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Induced defence in wild cabbage : integrating genes, volatiles & insect behaviour
Abstract
p>This study integrates three methods for detection of an induced response to aphid feeding. Firstly, it is shown that volatile organic compounds emitted from wild cabbage infested with the cabbage aphid Brevicoryne brassicae attract the aphid parasitoid Diaeretiella rapae and could constitute a reliable and detectable cue during parasitoid host foraging. It is found that there is no difference in attraction to aphid-infested plants when comparing field-collected and lab-reared parasitoids. Secondly, collection and analysis of compounds emitted from the aphid and Plutella xylostella damaged plants reveals several compounds exhibiting significant induction when compared to uninfested plants. Univariate techniques and multivariate canonical discriminant analysis reveal distinct volatile profiles contain several terpenes and 3-butenyl isothiocyanate as induced compounds that could be involved in the attraction of the parasitoid. It is proposed that this multivariate technique may closer resemble the method of signal recognition used by foraging insects.
Finally, global gene expression analysis using Arabidopsis thaliana microarrays coupled to a Brassica oleracea specific software filter is used to assay the expression of genes induced by aphid and Plutella feeding. These experiments reveal the aphid- and Plutella- related induction of genes involved in isoprenoid biosynthesis, involvement of the octadecanoid pathway following Plutella feeding and that although the genes induced are different, number of genes induced by the different herbivores is roughly equal. AFLP analysis illustrates the high degree of genotypic variation in wild Brassica populations when compared to crop cultivars and Arabidopsis thaliana . This suggests that the wild populations are segregating with a high level of gene flow between populations.</p
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The defatted seed meal of crambe, Crambe abyssinica Hochst ex. R. E. Fries, was systematically analyzed for insecticidal activity against the house fly, Musca domestica L., and the active components were isolated and characterized. 2-(S)-1-Cyano-2-hydroxy-3-butene (SCHB) and phenylethyl cyanide (PEC) were identified as active components, whereas diacetone alcohol (DAA), which was identified in the extracts and tested, was not toxic to house flies. The presence of DAA and PEC in the extracts was confirmed by using gas chromatographic and mass-spectral (GC-MS) comparison with purchased reference compounds. 1,3-Benzodioxole-5-carboxaldehyde (piperonal, PIP) was also identified as a possible minor component, but was not tested. GC-MS analysis determined that the dichloromethane extract of defatted crambe seed meal contained SCHB and crambe oil at a ratio of 5:2, while DAA and PEC were present in trace amounts. Topical LD50 values for M. domestica were calculated for SCHB, PEC, DAA, crude crambe extract, crambe oil and an "artificial crambe extract" composed of SCHB and crambe oil in the proportions found in the crude crambe extract. SCHB was found to be the most toxic major component of the crambe extract. Although SCHB concentration accounted for the toxicity of the artificial extract, the natural crambe extract was significantly less toxic than would be expected based on SCHB concentration alone. 2-(R)-1-Cyano-2-hydroxy-3-butene (RCHB), an enantiomer of SCHB which does not occur in crambe, was extracted from canola, Brassica napus L., seed meal and tested as well. RCHB was found to be significantly less toxic to house flies than SCHB.
Herbivore-induced plant volatiles have been suggested to function as indirect defence signals that attract natural enemies of herbivores. Several insect parasitoids are known to exploit such plant-provided cues to locate hosts. It is unclear if individual plants benefit from the action of parasitoids. We investigated this question in maize plants under attack by Spodoptera littoralis larvae and found that parasitization by the endoparasitoids Cotesia marginiventris and Campoletis sonorensis significantly reduced feeding and weight gain in the host larvae. As a result, young maize plants attacked by a single parasitized larva suffered much less feeding damage and, at maturity, produced about 30% more seed than plants that were attacked by an unparasitized larva. Such fitness benefits may have contributed to selection pressures that shaped the evolution of herbivore-induced indirect defence signals in plants.
In response to insect herbivory, plants synthesize and emit blends of volatile compounds from their damaged and undamaged tissues, which act as important host-location cues for parasitic insects1, 2, 3. Here we use chemical and behavioural assays to show that these plant emissions can transmit herbivore-specific information that is detectable by parasitic wasps (parasitoids). Tobacco, cotton and maize plants each produce distinct volatile blends in response to damage by two closely related herbivore species, Heliothis virescens and Helicoverpa zea. The specialist parasitic wasp Cardiochiles nigriceps exploits these differences to distinguish infestation by its host, H. virescens, from that by H.zea. The production by phylogenetically diverse plant species and the exploitation by parasitoids of highly specific chemical signals, keyed to individual herbivore species, indicates that the interaction between plants and the natural enemies of the herbivores that attack them is more sophisticated than previously realized.
