J. Daniel Hare

Ecological Role of Volatiles Produced by Plants in Response to Damage by Herbivorous Insects

Plants often release a blend of volatile organic compounds in response to damage by herbivorous insects that may serve as cues to locate those herbivores by natural enemies. The blend of compounds emitted by plants may be more variable than is generally assumed. The quantity and the composition of the blends may vary with the species of the herbivore, the plant species and genotype within species, the environmental conditions under which plants are grown, and the number of herbivore species attacking the plant. Although it is often assumed that induced emission of these compounds is an adaptive tactic on the part of plants, the evidence that such responses minimize fitness losses of plants remains sparse because the necessary data on plant fitness rarely have been collected. The application of techniques of evolutionary quantitative genetics may facilitate the testing of widely held hypotheses about the evolution of induced production of volatile compounds under natural conditions.

Measuring plant protein with the Bradford assay

The suitability of the Bradford protein assay for measuring plant protein was evaluated and a standard method developed. The assay involves extraction of dried, fresh, or frozen plant material in 0.1 NaOH for 30 min. Replicate 100-μl aliquots of centrifuged supernatant are assayed with 5 ml Bio-Rad Bradford dye reagent (Coomassie brilliant blue G-250) diluted 1:4 and containing 3 mg/ml soluble polyvinylpyrollidone. Absorbance at 595 nm is recorded after 15 min against an NaOH blank. Samples are calibrated against a ribulose 1,5-diphosphate carboxylase-oxygenase standard in NaOH. Procedures for plant preparation, extraction stability, the effects of phenol removal and quinone formation, and assay recovery are evaluated. Assay absorbance stability and techniques for increasing absorbance stability are reported. Changes in protein quality are briefly discussed.

GENETIC VARIATION AND HOST PLANT RELATIONS IN A PARTHENOGENETIC MOTH

Ecological genetics is the investigation of the influence of ecological factors on the genetic properties of populations, and the influence of genetic structure on their ecological properties. Ideally, these studies should determine how genetic variation is affected by selection and by the size, dynamics, and degree of subdivision of populations; what ecological factors determine the relative fitness of genotypes; and what effect the genetic composition of a population has on such ecological parameters as its density, stability, and ecological amplitude. Many studies have demonstrated selection in natural populations; indeed, the widespread existence of selection is perhaps the only generalization that can be made about ecological genetics. But most studies are incomplete. A few, such as the analysis of inversion karyotypes in Drosophila pseudoobscura by Dobzhansky

Learned and naïve natural enemy responses and the interpretation of volatile organic compounds as cues or signals

In response to arthropod herbivory, plants release volatile organic compounds (VOCs), which are attractive to natural enemies. Consequently, VOCs have been interpreted as co-evolved plant-natural enemy signals. This review argues that, while these data are necessary, they are not sufficient to demonstrate a VOC plant-natural enemy signaling function. We propose that evidence that (1) plant fitness is increased as a consequence of natural enemy recruitment, and either (2A) natural enemies preferentially learn prey-induced VOCs or (2B) natural enemies respond innately to the VOCs of the prey-host plant complex, is also required. Whereas there are too few studies to rigorously test hypotheses 1 and 2A, numerous studies are available to test hypothesis 2B. Of 293 tests of natural enemy responses to VOCs, we identified only 74 that were unambiguous tests of naïve natural enemies; in the remainder of the tests either natural enemies were experienced with their host in the presence of VOCs, or experience could not be ruled out. Of those 74 tests with naïve natural enemies, attraction was observed in 41 and not in 33. This review demonstrates that empirical support for the hypothesized VOC plant-natural enemy signaling function is not universal and presents alternative hypotheses for VOC production.

