba plants and (b) the differential occlusion reactions to burning and cutting in various plant species. A correlation between stimulus perception and mode of phloem loading is discussed.Key words: callose, electrical potential waves, forisome, membrane potential, phloem transport, sieve-element occlusion, wound potentials 相似文献
58.
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 相似文献
59.
Anisogamy refers to gametes that differ in size, and characterizes the difference between males and females. The evolution of aniosgamy is widely interpreted as involving conflict between gamete producers with small sperm parasitizing on the investment made by the eggs. Using a population genetic model for evolution at a locus that codes jointly for sperm and egg sizes of a hermaphrodite, we show that the origin of anisogamy in an externally spawning population need not involve conflict between gamete producers. Gamete size dimorphism may be an adaptation that increases gamete encounter rates when large zygotes are selected, and we show this in a mechanistically general individual selection model. We use the Vance survival function without specific allometric assumptions to model the zygote fitness dependence on its size, and hence obtain ecological and life-history correlates of isogamy and anisogamy, which we successfully compare with data from Volvocales. 相似文献
60.
The effect of surgery (insertion of an ileo-caecal cannula) and a subsequent parenteral treatment with oxytetracycline on the ileo-caecal and rectal coliform populations in 7 Swedish Yorkshire castrates were studied. Samples were collected during surgery as well as 3, 7, 14 and 20 days post surgery. The diversity of the enteric coliform flora was initially high both in the ileo-caecal ostium and in rectum. No alteration in the diversity of the enteric coliform flora was observed following surgery and treatment with oxytetracycline. As the insertion of ileo-caecal cannulas did not affect the intestinal coliform flora this study gives support to the use of this technique to mirror processes in the small intestine of pigs. Further, the diversity of the enteric coliform flora was unaffected by the parenteral treatment with oxytetracycline. 相似文献