To understand the role of allelochemicals in predator-prey interactions it is not sufficient to study the behavioral responses of predator and prey. One should elucidate the origin of the allelochemicals and be aware that it may be located at another trophic level. These aspects are reviewed for predator-prey interactions in general and illustrated in detail for interactions between predatory mites and herbivorous mites. In the latter system there is behavioral and chemical evidence for the involvement of the host plant in production of volatile allelochemicals upon damage by the herbivores with the consequence of attracting predators. These volatiles not only influence predator behavior, but also prey behavior and even the attractiveness of nearby plants to predators. Herbivorous mites disperse away from places with high concentrations of the volatiles, and undamaged plants attract more predators when previously exposed to volatiles from infested conspecific plants rather than from uninfested plants. The latter phenomenon may well be an example of plant-to-plant communication. The involvement of the host plant is probably not unique to the predator-herbivore-plant system under study. It may well be widespread since it makes sense from an evolutionary point of view. If so, prospects for application in pest control are wide open. These are discussed, and it is concluded that crop protection in the future should include tactics whereby man becomes an ally to plants in their strategies to manipulate predator-prey interactions through allelochemicals.
Plants may defend themselves against herbivores by enhancing the effectiveness of natural enemies of herbivores. This is termed "indirect defense," which may be induced by herbivore damage. An important aspect of induced indirect defense is the attraction of the herbivore's natural enemies to infested plants by the plant emitting so-called "herbivore-induced synomone" (HIS) in response to herbivore damage. In this paper, we review the role of terpenoids in the induced indirect defense of plants against herbivorous mites. HIS are emitted from both damaged and undamaged areas of infested plants, and the composition of HIS varies among different plant species. The emission of HIS may also vary within a plant species, depending upon: (1) plant cultivar, (2) leaf growth stage, (3) the herbivore species that is attacking, and (4) abiotic conditions (light intensity, time of year, and water stress). Predatory mites cope with this variation of HIS by innate recognition as well as temporary specialization to a certain HIS via learning.
The female parasitic waspCotesia kariyai discriminated between the volatiles of corn leaves infested by younger host larvaePseudaletia separata (first to fourth instar) and uninfested leaves in a Y-tube olfactometer; the wasps were attracted to the infested leaves. In contrast, when corn plants were infested by the later stages (fifth and sixth instar) of the armyworm, the wasps did not distinguish between infested corn leaves and uninfested corn leaves in the olfactometer. Mechanically damaged leaves were no more attractive than undamaged leaves, and host larvae or their feces were not attractive to the parasitoid. Through chemical analysis, the herbivore-induced plant volatiles were identified in the headspace of infested corn leaves. The herbivore-induced volatiles (HIVs) constituted a larger proportion of the headspace of corn leaves infested by early instar armyworms than of corn leaves infested by late instar armyworms. Application of third-instar larval regurgitant onto artificially damaged sites of leaves resulted in emission of parasitoid attractants from the leaf, whereas leaves treated with sixth-instar regurgitant did not. The function of this herbivore-stage related specificity of herbivore-induced synomones is discussed in a tritrophic context.
The effect of experience on the responsiveness of the braconid parasitoidAphidius ervi to host(Acyrthosiphon pisum)-associated cues was investigated on bean plants(Vicia faba) using a wind tunnel bioassay. Oviposition experience on the plant-host complex significantly increased the oriented flight
and landing responses ofA. ervi females to an undamaged plant and to a plant-host(A. pisum) complex. However, oviposition experience onA. pisum aphids when isolated from the plant did not change their responses to the intact plant and the complex. Searching on an unwashed
plant which had been previously damaged byA. pisum also increased their response to an undamaged plant and a host-damaged plant, whereas the experience of searching on an undamaged
plant did not significantly change their responses to undamaged plants. However, when parasitoids were allowed to search on
an undamaged plant which had been sprayed withA. pisum honeydew, this significantly increased their response to an undamaged plant. Oviposition experience on the plant-host complex
and foraging experience on a host-damaged plant or an undamaged plant sprayed with honeydew also significantly reduced the
mean time taken by the parasitoids to respond in the wind tunnel. The behavioral changes associated with such experience were
acquired within 30 min and persisted for at least 3 days. The results demonstrate the capacity ofA. ervi to learn associatively olfactory cues from plants, host-damaged plants, and plant-host complexes and confirm the role of
aphid honeydew as a host recognition kairomone forA. ervi.
cis-Jasmone, or (Z)-jasmone, is well known as a component of plant volatiles, and its release can be induced by damage, for example during insect
herbivory. Using the olfactory system of the lettuce aphid to investigate volatiles from plants avoided by this insect, (Z)-jasmone was found to be electrophysiologically active and also to be repellent in laboratory choice tests. In field studies,
repellency from traps was demonstrated for the damson-hop aphid, and with cereal aphids numbers were reduced in plots of winter
wheat treated with (Z)-jasmone. In contrast, attractant activity was found in laboratory and wind tunnel tests for insects acting antagonistically
to aphids, namely the seven-spot ladybird and an aphid parasitoid. When applied in the vapor phase to intact bean plants,
(Z)-jasmone induced the production of volatile compounds, including the monoterpene (E)-β-ocimene, which affect plant defense, for example by stimulating the activity of parasitic insects. These plants were more
attractive to the aphid parasitoid in the wind tunnel when tested 48 h after exposure to (Z)-jasmone had ceased. This possible signaling role of (Z)-jasmone is qualitatively different from that of the biosynthetically related methyl jasmonate and gives a long-lasting effect
after removal of the stimulus. Differential display was used to compare mRNA populations in bean leaves exposed to the vapor
of (Z)-jasmone and methyl jasmonate. One differentially displayed fragment was cloned and shown by Northern blotting to be up-regulated
in leaf tissue by (Z)-jasmone. This sequence was identified by homology as being derived from a gene encoding an α-tubulin isoform.