Seasonal Variation in Plant-Insect Associations: Utilization of Solanum Dulcamara by Leptinotarsa Decemlineata

In laboratory studies, Solanum dulcamara (nightshade) is one of the better host species for survival and growth of the Colorado potato beetle, Leptinotarsa decemlineata. The dynamics of L. decemlineata on S. dulcamara in the field, however, are far different than would be predicted from laboratory results. Therefore, a number of experiments were performed to determine the suitability of S. dulcamara for L. decemlineata under seasonal, field conditions. Field experiments showed that between 40 and 67% of first—generation adult females entered diapause without reproducing when fed S. dulcumara as both larvae and adults, although no adults entered diapause when fed S. tuberosum in both life—stages. Dispersal by first—generation adults from S. dulcamara was not observed in mark—recapture experiment. Survival of L. decemlineata on S. dulcamara did not differ significantly from survival on S. tuberosum during the part of the season corresponding to the development period of first—generation larvae. Survival on S. dulcamara was two—thirds lower during part of the season corresponding to the development period of the second—generation larvae, however. Emergence mass and larval development time varied less over the season on S. dulcamara than did survival. Analyses of extractable leaf protein, free amino acids, and glycoalkaloids showed significant seasonal variation in the concentrations of these S. dulcamara phytochemicals which are known to affect L. decemlineata feeding growth and survival. Multiple regression analyses between phytochemical concentration and larval survival showed that survival was significantly] positively associated with leaf nutritional quality (extractable protein and one free amino acid) during the first generation, and significantly negatively associated with leaf glycoalkaloid content during the second generation. Thus the seasonal variation in the suitability of S. dulcamara for L. decemlineata survival was most strongly associated with the two seasonally most variable S. dulcamara phytochemical factors. The unique and independent contribution of variation in either one to the variation in L. decemlineata survival, however, could not accurately be predicted without considering the simultaneous, independent variation in the other.

The impact of rapid evolution on population dynamics in the wild: experimental test of eco-evolutionary dynamics

Rapid evolution challenges the assumption that evolution is too slow to impact short-term ecological dynamics. This insight motivates the study of Eco-Evolutionary Dynamics or how evolution and ecological processes reciprocally interact on short time scales. We tested how rapid evolution impacts concurrent population dynamics using an aphid (Myzus persicae) and an undomesticated host (Hirschfeldia incana) in replicated wild populations. We manipulated evolvability by creating non-evolving (single clone) and potentially evolving (two-clone) aphid populations that contained genetic variation in intrinsic growth rate. We observed significant evolution in two-clone populations whether or not they were exposed to predators and competitors. Evolving populations grew up to 42% faster and attained up to 67% higher density, compared with non-evolving control populations but only in treatments exposed to competitors and predators. Increased density also correlates with relative fitness of competing clones suggesting a full eco-evolutionary dynamic cycle defined as reciprocal interactions between evolution and density.

Indirect cost of a defensive trait: variation in trichome type affects the natural enemies of herbivorous insects on Datura wrightii

The costs and benefits of defensive traits in plants can have an ecological component that arises from the effect of defenses on the natural enemies of herbivores. We tested if glandular trichomes in Datura wrightii, a trait that confers resistance to several species of herbivorous insects, impose an ecological cost by decreasing rates of predation by the natural enemies of herbivores. For two common herbivores of D. wrightii, Lema daturaphila and Tupiocoris notatus, several generalized species of natural enemies exhibited lower rates of predation on glandular compared to non-glandular plants. Lower rates of predation were associated with reductions in the residence time and foraging efficiency of natural enemies on plants with glandular trichomes, but not with direct toxic effects of glandular exudate. Our results suggest that the benefit of resistance to herbivores conferred by glandular trichomes might be offset by the detrimental effect of this trait on the natural enemies of herbivores, and that the fitness consequences of this trichome defense might depend on the composition and abundance of the natural-enemy community.

