Eddy van der Meijden

Plant defence, an evolutionary dilemma: contrasting effects of (specialist and generalist) herbivores and natural enemies

When estimating the relative importance of herbivores with regard to the amount of plant material they consume, insects are among the most voracious herbivores. A few plant species appear to be relatively free from insect herbivory and some secondary metabolites, such as nicotine and azadirachtin, are extremely poisonous to most insects. Yet it is obvious that not all plants are well defended and that herbivory sometimes leads to considerable damage and fitness loss (see Marquis, 1992, for a review).

Size-specific interaction patterns and size matching in a plant–pollinator interaction web

BACKGROUND AND AIMS Many recent studies show that plant-pollinator interaction webs exhibit consistent structural features such as long-tailed distributions of the degree of generalization, nestedness of interactions and asymmetric interaction dependencies. Recognition of these shared features has led to a variety of mechanistic attempts at explanation. Here it is hypothesized that beside size thresholds and species abundances, the frequency distribution of sizes (nectar depths and proboscis lengths) will play a key role in determining observed interaction patterns. METHODS To test the influence of size distributions, a new network parameter is introduced: the degree of size matching between nectar depth and proboscis length. The observed degree of size matching in a Spanish plant-pollinator web was compared with the expected degree based on joint probability distributions, integrating size thresholds and abundance, and taking the sampling method into account. KEY RESULTS Nectar depths and proboscis lengths both exhibited right-skewed frequency distributions across species and individuals. Species-based size matching was equally close for plants, independent of nectar depth, but differed significantly for pollinators of dissimilar proboscis length. The observed patterns were predicted well by a model considering size distributions across species. Observed size matching was closer when relative abundances of species were included, especially for flowers with openly accessible nectar and pollinators with long proboscises, but was predicted somewhat less successfully by the model that included abundances. CONCLUSIONS The results suggest that in addition to size thresholds and species abundances, size distributions are important for understanding interaction patterns in plant-pollinator webs. It is likely that the understanding will be improved further by characterizing for entire communities how nectar production of flowers and energetic requirements of pollinators covary with size, and how sampling methods influence the observed interaction patterns.

Asymmetric specialization and extinction risk in plant–flower visitor webs: a matter of morphology or abundance?

A recently discovered feature of plant–flower visitor webs is the asymmetric specialization of the interaction partners: specialized plants interact mainly with generalized flower visitors and specialized flower visitors mainly with generalized plants. Little is known about the factors leading to this asymmetry and their consequences for the extinction risk of species. Previous studies have proposed random interactions proportional to species abundance as an explanation. However, the simulation models used in these studies did not include potential biological constraints. In the present study, we tested the potential role of both morphological constraints and species abundance in promoting asymmetric specialization. We compared actual field data of a Mediterranean plant–flower visitor web with predictions of Monte Carlo simulations including different combinations of the potential factors structuring the web. Our simulations showed that both nectar-holder depth and abundance were able to produce asymmetry; but that the expected degree of asymmetry was stronger if based on both. Both factors can predict the number of interaction partners, but only nectar-holder depth was able to predict the degree of asymmetry of a certain species. What is more, without the size threshold the influence of abundance would disappear over time. Thus, asymmetric specialization seems to be the result of a size threshold and, only among the allowed interactions above this size threshold, a result of random interactions proportional to abundance. The simulations also showed that asymmetric specialization could not be the reason that the extinction risk of specialists and generalists is equalized, as suggested in the literature. In asymmetric webs specialists clearly had higher short-term extinction risks. In fact, primarily generalist visitors seem to profit from asymmetric specialization. In our web, specialists were less abundant than generalists. Therefore, including abundance in the simulation models increased the difference between specialists and generalists even more.

Species by Environment Interactions Affect Pyrrolizidine Alkaloid Expression in Senecio jacobaea, Senecio aquaticus, and Their Hybrids

We examined the effects of water and nutrient availability on the expression of the defense pyrrolizidine alkaloids (PAs) in Senecio jacobaea and S. aquaticus. Senecio jacobaea, and S. aquaticus are adapted to different natural habitats, characterized by differing abiotic conditions and different selection pressures from natural enemies. We tested if PA concentration and diversity are plastic over a range of water and nutrient treatments, and also whether such plasticity is dependent on plant species. We also tested the hypothesis that hybridization may contribute to PA diversity within plants, by comparing PA expression in parental species to that in artificially generated F1 hybrids, and also in later generation natural hybrids between S. jacobaea and S. aquaticus. We showed that total PA concentration in roots and shoots is not dependent on species, but that species determines the pattern of PA diversification. Pyrrolizidine alkaloid diversity and concentration are both dependent on environmental factors. Hybrids produce a putatively novel PA, and this PA is conserved in natural hybrids, that are backcrossed to S. jacobaea. Natural hybrids that are backcrossed several times to S. jacobaea are with regard to PA diversity significantly different from S. jacobaea but not from S. aquaticus, while F1 hybrids are in all cases more similar to S. jacobaea. These results collectively suggest that PA diversity is under the influence of natural selection.

