Insect herbivory and plant adaptation in an early successional community*

Development and reproduction of Scirtothrips dorsalis (Thysanoptera: Thripidae) on six host plant species

Host plants can strongly influence the population performance of insects. Here, we investigated the development, survival, and oviposition of Scirtothrips dorsalis Hood on 6 host plants-Camellia sinensis ( L.) Kuntze (Ericales: Theaceae), Rosa chinensis Jacq. (Rosales: Rosaceae), Capsicum annuum L. (Solanales: Solanaceae), Eustoma grandiflorum (Hook.) G.Don (Gentianales: Gentianaceae), Glycine max (L.) Merr. (Fabales: Fabaceae), and Cucumis sativus L. (Cucurbitales: Cucurbitaceae), and constructed life tables for S. dorsalis on each plant. Significant differences in S. dorsalis development on the host species were observed. The mean developmental period from egg to adult was 11.45 ± 0.12 days, 11.24 ± 0.13 days, 12.08 ± 0.15 days, 12.28 ± 0.12 days, 12.67 ± 0.10 days, and 13.03 ± 0.11 days on C. sinensis, R. chinensis, C. annuum, E. grandiflorum, G. max, and C. sativus, respectively. Significant differences in survival of S. dorsalis were observed, namely, C. sinensis ≈ R. chinensis > E. grandiflorum ≈ C. annuum > G. max > C. sativus. The highest and lowest fecundities of S. dorsalis were recorded on R. chinensis (60.44 ± 1.53) and C. sativus (28.64 ± 1.02), respectively. Both of the net reproductive rate (R0) and intrinsic rate of increase (rm) of S. dorsalis were the highest on R. chinensis, with the values of 27.63 ± 0.58 and 0.142 ± 0.002, respectively; while the lowest on C. sativus, with the values of 8.81 ± 0.12 and 0.092 ± 0.003, respectively. Thus, R. chinensis was found to be the most suitable host, but C. sativus was the least suitable, for population development of S. dorsalis. Our results provide important information for the key control of S. dorsalis among different host plants.

Correlation of microbiomes in "plant-insect-soil" ecosystem

Introduction

Traditional chemical control methods pose a damaging effect on farmland ecology, and their long-term use has led to the development of pest resistance.

Methods

Here, we analyzed the correlations and differences in the microbiome present in the plant and soil of sugarcane cultivars exhibiting different insect resistance to investigate the role played by microbiome in crop insect resistance. We evaluated the microbiome of stems, topsoil, rhizosphere soil, and striped borers obtained from infested stems, as well as soil chemical parameters.

Results and Discussion

Results showed that microbiome diversity was higher in stems of insect-resistant plants, and contrast, lower in the soil of resistant plants, with fungi being more pronounced than bacteria. The microbiome in plant stems was almost entirely derived from the soil. The microbiome of insect-susceptible plants and surrounding soil tended to change towards that of insect-resistant plants after insect damage. Insects' microbiome was mainly derived from plant stems and partly from the soil. Available potassium showed an extremely significant correlation with soil microbiome. This study validated the role played by the microbiome ecology of plant-soil-insect system in insect resistance and provided a pre-theoretical basis for crop resistance control.

Experimental insect suppression causes loss of induced, but not constitutive, resistance in Solanum carolinense