Variation in Herbivore and Methyl Jasmonate-Induced Volatiles Among Genetic Lines of Datura wrightii

Many plant species produce volatile organic compounds after being damaged by herbivores. The production of volatiles also may be induced by exposing plants to the plant hormone, jasmonic acid, or its volatile ester, methyl jasmonate. This study addresses the induction of the production volatile organic compounds among genetic lines of Datura wrightii. Within populations, some plants produce glandular trichomes, whereas others produce nonglandular trichomes, and trichome phenotype is controlled by a single dominant gene. Glandular trichomes not only confer resistance to some herbivorous insects, but they also inhibit many natural enemies of those herbivores. Because of the potential benefit of natural enemies that use volatile cues to find individuals of the non-glandular phenotype, it is reasonable to ask if plants of D. wrightii that differ in trichome morphology might produce different blends of volatile compounds. Volatile compounds were collected from eight genetic lines of plants that had been backcrossed for three generations. Volatiles were collected from pairs of sibling plants before and after insect damage or treatment with methyl jasmonate. Within each pair, one sib expressed glandular trichomes and the other expressed nonglandular trichomes. Overall, plants produced an array of at least 17 compounds, most of which were sesquiterpenes. Total production of volatiles increased from 3.9- to 16.2-fold among genetic lines after insect damage and from 3.6- to 32-fold in plants treated with methyl jasmonate. The most abundant compound was (E)-β-caryophyllene. This single compound comprised from 17 to 59% of the volatiles from insect-damaged plants and from 24 to 88% of the volatiles from plants treated with methyl jasmonate, depending upon genetic line. The production of (E)-β-caryophyllene by the original male parents of the eight genetic lines was significantly related to the mean production of their third-generation backcross progeny indicating that the variation in the production of (E)-β-caryophyllene was inherited. Blends did not differ qualitatively or quantitatively between sibs expressing glandular or nonglandular trichomes.

INDIRECT EFFECTS OF CITRUS CULTIVARS ON LIFE HISTORY PARAMETERS OF A PARASITIC WASP

We determined sex ratio, initial egg complement, and size of adult Aphytis melinus, an ectoparasitoid of California red scale, Aonidiella aurantii. Host scales were reared on leaves of four citrus cultivars previously shown to differ in suitability for scale growth and survival. Wasps from scales reared on leaves of lemon (Citrus limon) produced nearly twice the proportion of female progeny as did wasps from scales reared on leaves of grapefruit (Citrus paradisi), orange (Citrus sinensis), or mandarin (Citrus unshiu). More- over, female wasps from lemon-reared scales had highest initial egg complement, followed by wasps reared from grapefruit, mandarin, and orange. The quality of scales for the production of fecund, female wasp progeny also varied seasonally over all cultivars pooled. Scales reached largest size when reared in the cooler autumn temperatures and were more than twice as suitable for the production of fecund, female A. melinus as scales reared in early summer or midsummer. Results suggest that the mechanisms allowing persistence of the interaction between California red scale and A. melinus may vary among different citrus cultivars.

Constitutive and Jasmonate-Inducible Traits of Datura wrightii

Plants in the family Solanaceae possess numerous traits that are induced from damage from herbivores. Many of these also can be induced by exposing plants to the plant hormone jasmonic acid or its volatile ester methyl jasmonate. Datura wrightii (Solanaceae) is dimorphic for leaf trichome morphology in most southern California populations. Trichome phenotype is governed by a single gene, and the glandular trichome condition is dominant and under developmental control. This study addressed two major objectives. The first was to determine if mature plants with glandular or nonglandular trichomes responded differentially to methyl jasmonate. The second objective was to determine if exposure of seedlings to methyl jasmonate during the period of trichome differentiation altered either the phenotype or the density of trichomes that mature plants expressed. Methyl jasmonate induced from 200 to 800xa0μg/ml of proteinase inhibitor activity and increased the activity of polyphenol oxidase by more than threefold depending on the experiment. These increases did not differ significantly between plants expressing glandular or nonglandular trichomes. Methyl jasmonate exposure did not increase the activity of peroxidase or the concentration of scopolamine or hyoscyamine, the two major alkaloids of Datura. Exposure to methyl jasmonate during trichome differentiation did not affect either the final trichome phenotype or the density of either type of trichome, but did increase the production of acylsugars in glandular trichomes by 44%. Because trichome phenotype was not inducible, and because both trichome phenotypes showed similar increases in proteinase inhibitors and polyphenol oxidase activity, the methyl-jasmonate-inducible responses of D. wrightii are independent of trichome phenotype in D. wrightii.