Jetske G. de Boer

Inducible indirect defence of plants: from mechanisms to ecological functions

Abstract Inducible defences allow plants to be phenotypically plastic. Inducible indirect defence of plants by attracting carnivorous enemies of herbivorous arthropods can vary with plant species and genotype, with herbivore species or instar and potentially with other environmental conditions. So far, inducible indirect defence has mostly been studied for simple linear food chains. However, ultimately, ecologists should address inducible indirect defence in a food web context, where more than one organism (different herbivores and pathogens) may attack a plant and where a plant that emits herbivore-induced volatiles is surrounded by other plants that emit odours that can mix with the herbivore-induced volatiles from the attacked plant. Evolutionary ecologists are interested in the costs and benefits of interactions between plants and their attackers. These may be investigated by comparing different plant genotypes. The best comparison is between plant individuals that differ in only a single or restricted number of known traits. Such genotypes are difficult to obtain by conventional methods. However, rapid progress in the study of mechanisms of plant-attacker interactions and in the field of molecular genetics and genomics provides new tools that can be exploited by ecologists. For instance, genomic knowledge on Arabidopsis thaliana and the availability of characterized mutants and transgenes that are altered in one or a restricted number of genes can be exploited to address functional aspects of inducible indirect defence. In this paper we review progress in the knowledge of mechanisms of inducible indirect defence of plants and its importance for investigating the functional aspects of plant responses to herbivorous arthropods. Finally we identify some of the ecological questions that can be addressed by exploiting mechanistic aspects of inducible indirect defence. Induzierbare Abwehr ermoglicht Pflanzen phanotypische Plastizitat. Bei der induzierbaren indirekten Abwehr von Pflanzen gegen Herbivore werden carnivore Feinde der herbivoren Arthropoden angelockt. Diese indirekte Abwehrreaktion kann je nach Art und Genotyp der Pflanze variieren, je nach Art und Stadium der Herbivoren und potentiell auch mit anderen Umweltfaktoren. Bisher wurde induzierbare indirekte Abwehr zumeist in einfachen, linearen Nahrungsketten untersucht. Letztlich sollten Okologen induzierbare indirekte Abwehr aber im Kontext von Nahrungsnetzen untersuchen und berucksichtigen, dass mehr als ein Organismus (verschiedene Herbivore und Pathogene) die Pflanze attackieren konnen. Weiterhin ist eine Pflanze, die durch Herbivorenbefall induzierte Dufte freisetzt, auch noch umgeben von anderen Pflanzen, die ebenfalls fluchtige Verbindungen abgeben, die sich mit den Duften der induzierten Pflanze mischen konnen. Evolutionare Okologen sind an den Kosten und Nutzen der Interaktionen zwischen Pflanzen und ihrer Angreifern interessiert. Kosten-Nutzen-Analysen konnen anhand eines Vergleichs verschiedener Genotypen durchgefuhrt werden. Am besten werden Pflanzenindividuen verglichen, die sich nur in einem oder einer sehr begrenzten Zahl von Merkmalen unterscheiden. Solche Genotypen sind aber mit konventionellen Methoden schwer erhaltlich. Aber die schnellen Fortschritte sowohl bei den Studien der Mechanismen von Pflanzen-Angreifer-Interaktionen als auch in der Molekulargenetik und Genomforschung eroffnen neue Moglichkeiten, die auch von Okologen genutzt werden konnen. Um funktionale Aspekte der induzierbaren indirekten Verteidigung zu analysieren, kann man z.B. die Kenntnisse zum Genom von Arabidopsis thaliana nutzen sowie die Verfugbarkeit bestimmter Mutanten und transgener Individuen von A. thaliana, die nur in einem Gen oder einer bestimmten Anzahl von Genen verandert sind. Diese Publikation gibt einen Uberblick der Kenntnisse uber die Mechanismen der induzierbaren indirekten Verteidigung von Pflanzen und ihre Bedeutung fur Analysen funktionaler Aspekte der pflanzlichen Reaktionen auf herbivore Arthropoden. Abschliesend verweisen wir auf okologische Fragen, die man untersuchen kann, wenn man mechanistische Aspekte der induzierbaren indirekten Antwort nutzt.

Identification of volatiles that are used in discrimination between plants infested with prey or nonprey herbivores by a predatory mite

Carnivorous arthropods can use herbivore-induced plant volatiles to locate their herbivorous prey. In the field, carnivores are confronted with information from plants infested with herbivores that may differ in their suitability as prey. Discrimination by the predatory mite Phytoseiulus persimilis between volatiles from lima bean plants infested with the prey herbivore Tetranychus urticae, or plants infested with the nonprey caterpillar Spodoptera exigua, depends on spider mite density. In this article, we analyzed the chemical composition of the volatile blends from T. urticae-infested lima bean plants at different densities of spider mites, and from S. exigua-infested plants. Based on the behavioral preferences of P. persimilis and the volatile profiles, we selected compounds that potentially enable the mite to discriminate between T. urticae-induced and S. exigua-induced volatiles. Subsequently, we demonstrated in Y-tube olfactometer assays that the relatively large amounts of methyl salicylate and (3E, 7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene emitted by T. urticae-infested bean plants compared to S. exigua-infested plants enable the predators to discriminate. Our data show that specific compounds from complex herbivore-induced volatile blends can play an important role in the selective foraging behavior of natural enemies of herbivorous arthropods.