Spatiotemporal variation in herbivory is a major driver of intraspecific variation in plant defense. Comparatively little is known, however, about how changes in herbivory regime affect the balance of constitutive and induced resistance, which are often considered alternative defensive strategies. Here, we investigated how nearly a decade of insect herbivore suppression affected constitutive and induced resistance in horsenettle (Solanum carolinense), a widespread herbaceous perennial. We allowed replicated horsenettle populations to respond to the presence or absence of herbivores by applying insecticide to all plants in half of 16 field plots. Horsenettle density rapidly increased in response to insecticide treatment, and this effect persisted for at least four years after the cessation of herbivore suppression. We subsequently grew half-sibling families from seeds collected during and shortly after insecticide treatment in a common garden and found strong effects of insect suppression on induced resistance. Feeding trials in field mesocosms with false Colorado potato beetles (Leptinotarsa juncta), a common specialist herbivore, revealed that multi-year herbivore suppression drove rapid attenuation of induced resistance: offspring of plants from insect-suppression plots exhibited a near-complete loss of induced resistance to beetles, while those from control plots incurred ~70% less damage after experimental induction. Plants from insect-suppression plots also had ~40% greater constitutive resistance than those from control plots, although this difference was not statistically significant. We nonetheless detected a strong trade-off between constitutive and induced resistance across families. In contrast, the constitutive expression of trypsin inhibitors (TI), an important chemical defense trait in horsenettle, was reduced by 20% in the offspring of plants from insect-suppression plots relative to those from control plots. However, TIs were induced to an equal extent whether or not insect herbivores had been historically suppressed. While several defense and performance traits (prickle density, TI concentration, resistance against false Colorado potato beetles and flea beetles, biomass, and seed mass) varied markedly across families, no traits exhibited significant pairwise correlations. Overall, our results indicate that, while the divergent responses of multiple defense traits to insect suppression led to comparatively small changes in overall constitutive resistance, they significantly reduced induced resistance against false Colorado potato beetle.

Population Performance of Thrips hawaiiensis (Thysanoptera: Thripidae) on Different Vegetable Host Plants

Thrips hawaiiensis (Morgan) is a flower-inhabiting thrips with a wide range of host plants, but little is known regarding its biological and ecological characteristics on vegetable hosts. Here, we evaluated the development, survival, and oviposition of T. hawaiiensis on five vegetable species (Capsicum annuum, Solanum melongena, Cucurbita moschata, Lablab purpureus, and Brassica oleracea), and constructed its life tables on these vegetables. There were significant differences in the development of T. hawaiiensis on the five vegetables, and the developmental times from egg to adult were 12.19 days, 11.59 days, 11.12 days, 10.78 days, and 10.51 days on C. moschata, B. oleracea, L. purpureus, C. annuum, and S. melongena, respectively. There were also significant differences in T. hawaiiensis' survival rate on these plants, with S. melongena (71.00%) > C. annuum (67.33%) > L. purpureus (63.33%) > B. oleracea (57.00%) > C. moschata (49.33%). The greatest and lowest fecundity levels of T. hawaiiensis were found on S. melongena (44.28) and C. moschata (30.16), respectively. T. hawaiiensis had the greatest net reproductive rate on S. melongena (19.22), followed by C. annuum (16.11), L. purpureus (15.17), B. oleracea (11.10), and C. moschata (8.47), and the intrinsic rate of increase showed a similar trend, with values of 0.140, 0.125, 0.121, 0.112, and 0.093, respectively. Thus, S. melongena and C. moschata were the most and least suitable hosts for the population development of T. hawaiiensis among the five tested vegetable hosts. This study could provide important information for the key control of T. hawaiiensis on different crops.

Herbivore-Mediated Selection on Floral Display Covaries Nonlinearly With Plant-Antagonistic Interaction Intensity Among Primrose Populations

Quantifying the relations between plant-antagonistic interactions and natural selection among populations is important for predicting how spatial variation in ecological interactions drive adaptive differentiation. Here, we investigate the relations between the opportunity for selection, herbivore-mediated selection, and the intensity of plant-herbivore interaction among 11 populations of the insect-pollinated plant Primula florindae over 2 years. We experimentally quantified herbivore-mediated directional selection on three floral traits (two display and one phenological) within populations and found evidence for herbivore-mediated selection for a later flowering start date and a greater number of flowers per plant. The opportunity for selection and strength of herbivore-mediated selection on number of flowers varied nonlinearly with the intensity of herbivory among populations. These parameters increased and then decreased with increasing intensity of plant-herbivore interactions, defined as an increase in the ratio of herbivore-damaged flowers per individual. Our results provide novel insights into how plant-antagonistic interactions can shape spatial variation in selection on floral traits and contribute toward understanding the mechanistic basis of geographic variation in angiosperm flowers.