Genetic variation in chrysanthemum for resistance to Frankliniella occidentalis

In a choice‐experiment, 42 chrysanthemum cultivars were screened for resistance to Frankliniella occidentalis (Pergande). Oviposition preference, two types of feeding damage and thrips numbers per flower were recorded as measures of resistance. A large genetic variation in thrips resistance was found among the cultivars screened. The amount of feeding damage was strongly determined by oviposition preference. Besides, a positive correlation was found between the oviposition preference in non‐flowering chrysanthemums (number of eggs) and flowering chrysanthemums (number of thrips per flower). Thrips feeding on young, developing tissues, causes growth damage because affected cells are unable to expand and leaves become distorted. Thrips feeding on older, expanded leaves causes cells to become filled with air, resulting in ‘silver’ damage. The amounts of growth‐ and ‘silver’ damage were negatively correlated suggesting that thrips chose either young or older leaves to feed on. The order of resistance among cultivars did not change during the experiment. In order to get more insight in resistance mechanisms the influence of some plant‐ and flower characters on resistance was examined. The plant characters height, number of leaves, flower production and flower weight were all negatively correlated with resistance. It is suggested that tall chrysanthemum cultivars with many and large flowers may invest less in defence than smaller cultivars, and therefore are more damaged by thrips.

Control and biological implications of alkaloid synthesis inCinchona seedlings

Abstract During germination of seeds of Cinchona ledgeriana , a rapid increase in alkaloid content occurs, which stops ca 4 days after the onset of germination. Cinchona alkaloids are derived from the precursor strictosidine, which itself is synthesized by enzymatic condensation of tryptamine and secologanin. When the increase in alkaloid content of the seedlings stops, both types of substrates and the necessary enzymatic activity for strictosidine synthesis are still present in the seedlings, suggesting that different intracellular compartmentation of these compounds prohibits further alkaloid biosynthesis. Indeed, upon homogenization and extraction in buffer of the seedlings, a pronounced increase in the amount of strictosidine was observed. External application of the alkaloids at concentrations higher than present in the seedlings showed autotoxic effects, which may explain why all substrate is not converted into alkaloid in the seedlings. At the concentration actually present in the seedlings, the alkaloids showed strong deterrence of feeding by slugs, indicating that this concentration is sufficient for ecological interactions. Moreover, the metabolic cost of carbon for alkaloid synthesis was estimated to be low.

Population growth and survival of western flower thrips Frankliniella occidentalis Pergande (Thysanoptera, Thripidae) on different chrysanthemum cultivars: Two methods for measuring resistance

Two methods were used to measure the resistance of 5 chrysanthemum cultivars to thrips Frankliniella occidentalis Pergande (Thysanoptera, Thripidae). As a measure of resistance of a cultivar, growth and survival of the insect population were taken.

Pyrrolizidine alkaloid content in Senecio: ontogeny and developmental constraints.

Summary.The ontogeny of pyrrolizidine alkaloid (PA) synthesis and constraints on defence level during the seedling stage were examined in the annual Senecio vulgaris and the monocarpic perennial Senecio jacobaea. In both species, PAs were actively synthesized from the onset of seedling growth so that juvenile stages did not go through an undefended stage. Roots are known to be the exclusive sites where PAs are produced. Root biomass was the single most important biomass parameter explaining variation in total PAs per seedling. All correlation coefficients between—relative growth rate and PA concentration were negative, but none was significant. However, a significant negative—correlation was found between shoot to root ratio and PA concentration in S. jacobaeaseedlings, suggesting a dilution effect of the PAs. Earlier studies have shown that the shoot to root ratio is positively correlated with relative growth—rate of established S. jacobaea plants. It is therefore suggested that young S. jacobaea plants with a high shoot to root ratio and hence a high growth capacity necessarily have lower PA defence levels than plants with a low shoot to root ratio.

Detrimental effects ofCinchona leaf alkaloids on larvae of the polyphagous insectSpodoptera exigua

YoungCinchona ledgeriana plants contain two types of alkaloid: indole alkaloids in the leaves and quinoline alkaloids in the root. FromCinchona leaves, a crude alkaloid extract was made, containing the cinchophylline type of indole alkaloids and a small amount of 5-methoxytryptamine. The leaf alkaloid extract exerted a strong detrimental effect on the growth of larvae of the polyphagous beet armyworm,Spodoptera exigua (Lepidoptera). Feeding of larvae on an artificial diet containing the leaf alkaloids at the same concentrations as those found in the plant resulted in significant growth reduction, retardation in development, and mortality of the larvae. Cinchophyllines are composed of 5-methoxytryptamine coupled to a corynantheal unit. When incorporated into the artificial diet, 5-methoxytryptamine alone had no effect on the 5.exigua larvae. Corynantheal, however, had a strong detrimental effect on growth of the larvae, its effect being comparable to that of the leaf alkaloid extract. In contrast to the indole-type leaf alkaloids, the quinolinetypeCinchona root alkaloids did not affect growth and development of the larvae. These results suggest that the indole-type alkaloids, which inCinchona plants are present at the highest concentrations in the young, vulnerable leaflets, are involved in the chemical defense of the plant against herbivorous insects.

Allelopathic inhibition of seed germination by Cinchona alkaloids

Abstract The inhibition of seed germination by quinoline alkaloids synthesized by plants of the tropical genus Cinchona was studied. The germination of Ocimum (a tropical herb), of Spermacoce and Catharanthus (two tropical, alkaloid-producing plants), and of Cinchona itself was strongly inhibited by the alkaloids when applied at concentrations higher than about 0.3 mM. To test for the possible allelopathic significance of this finding, the soil in which two-year-old Cinchona plants were grown was examined for its quinoline alkaloid content. Although the roots of the plants contain high concentrations of these alkaloids ( ca 10 mM), in the soil only very low concentrations were found ( ca 0.02 mM). Upon germination of seeds sown close by the plants, no toxic effects were observed. So, although several studies have reported inhibition of seed germination by Cinchona alkaloids under laboratory conditions, our results indicate that this property does not play a role under natural circumstances at realistic concentrations.