Parasitoid‐plant mutualism: parasitoid attack of herbivore increases plant reproduction

We tested whether a plants life time seed production is increased by parasitization of herbivores in a tritrophic system, Arabidopsis thaliana (Brassicaceae) plants, Pieris rapae (Lepidoptera: Pieridae) caterpillars and the solitary endoparasitoid Cotesia rubecula (Hymenoptera: Braconidae). We established seed production for intact A. thaliana plants, plants that were mechanically damaged, plants fed upon by parasitized caterpillars and plants fed upon by unparasitized caterpillars. In the first experiment, with ecotype Landsberg (erecta mutant), herbivory by unparasitized P. rapae caterpillars resulted in a strongly reduced seed production compared to undamaged plants. In contrast, damage by P. rapae caterpillars that had been parasitized by C. rubecula did not result in a significant reduction in seed production. For the second experiment with the ecotype Columbia, the results were identical. Plants damaged by unparasitized caterpillars only produced seeds on regrown shoots. Seed production of plants that had been mechanically damaged was statistically similar to that of undamaged plants. Production of the first ripe siliques by plants fed upon by unparasitized caterpillars was delayed by 18–22 days for Landsberg and 9–10 days for Columbia. We conclude that parasitization of P. rapae by C. rubecula potentially confers a considerable fitness benefit for A. thaliana plants when compared to plants exposed to feeding damage by unparasitized P. rapae larvae. Plants that attract parasitoids and parasitoids that respond to herbivore‐induced plant volatiles will both experience selective advantage, justifying the use of the term mutualism for this parasitoid‐plant interaction. This type of mutualism is undoubtedly very common in nature.

Prey and Non-prey Arthropods Sharing a Host Plant: Effects on Induced Volatile Emission and Predator Attraction

It is well established that plants infested with a single herbivore species can attract specific natural enemies through the emission of herbivore-induced volatiles. However, it is less clear what happens when plants are simultaneously attacked by more than one species. We analyzed volatile emissions of lima bean and cucumber plants upon multi-species herbivory by spider mites (Tetranychus urticae) and caterpillars (Spodoptera exigua) in comparison to single-species herbivory. Upon herbivory by single or multiple species, lima bean and cucumber plants emitted volatile blends that comprised mostly the same compounds. To detect additive, synergistic, or antagonistic effects, we compared the multi-species herbivory volatile blend with the sum of the volatile blends induced by each of the herbivore species feeding alone. In lima bean, the majority of compounds were more strongly induced by multi-species herbivory than expected based on the sum of volatile emissions by each of the herbivores separately, potentially caused by synergistic effects. In contrast, in cucumber, two compounds were suppressed by multi-species herbivory, suggesting the potential for antagonistic effects. We also studied the behavioral responses of the predatory mite Phytoseiulus persimilis, a specialized natural enemy of spider mites. Olfactometer experiments showed that P. persimilis preferred volatiles induced by multi-species herbivory to volatiles induced by S. exigua alone or by prey mites alone. We conclude that both lima bean and cucumber plants effectively attract predatory mites upon multi-species herbivory, but the underlying mechanisms appear different between these species.

Experience with methyl salicylate affects behavioural responses of a predatory mite to blends of herbivore-induced plant volatiles

Many natural enemies of herbivorous arthropods use herbivore‐induced plant volatiles to locate their prey. These foraging cues consist of mixtures of compounds that show a considerable variation within and among plant–herbivore combinations, a situation that favours a flexible approach in the foraging behaviour of the natural enemies. In this paper, we address the flexibility in behavioural responses of the predatory mite Phytoseiulus persimilis Athias‐Henriot (Acari: Phytoseiidae) to herbivore‐induced plant volatiles. In particular, we investigated the effect of experience with one component of a herbivore‐induced volatile blend: methyl salicylate (MeSA). We compared the responses of three groups of predatory mites: (1) those reared from egg to adult on Tetranychus urticae Koch (Acari: Tetranychidae) on lima bean plants (Phaseolus lunatus L. that produces MeSA), (2) those reared on T. urticae on cucumber (Cucumus sativus L. that does not produce MeSA), and (3) those reared on T. urticae on cucumber in the presence of synthetic MeSA. Exposure to MeSA during the rearing period (groups 1 and 3) resulted in an attraction to the single compound MeSA in a Y‐tube olfactometer. Moreover, exposure to MeSA affected the choice of predatory mites between two volatile blends that were similar, except for the presence of MeSA. Predators reared on lima bean plants preferred the volatile blend from T. urticae‐induced lima bean (including MeSA) to the volatile blend from jasmonic‐acid induced lima bean (lacking MeSA), but predators reared on cucumber preferred the volatile blend from the latter. Predatory mites reared on cucumber in the presence of synthetic MeSA did not discriminate between these two blends. Exposure to MeSA for 3 days in the adult phase, after rearing on cucumber, also resulted in attraction to the single compound MeSA. We conclude that a minor difference in the composition of the volatile blend to which a predatory mite is exposed can explain its preferences between two odour sources.