Evolution and seed dormancy shape plant genotypic structure through a successional cycle

Dormancy has repeatedly evolved in plants, animals, and microbes and is hypothesized to facilitate persistence in the face of environmental change. Yet previous experiments have not tracked demography and trait evolution spanning a full successional cycle to ask whether early bouts of natural selection are later reinforced or erased during periods of population dormancy. In addition, it is unclear how well short-term measures of fitness predict long-term genotypic success for species with dormancy. Here, we address these issues using experimental field populations of the plant Oenothera biennis, which evolved over five generations in plots exposed to or protected from insect herbivory. While populations existed above ground, there was rapid evolution of defensive and life-history traits, but populations lost genetic diversity and crashed as succession proceeded. After >5 y of seed dormancy, we triggered germination from the seedbank and genotyped >3,000 colonizers. Resurrected populations showed restored genetic diversity that reduced earlier responses to selection and pushed population phenotypes toward the starting conditions of a decade earlier. Nonetheless, four defense and life-history traits remained differentiated in populations with insect suppression compared with controls. These findings capture key missing elements of evolution during ecological cycles and demonstrate the impact of dormancy on future evolutionary responses to environmental change.

Combining biotechnology and evolution for understanding the mechanisms of pollinator attraction

Many flowering plants rely on pollinators for their reproductive success. Plant-pollinator interactions usually depend on a complex combination of traits based on a fine-tuned biosynthetic machinery, with many structural and regulatory genes involved. Yet, the physiological mechanisms in plants are the product of evolutionary processes. While evolution has been modifying flowers through millions of years, it is also a rapid process that can change plant traits within few generations. Here we discuss both mechanistic and evolutionary aspects of pollinator attraction. We also propose how latest advances in biotechnology and evolutionary studies, and their combination, will improve the elucidation of molecular mechanisms and evolutionary dynamics of pollinator attraction in changing environments.

Assessing the genetic and chemical diversity of Taraxacum species in the Korean Peninsula

The genetic relationship between Taraxacum species, also known as the dandelion, is complicated because of asexual and mixed sexual apomictic reproduction. The usage of Taraxacum species in traditional medicines make their specialized metabolism important, but interspecific chemical difference has rarely been reported for the genus. In this study, we assembled the chloroplast genome and 45S rDNA of six Taraxacum species that occur in Korea (T. campylodes, T. coreanum, T. erythrospermum, T. mongolicum, T. platycarpum, and T. ussuriense), and performed a comparative analysis, which revealed their phylogenetic relationships and possible natural hybridity. We also performed a liquid chromatography-mass spectrometry-based phytochemical analysis to reveal interspecific chemical diversity. The comparative metabolomics analysis revealed that Taraxacum species could be separated into three chemotypes according to their major defensive specialized metabolites, which were the sesquiterpene lactones, the phenolic inositols, and chlorogenic acid derivatives. The CP DNA- and 45S rDNA-based phylogenetic trees showed a tangled relationship, which supports the notion of ongoing hybridization of wild Taraxacum species. The untargeted LC-MS analysis revealed that each Taraxacum plant exhibits species-specific defensive specialized metabolism. Moreover, 45S rDNA-based phylogenetic tree correlated with the hierarchical cluster relied on metabolite compositions. Given the coincidence between these analyses, we represented that 45S rDNA could well reflect overall nuclear genome variation in Taraxacum species.