The compoundN-(17-hydroxylinolenoyl)-l-glutamine (named here volicitin) was isolated from oral secretions of beet armyworm caterpillars. When applied to damaged
leaves of corn seedlings, volicitin induces the seedlings to emit volatile compounds that attract parasitic wasps, natural
enemies of the caterpillars. Mechanical damage of the leaves, without application of this compound, did not trigger release
of the same blend of volatiles. Volicitin is a key component in a chain of chemical signals and biochemical processes that
regulate tritrophic interactions among plants, insect herbivores, and natural enemies of the herbivores.
This study addressed whether prior damage to black mustard, Brassica nigra (L.) Koch, would reduce growth, herbivory, or mortality of two specialist herbivores on new leaves. Plants received either no initial damage or 12 h of feeding by two third instars of Pieris rapae (L.) or 50 adult Phyllotreta cruciferae (Goeze) when plants had four leaves. Later, the seventh leaf of plants was either harvested for measurement of trichome density and glucosinolate concentration or enclosed in a mesh cage containing two neonate P. rapae or 10 adult P. cruciferae. Caged herbivores were measured for mass gain, leaf consumption, and mortality after 1 wk. Damage by P. rapae caused substantial increases in trichome density and sinigrin concentration, whereas damage by P. cruciferae had no effect. Larvae of P. rapae grew 30% more slowly on plants initially damaged by conspecifics than on control plants. Percent herbivory by P. rapae was 33% lower on plants initially damaged by either P. rapae or P. cruciferae than on control plants. Growth rate and percent herbivory by P. cruciferae were not generally affected by prior plant damage. However, mortality of P. cruciferae was 84% higher on plants previously damaged by conspecifics than on control plants. Together, the data demonstrate that induction responses negatively affected both Pieris and Phyllotreta and suggest that trichomes may be relatively important in the increased resistance. Test herbivores generally performed similarly on plants damaged by either herbivore, suggesting a low specificity of effect for the induction response.
The effect of barometric pressure changes on flight initiation of female Trichogramma evanescens Riley and Trichogramma pretiosum Westwood (Hymenoptera: Trichogrammatidae) was studied in a flight chamber over a 6-h period. Six different barometric pressure
regimens were tested for their impact on flight initiation of both species: constant 975 and 1,025 mbar (stable regimens),
and 50 mbar increase or decrease in pressure between 975 and 1,025 mbar either within 1 (rapid increase or decrease) or 6
h (slow increase or decrease). Both Trichogramma species did not respond to stable or slow changes in barometric pressure, but rapid barometric changes significantly reduced
the flight initiation of females. The effect of rapid barometric pressure changes was almost immediate and independent of
its direction, whether it was increasing or decreasing. The reduction in flight initiation was lifted rapidly when the barometric
pressure became stable again, immediately for T. pretiosum and 1 h later for T. evanescens. These results are in agreement with the hypothesis of an adaptive response to rapidly changing climatic conditions and associated
higher risks of mortality during flight for small insects like Trichogramma. The higher flight initiation observed for T. evanescens females also agrees with field results where they displayed higher dispersal capacity than T. pretiosum females under adverse climatic conditions. The effects of barometric pressure on parasitoid foraging behavior deserve more
attention because this climatic variable may play a significant role in their population dynamics.
An in vitro study was conducted of the nematocidal effect of some glucosinolates and the products of their myrosinase-mediated enzymatic hydrolysis on second-stage juveniles of the sugar beet cyst nematode Heterodera schachtii. The glucosinolates tested were purified from the seeds or from plant organs of some crucifers cited as hosts of the nematode: Brassica napus, Lepidium sativum, Brassica rapa, Brassica carinata, Raphanus sativus, and Sinapis alba. The glucosinolates were dissolved in various concentrations in both the absence and presence of the enzyme myrosinase, and their nematocidal effect in time on second-stage juveniles was then determined. All of the glucosinolates tested as such (i.e., in their native form) showed no nematocidal effect, whereas the products of the enzymatic hydrolysis at pH 7.0 (essentially isothiocyanate) of some of them (sinigrin, gluconapin, glucotropeolin, glucodehydroerucin, and the whole glucosinolate group extracted from rapeseed) demonstrated a mortality rate that varied as a function of both the concentration of product and the exposure time. The reaction products of glucoraphenin and sinalbin did not show any nematocidal activity at the concentrations tested.
p>Plant defence highlights the need to approach plant-insect interactions from a tritrophic perspective. Glucosinolates are a group of defensive chemicals produced extensively by members of the Brassicaceous family. The constitutive production of glucosinolates directly deters generalist herbivores. However, special herbivores are attracted to these chemicals and utilise them to locate suitable host plants. Upon herbivore attach, glucosinolates are hydrolysed by the enzyme myrosianase, which causes the release of a specific blend of volatile products. These herbivore-induced volatiles are thought to form an indirect method of plant defence via the action of natural enemies.
A wild Brassica system was identified to investigate the differences in glucosinolate genotype on a specialist herbivore and its parasitoid. HPLC analysis revealed that Brassica oleracea subsp. Oleracea (wild cabbage) at Kimmeridge and Durdle Door produce quantitative and qualitative differences in aliphatic glucosinolates. Air entertainment experiments revealed that 3-butenylisothiocyanate was released. Laboratory and field investigations concurred to reveal that Brevicoryne brassicae and Diaeretiella rapae representing the second and third trophic level respectively were unable to differentiate between the glucosinolate differences produced by Brassica oleracea subsp. oleracea from Kimmeridge and Durdle Door. However, "host-line" variation occurred in Brevicoryne brassicae, whilst behavioural differences between laboratory-reared and field-collected Diaeretiella rapae were observed. The results of all of the investigations are discussed in relation to the evolution of plant chemical defence and factors responsible for the glucosinolate polymorphism observed in these wild Brassica populations.</p
Designing PCR and sequencing primers are essential activities for molecular biologists around the world. This chapter assumes acquaintance with the principles and practice of PCR, as outlined in, for example, refs. 1, 2, 3, 4.
Plants that are damaged by herbivorous arthropods provide carnivorous enemies of the herbivores with important information. They emit an induced volatile blend that is highly detectable to the carnivores from a distance. Such detectable signals that indicate herbivore presence are important for the carnivores because herbivores themselves are under strong selection not to expose themselves. In addition, carnivores would benefit from a specificity of the induced plant volatiles. Whether herbivore-induced plant volatiles are reliable indicators of herbivore identity, however, has not been resolved unambiguously. Some studies support the reliability of herbivore-induced plant volatiles, while others do not. Different approaches have been used such as chemical analysis, behavioural analysis or a combination of the two. Based on the total of chemical studies one might conclude that in most cases herbivore-induced plant volatiles are not very specific for the herbivore that damages the plant. However, arthropod chemosensors are much more sensitive than the detectors of analytical instruments. Therefore, chemical analyses are not suitable to demonstrate whether or not herbivore-induced plant volatiles are reliable indicators of herbivore identity to carnivores. Behavioural studies should provide this information. In analysing carnivore behaviour it should be realised, however, that arthropod behaviour can be highly variable. Arthropod foraging decisions are affected by external and internal factors such as (a) abiotic environmental factors, (b) presence of competitors or enemies, (c) deprivation of food or oviposition sites, (d) specific deprivation of certain nutrients or (e) learning. In this paper their effect on discrimination of carnivores between volatile blends emitted by plants infested by different herbivores is reviewed. This provides testable hypotheses of why discrimination was not found in some studies. The ability of carnivores to discriminate is likely to be more common than is clear to date, which should invoke functional studies of the conditions that influence the occurrence of this discrimination.
A new method called the neighbor-joining method is proposed for reconstructing phylogenetic trees from evolutionary distance data. The principle of this method is to find pairs of operational taxonomic units (OTUs [= neighbors]) that minimize the total branch length at each stage of clustering of OTUs starting with a starlike tree. The branch lengths as well as the topology of a parsimonious tree can quickly be obtained by using this method. Using computer simulation, we studied the efficiency of this method in obtaining the correct unrooted tree in comparison with that of five other tree-making methods: the unweighted pair group method of analysis, Farris's method, Sattath and Tversky's method, Li's method, and Tateno et al.'s modified Farris method. The new, neighbor-joining method and Sattath and Tversky's method are shown to be generally better than the other methods.
Inheritance of three major genes involved in synthesis of aliphatic glucosinolates (GSL) was followed in segregating populations of Brassica oleracea L. generated from three crosses: broccoli x cauliflower, collard x broccoli, and collard x cauliflower. Two of these genes, GSL-PRO and GSL-ELONG, regulate sidechain length. The action of the former results in three-carbon GSL, whereas action of the latter produces four-carbon GSL. We determined that these two genes act and segregate independently from each other in B. oleracea. The double recessive genotype produces only trace amounts of aliphatic GSL. The third gene, GSL-ALK controls sidechain desaturation and, as it has been observed in Arabidopsis thaliana (L.) Heynh., we found that this gene cosegregates with a fourth gene, GSL-OH, that is responsible for sidechain hydroxylation. Elucidation of the inheritance of major genes controlling biosynthesis of GSL will allow for manipulation of these genes and facilitate development of lines with specific GSL profiles. This capability will be important for improvement of Brassica breeding lines with high content of desirable GSL, like glucoraphanin, a demonstrated precursor of anticarcinogenic compounds. Additionally, this work is the first step towards cloning the major genes of the aliphatic GSL pathway, and to use these clones in transformation strategies for further crop enhancement.