Population-wide shifts in herbivore resistance strategies over succession

As a strategic cost-saving alternative to constitutive resistance, induction of resistance against herbivores in plants can be especially beneficial when enemies are scarce or variable in abundance. Although probably describing the two ends of a continuum, constitutive and induced resistance strategies have long been observed to trade off within species. Examining these traits among populations along a successional gradient can help explain how temporally variable environments can maintain genetic variation and how ecosystem processes are affected by shifting plant resistance trait expression over time. Here we leverage large experimental plots that represent a chronosequence of succession up to 15 yr in combination with common garden experiments to examine changes in the selective environment and genetic differences in tall goldenrod's (Solidago altissima) constitutive and induced resistance. We show that resistance against a specialist herbivore Trirhabda virgata was inducible in the plants originating from midsuccession, which coincides with the largest loads of herbivores. The flavonoid compound content of the leaves varied with successional stage of the population of origin, which is indicative of constitutive differences in secondary metabolite production. Finally, there was a clear trade-off between constitutive and induced resistance. Our study indicates that selection for resistance traits within a population can be highly variable over time and likely result in genetically determined shifts of resistance strategies over relatively short time periods via genotype sorting.

Impact of Seasonal and Temperature-Dependent Variation in Root Defense Metabolites on Herbivore Preference in Taraxacum officinale

Plants experience seasonal fluctuations in abiotic and biotic factors such as herbivore attack rates. If and how root defense expression co-varies with seasonal fluctuations in abiotic factors and root herbivore attack rates is not well understood. Here, we evaluated seasonal changes in defensive root latex chemistry of Taraxacum officinale plants in the field and correlated the changes with seasonal fluctuations in abiotic factors and damage potential by Melolontha melolontha, a major natural enemy of T. officinale. We then explored the causality and consequences of these relationships under controlled conditions. The concentration of the defensive sesquiterpene lactone taraxinic acid β-D glucopyranosyl ester (TA-G) varied substantially over the year and was most strongly correlated to mean monthly temperature. Both temperature and TA-G levels were correlated with annual fluctuations in potential M. melolontha damage. Under controlled conditions, plants grown under high temperature produced more TA-G and were less attractive for M. melolontha. However, temperature-dependent M. melolontha feeding preferences were not significantly altered in TA-G deficient transgenic lines. Our results suggest that fluctuations in temperature leads to variation in the production of a root defensive metabolites that co-varies with expected attack of a major root herbivore. Temperature-dependent herbivore preference, however, is likely to be modulated by other phenotypic alterations.

Foliar-feeding insects acquire microbiomes from the soil rather than the host plant

Microbiomes of soils and plants are linked, but how this affects microbiomes of aboveground herbivorous insects is unknown. We first generated plant-conditioned soils in field plots, then reared leaf-feeding caterpillars on dandelion grown in these soils, and then assessed whether the microbiomes of the caterpillars were attributed to the conditioned soil microbiomes or the dandelion microbiome. Microbiomes of caterpillars kept on intact plants differed from those of caterpillars fed detached leaves collected from plants growing in the same soil. Microbiomes of caterpillars reared on detached leaves were relatively simple and resembled leaf microbiomes, while those of caterpillars from intact plants were more diverse and resembled soil microbiomes. Plant-mediated changes in soil microbiomes were not reflected in the phytobiome but were detected in caterpillar microbiomes, however, only when kept on intact plants. Our results imply that insect microbiomes depend on soil microbiomes, and that effects of plants on soil microbiomes can be transmitted to aboveground insects feeding later on other plants.

Leaf-feeding insect microbiomes could be influenced by the soil, the plant, or a product of the two. Here, the authors conduct a series of experiments to show that an herbivorous insect predominantly acquires its microbiome from the soil rather than the plant, and that these insect microbiomes reflect soil legacies of earlier growing plants.

Digest: Plants adapt under attack: genotypic selection and phenotypic plasticity under herbivore pressure

Plant species adapt to changing environmental conditions through phenotypic plasticity and natural selection. Agrawal et al. (2018) found that dandelions responded to the presence of insect pests by producing higher levels of defensive compounds. This defensive response resulted both from phenotypic plasticity, with individual plants' defenses triggered by insect attack, and from evolution by natural selection acting on genetic variation in the plant population.