Examined the interaction between the aphid Aphis varians (Homoptera: Aphididae) and its predator Hippodamia convergens (Coleoptera: Coccinellidae). Aphids tended to stay within clumps of conspecifics. Regarding the per capita population growth rates for aphid colonies of differing sizes, individual aphids benefited by being clumped, but a predator-exclusion experiment indicated that clumping was beneficial to aphids only in the presence of predators. Predators exhibited a strong aggregating response: large colonies of aphids were attacked by more predators than were small colonies, and ladybird feeding rate increased with increasing colony size. The total number of prey taken from colonies by predators, however, could not keep pace with the increase in aphid colony size, and thus the risk of predation to individual prey was "diluted' by prey numbers. -from Authors
Two distinctive leaf feeding patterns, termed "edge" and "random", by flea beetles, Phyllotreta cruciferae (Goeze) were identified in 38 Brassicaceae belonging to nine groups of known genomic composition. Edge feeding occurred in the monogenomic diploid Brassica oleracea group (cc genome) and the digenomic amphidiploid B. napus group (aacc genome). All other genomic groups were fed upon randomly. Experimental manipulation of the plant (by leaf edge removal and new edge creation) and the insect (by antennectomy) and paired feeding tests with plants selected for type of feeding pattern revealed some of the salient features of pattern feeding by P. cruciferae. Leaf edge feeding was correlated with a feeding rate one-tenth of that observed for random feeding, suggesting that edge feeding may be determined by physical and/or chemical factors that render the leaf unpalatable or difficult to feed upon or both.Key words: Insecta, Phyllotreta cruciferae, Brassicaceae, feeding pattern, feeding rate
New combinations in Arabidopsis are proposed. Species previously placed in Cardaminopsis are here transferred to Arabidopsis and taxa previously recognized in Arabidopsis, other than A. thaliana and A. suecica, are excluded from the genus. Distributions and a key to the nine species and five subspecies are presented.
Nucleotide sequences for the chloroplast gene rbcL have been obtained for three species of Salvadoraceae and for the unispecific Pentadiplandraceae, thus completing a survey of all known mustard-oil families. Phylogenetic analyses of the sequences based on maximum parsimony reveal two separate lineages of plants that produce mustard oil glucosides. A major mustard-oil clade comprises the core capparalean families of Brassicaceae, Capparaceae, Gyrostemonaceae, Resedaceae, and Tovariaceae and includes Akaniaceae, Bataceae, Bretschneideraceae, Caricaceae, Koeberliniaceae, Limnanthaceae, Moringaceae, Pentadiplandraceae, Salvadoraceae, and Tropaeolaceae. A second unrelated lineage consists of the genus Drypetes, traditionally placed in Euphorbiaceae. The major mustard-oil clade recovered from the rbcL analysis is congruent in many topological aspects with that found in a recent cladistic study of morphological characters, and in turn the molecular and morphological results confirm Dahlgren's radically expanded Capparales. An approach combining molecular and morphological evidence yields robust support for several lineages within this major mustard-oil clade.
The history of wild and cultivated cabbages in the British Isles is reviewed. There have been, considerable fluctuations in the number and sizes of wild populations, and the species seems to be declining. Local Floras show that 'wild' sites are usually associated with the activities of man. The possible role of sea-birds in the distribution of wild cabbages is discussed. It is concluded that Br(lSsic;a oleracea, even when growing wild on the coasts of the British Isles, should probably be considered as an introduction ..
Mixtures of polychlorinated biphenyls, Aroclor 1242 and 1254, had different effects on larval survival of strains of the housefly Musca domestica recently derived from nature, a long-established laboratory strain, and a strain selected for insecticide resistance. All strains tolerated high levels of PCBs; however, both wild-derived strains exhibited unexpected increased survival upon exposure to moderate levels of PCBs alone, whereas the long-established laboratory strain exhibited lowered larval survival in the presence of PCBs. This was not related to a lack of uptake of PCBs by the wild-derived flies, since uptake was high and all major Aroclor GC fractions were represented in the emerging flies. There was increased effectiveness of insecticides in the presence of PCBs on the long-established laboratory strain, but there was no such synergism with insecticides for the wild-derived flies. Recent evolutionary modification of the effects of PCBs alone or in conjunction with insecticides has apparently occurred in this species.
Alate and apterous virginoparae ofAphis fabae Scop, and alate virginoparae ofBrevicoryne brassicae (L.), walking in a linear track olfactometer, were attracted by odor from leaves of their host plants.A. fabae responded to odor from undamaged but not damaged bean leaves. Gynoparae (autumn migrants) ofA. fabae, however, did not respond to their host plant (spindle,Euonymus europaeus) odor. Odors of certain nonhost plants masked the attractiveness of the host plant leaves, but tansy (Tanacetum vulgare) and summer savory (Satureja hortensis) volatiles repelledB. brassicae andA. fabae, respectively. 3-Butenyl isothiocyanate attractedB. brassicae andLipaphis erysimi (Kalt.), the latter species being more sensitive in both behavioral and electrophysiological studies. Isothiocyanate receptors were found on the antennae ofA. fabae, which was repelled by these compounds, 4-pentenyl isothiocyanate being the most active.
Induction of plant defense in response to herbivory includes the emission of synomones that attract the natural enemies of herbivores. We investigated whether mechanical damage to Brussels sprouts leaves (Brassica oleracea var.gemmifera) is sufficient to obtain attraction of the parasitoidCotesia glomerata or whether feeding byPieris brassicae caterpillars elicits the release of synomones not produced by mechanically damaged leaves. The response of the parasitoidCotesia glomerata to different types of simulated herbivory was observed. Flight-chamber dual-choice tests showed that mechanically damaged cabbage leaves were less attractive than herbivore-damaged leaves and mechanically damaged leaves treated with larval regurgitant. Chemical analysis of the headspace of undamaged, artificially damaged, caterpillar-infested, and caterpillar regurgitant-treated leaves showed that the plant responds to damage with an increased release of volatiles. Greenleaf volatiles and several terpenoids are the major components of cabbage leaf headspace. Terpenoids are emitted in analogous amounts in all treatments, including undamaged leaves. On the other hand, if the plant is infested by caterpillars or if caterpillar regurgitant is applied to damaged leaves, the emission of green-leaf volatiles is highly enhanced. Our data are in contrast with the induction of more specific synomones in other plant species, such as Lima bean and corn.
The irregular C11 homoterpene 4,8-dimethylnona-1,3,7-triene 1 originates in higher plants (Angiospermae) from nerolidol 3 by sequential oxidative degradation. The degradative pathway is established by feeding of deuterium labelled precursors to plantlets of the lima bean Phaseolus lunatus followed by mass spectrometric analysis of the collected volatile metabolites. The successful conversion of nerolidol 3, geranylacetone 4, 2,3-dihydroxy-3,7,11-trimethyldodeca-6, 10-diene 6, 1,2-epoxinerolidol 7 and 1,2,3-trihydroxy- 3,7,11-trimethyldodeca-6,10-diene 8 into 1 may be indicative for a sequential degradation in the order 3 → 7 → 8 → 4 → 1 or 3 → 6 → 4 → 1. The chiral centres of the putative intermediates have been characterized and a high degree of enantioselectivity of the involved enzymes has been found. The sequence exhibits a formal analogy to cytochrome P-450 linked dealkylation reactions in the field of steroids.
Plants can be treated with natural elicitors to induce resistance to herbivores. The effects of induced resistance to cotton aphids Aphis gossypii Glover (Homoptera: Aphididae), two-spotted spider mites Tetranychus urticae Koch (Acari: Tetranychidae), and western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) in cotton plants were investigated using applications of the natural plant inducer, jasmonic acid (JA). Preference was reduced by more than 60% for aphids and spider mites, and more than 90% for thrips on JA-induced leaves compared with control leaves. Survival of aphid nymphs was reduced by two-fold and percentage of nymphs attaining the adult stage was reduced by about eight-fold on induced leaves compared with controls. Induction reduced survival and reproduction of adult aphids by about 40% and 75%, respectively. Induction did not affect survival in spider mites or thrips. However, induction reduced egg production in spider mites by more than 75% and feeding in thrips by up to 80% compared with controls. Induction reduced leaf area in cotton seedlings by about 28%. The implications of our results for cotton pest management are discussed.
AFLPs (amplified fragment length polymorphisms) were used to estimate the genetic diversity of seven populations of Brassica campestis L. ssp. napus var. nippo‐oleifera Makino between naturalized and cultivated populations. The seven Korean populations maintained a high level of genetic diversity. For example, all eight primers were high polymorphic, with an average of 3.2 effective alleles per primer set, and the expected heterozygosity was also high. The majority of genetic variance resided within populations. The combinations of an insect‐pollinated, outcrossing breeding system, large population sizes, a high degree of gene flow and a propensity for high fecundity may explain the high level of genetic diversity within cultivated populations. Estimates of genetic similarity on the proportion of shared fragments ranged from 0.952 to 0.999. The high level of gene flow in Korean naturalized populations is mainly caused by seed dispersal via sea tide and the gene flow of cultivated populations may be enhanced in part by artificial pollen dispersal.
Species in the family Aphidiidae (Hymenoptera) parasitize exclusively ovoviviparous aphids. Females use a variety of information to detect and evaluate suitable hosts. Olfactory cues associated with aphids, or the aphids’ host plant, are important for host location. Visual cues including aphid colour, shape, and movement can be evaluated from a distance without physical contact; aphid movement may act as a releasing stimulus for attack. Contact chemosensory cues (gustatory cues) are evaluated by antennation of the host cuticle and during ovipositor probing. A potential host must conform to the wasp’s response profile and satisfy minimum physiological and dietary requirements for immature development and growth. Host quality is determined in part by attributes specific to each aphid species and in part by each aphid’s individual-specific growth potential. Host quality for male and female progeny may vary as a result of different patterns of resource allocation and sexual size dimorphism. For an encountered aphid to be accepted as a host, its perceived value must exceed the wasp’s response threshold for oviposition. Host value, as opposed to host quality, varies dynamically with parasitoid state variables such as age, egg load, and prior experience. A conceptual model of host choice by aphidiid wasps is presented.
ABSTRACT Resistance to herbivore attack is elicited by damage- and herbivore-specific signals. To understand the role of salicylic acid (SA) and jasmonic acid (JA) signalling in the herbivore-induced transcriptional responses of the native tobacco Nicotiana attenuata, a comparison was made of the responses in undamaged systemic leaves after attack by five herbivore species (Manduca sexta, Spodoptera littoralis, Trichoplusia ni, Tupiocoris notatus and Myzus nicotianae) from three feeding guilds and to elicitation by SA, BTH and methyl-JA with a 789-gene oligo-microarray and measured endogenous SA and JA levels. Systemic responses to M. sexta attack were similar to the responses in locally attacked tissues; however, the up-regulation of genes (such as proteinase inhibitor, PI) was more attenuated and resembled the responses to attack from the other Lepidopteran, S. littoralis, and to methyl-JA elicitation. Responses to attack from the puncture-feeding T. notatus repressed some JA-elicited genes (PI) elicited by the leaf-chewing Lepidopterans and most closely resembled SA elicitation. Attack from the phloem-feeding M. nicotianae aphids elicited few responses. Treatment with the SA-mimic BTH induced both SA- and JA-elicited genes and most closely resembled the aphid response. SA concentrations were uniformly increased by herbivore attack and correlated with the down-regulation of photosynthetic genes in all treatments.
A phenolase that will mediate the oxidation of DOPA and catechol occurs in the cells of the principal salivary duct and the accessory apparatus of the salivary glands of various Heteroptera: Pentatomorpha, and in the accessory apparatus only of a mirid, a ploiariid, and a nabid of the Cimicomorpha. Similar enzyme activity occurs in various parts of the salivary apparatus of the Heteroptera-Hydrocorisae and Homoptera. Whenever tested, the activity was found to be inhibited by cyanide and phenylthiourea, and to a lesser extent by azide. Ejected saliva from Pentatomorpha reacts variably with a sensitive amine reagent for oxidase and with DOPA. In Elasmolomus (Aphanus) sordidus (F.), this variability is at least partly dependent on diet.
Motoo Kimura, as founder of the neutral theory, is uniquely placed to write this book. He first proposed the theory in 1968 to explain the unexpectedly high rate of evolutionary change and very large amount of intraspecific variability at the molecular level that had been uncovered by new techniques in molecular biology. The theory - which asserts that the great majority of evolutionary changes at the molecular level are caused not by Darwinian selection but by random drift of selectively neutral mutants - has caused controversy ever since. This book is the first comprehensive treatment of this subject and the author synthesises a wealth of material - ranging from a historical perspective, through recent molecular discoveries, to sophisticated mathematical arguments - all presented in a most lucid manner.
The glucosinolate, sinigrin (allyl- or 2-propenyl glucosinolate),
present in several Cruciferae, was incorporated in varying
concentrations into four different diet background mixtures to study the
behavioural reactions of Mamestra configurata and Trichoplusia ni.
Sinigrin concentrations were chosen to mimic normal levels in naturally
occurring cruciferous plants, or to represent a plant during a
particular stage in its growth cycle. One diet background mixture
contained potassium chloride (KCl) and no stimulatory sugar or sugar
alcohol, two backgrounds contained KCl and a single sugar or sugar
alcohol (sucrose or inositol, respectively), and the fourth background
contained KCl and both sugar and sugar alcohol (sucrose and inositol,
respectively). Sinigrin acted primarily to reduce (deter) feeding in all
backgrounds, although the effect varied with sinigrin concentration,
background and species. When inositol or sucrose was included in the
mixture, the deterrent effect of sinigrin was decreased in both species.
When inositol and sucrose were present, suppression of the deterrent
effect of sinigrin was greatest for M. configurata. The effects of
mixtures were not predictable from a knowledge of the action of
individual components. Differences observed between species may reflect
different sensory capabilities.
The relationship between aggregation behavior, measured by mean crowding on the host plant, and mortality owing to a parasitoid fly is examined in kin groups of the beetle, Leptinotarsa juncta. Considerable group-to-group variation in the propensity to aggregate was found under both laboratory and field conditions. The behavior was shown to be repeatable in the laboratory by measuring the propensity to aggregate twice in succession for each of 15 groups. Partitioning of total variation into within- and among-group components by ANOVA resulted in a repeatability score of 0.62. The relationship between aggregation and mortality owing to the parasitoid was examined in three field studies. In an observational study of naturally occurring groups, a significant positive relationship between group-specific survival and degree of aggregation was shown by a multiple regression analysis that controlled statistically for natural variation in group size. Group size and dispersion pattern were manipulated in two experimental field studies. These studies also showed a significant positive regression of group-specific survival on degree of aggregation. It is argued that this result is an example of intrademic group selection since there is a significant among-group component of total selection as described in population genetic models of the process.
This is an invited expository article for The American Statistician. It reviews the nonparametric estimation of statistical error, mainly the bias and standard error of an estimator, or the error rate of a prediction rule. The presentation is written at a relaxed mathematical level, omitting most proofs, regularity conditions, and technical details.
Insect parasitoids have to disperse to locate host habitat and host, and individuals of small species, such as egg parasitoids, are expected to be very sensitive to climatic conditions during dispersal. The effect of 16 environmental variables on the spatio-temporal pattern of dispersion of Trichogramma evanescens Westwood, originating from Egypt, and Trichogramma pretiosum Riley, originating from Quebec, was quantified. In T. evanescens, the accumulation of solar radiation at temperatures >15°C had a significant effect on its dispersion. When >15,000 kJ/m2 were accumulated, more parasitism was observed in a larger area. When wind blew above 15 km/h for >4 h in a day, dispersion also decreased significantly. In T. pretiosum, accumulated solar radiation had no significant effect, whereas it took 8 h of wind >15 km/h to significantly reduce dispersion. T. evanescens, a warm climate species, thus appears to be more sensitive to temperature. Its relatively higher sensitivity to wind condition may indicate that this species is adapted to aggregated or rare hosts, which require more flight control for location.
SA has been shown to play an important signaling role in the activation of various plant defense responses following pathogen attack. These responses include the induction of local and systemic disease resistance, the potentiation of host cell death, and the containment of pathogen spread. The mechanisms through which SA mediates these effects are varied and can involve alterations in the activity or synthesis of certain enzymes, increased defense gene expression, potentiation of several defense responses, and/or the generation of free radicals. Through the analysis of mutant plants exhibiting aberrant responses to pathogen infection, many genes encoding products involved in the SA-mediated defense pathway(s) have been isolated. In addition, mounting evidence suggests that certain defense responses can be activated via a SA-independent pathway(s). This review focuses primarily on recent discoveries pertaining to the SA signaling pathway(s) leading to disease resistance; however, a very brief discussion of the SA-independent pathway (s) and its ability to cross-talk with the SA pathway is also presented.
The inheritance of aliphatic glucosinolates in Arabidopsis thaliana was studied. Analysis of F3 families from a cross between the ecotype Limburg-5 and the accession H51 (an inbred line derived from the ecotype Stockholm) suggested that alleles at a single locus regulate the conversion of methylsulphinylalkyl glucosinolates into alkenyl (and hydroxyalkenyl) glucosinolates. Likewise, analysis of recombinant inbred lines derived from a cross between the ecotypes Columbia and Landsberg erecta suggested that alleles at a single locus regulate the conversion of methylsulphinylpropyl glucosinolate into hydroxypropyl glucosinolate. Both loci mapped to a similar position on chromosome 4 and it is suggested that these alleles occur at the same locus. A genetic model is proposed in which the aliphatic glucosinolates of A. thaliana are determined by alleles at three loci. The ecological significance of variation in aliphatic glucosinolates is discussed.Keywords: Arabidopsis thaliana, gene mapping, genetic diversity, glucosinolates, pests
Significant differences occur in the levels and types of aliphatic glucosinolates in leaves of plants of four Brassica oleracea populations in Dorset. Plants in grassland at St Aldhelm's Head and Winspit have high levels of 3-butenyl glucosinolate, whereas plants of an adjacent population growing on and along the top of cliffs at Kimmeridge have low levels of 2-hydroxy-3-butenyl, 2-propenyl and methylsulphinylalkyl glucosinolates. Plants growing in a variable habitat at Worbarrow Tout have intermediate levels. The differences in occurrence of individual glucosinolates result from allelic variation at four loci. The level of total aliphatic glucosinolates is under more complex genetic control, but is shown to be highly heritable. Allele frequencies at isozyme loci indicate that genetic variation for glucosinolate production is unlikely to have arisen or to be maintained by founder effects or genetic drift. It is suggested that there is selection for high levels of butenyl glucosinolates at St Aldhelm's Head and Winspit because of grazing by generalist herbivores, whereas there is selection for low levels of 2-hydroxy-3-butenyl and other non-butenyl aliphatic glucosinolates at Kimmeridge because of two factors. First, plants effectively escape from generalist herbivores because of physical aspects of the habitat and association with other plant species which provide physical and chemical defences. Thus there is selection for individuals which do not carry the hypothetical metabolic costs of glucosinolate biosynthesis. Secondly, herbivory by specialist cruciferous insects at Kimmeridge, which is enhanced because of the local abundance of B. nigra, selects for individuals which have low levels of 2-hydroxy-3-butenyl glucosinolates.Keywords: Brassica, coevolution, F-statistics, generalist herbivores, glucosinolates, specialist herbivores