Olfactory learning by predatory arthropods

Many natural enemies of herbivorous arthropods can use herbivore-induced plant volatiles to locate their prey. The composition of herbivore-induced volatile blends is highly variable, e.g., for different plant or herbivore species. When this variation is predictable during the lifetime of an individual, learning is expected to be adaptive for natural enemies that use such information. Learning has indeed been demonstrated many times for parasitoid wasps that use herbivore-induced plant volatiles to locate their hosts. However, evidence for learning of plant volatiles by predatory mites and insects is scarce and this is the topic of the present paper. We first review previously published research that demonstrated that anthocorid bugs and the predatory mite Phytoseiulus persimilis have the capacity to learn. Next, we present new evidence for an effect of previous experiences of P. persimilis on its responses to mixtures of volatile blends, induced by prey or non-prey herbivores. Finally, we discuss the ecological relevance of olfactory learning by predatory arthropods and the need to address this topic in future research.

Experimental Support for Multiple Locus Complementary Sex Determination in the Parasitoid Cotesia vestalis

Despite its fundamental role in development, sex determination is highly diverse among animals. Approximately 20% of all animals are haplodiploid, with haploid males and diploid females. Haplodiploid species exhibit diverse but poorly understood mechanisms of sex determination. Some hymenopteran insect species exhibit single-locus complementary sex determination (sl-CSD), where heterozygosity at a polymorphic sex locus initiates female development. Diploid males are homozygous at the sex locus and represent a genetic load because they are inviable or sterile. Inbreeding depression associated with CSD is therefore expected to select for other modes of sex determination resulting in fewer or no diploid males. Here, we investigate an alternative, heretofore hypothetical, mode of sex determination: multiple-locus CSD (ml-CSD). Under ml-CSD, diploid males are predicted to develop only from zygotes that are homozygous at all sex loci. We show that inbreeding for eight generations in the parasitoid wasp Cotesia vestalis leads to increasing proportions of diploid males, a pattern that is consistent with ml-CSD but not sl-CSD. The proportion of diploid males (0.27 ± 0.036) produced in the first generation of inbreeding (mother–son cross) suggests that two loci are likely involved. We also modeled diploid male production under CSD with three linked loci. Our data visually resemble CSD with linked loci because diploid male production in the second generation was lower than that in the first. To our knowledge, our data provide the first experimental support for ml-CSD.

Sex determination meltdown upon biological control introduction of the parasitoid Cotesia rubecula

Natural enemies may go through genetic bottlenecks during the process of biological control introductions. Such bottlenecks are expected to be particularly detrimental in parasitoid Hymenoptera that exhibit complementary sex determination (CSD). CSD is associated with a severe form of inbreeding depression because homozygosity at one or multiple sex loci leads to the production of diploid males that are typically unviable or sterile. We observed that diploid males occur at a relatively high rate (8–13% of diploid adults) in a field population of Cotesia rubecula in Minnesota, USA, where this parasitoid was introduced for biological control of the cabbage white Pieris rapae. However, our laboratory crosses suggest two‐locus CSD in a native Dutch population of C. rubecula and moderately high diploid males survival (approximately 70%), a scenario expected to produce low proportions of diploid males. We also show that courtship behavior of diploid males is similar to that of haploid males, but females mated to diploid males produce only very few daughters that are triploid. We use our laboratory data to estimate sex allele diversity in the field population of C. rubecula and discuss the possibility of a sex determination meltdown from two‐locus CSD to effective single‐locus CSD during or after introduction.

Absence of complementary sex determination in the parasitoid wasp genus Asobara (Hymenoptera: Braconidae)

An attractive way to improve our understanding of sex determination evolution is to study the underlying mechanisms in closely related species and in a phylogenetic perspective. Hymenopterans are well suited owing to the diverse sex determination mechanisms, including different types of Complementary Sex Determination (CSD) and maternal control sex determination. We investigated different types of CSD in four species within the braconid wasp genus Asobara that exhibit diverse life-history traits. Nine to thirteen generations of inbreeding were monitored for diploid male production, brood size, offspring sex ratio, and pupal mortality as indicators for CSD. In addition, simulation models were developed to compare these observations to predicted patterns for multilocus CSD with up to ten loci. The inbreeding regime did not result in diploid male production, decreased brood sizes, substantially increased offspring sex ratios nor in increased pupal mortality. The simulations further allowed us to reject CSD with up to ten loci, which is a strong refutation of the multilocus CSD model. We discuss how the absence of CSD can be reconciled with the variation in life-history traits among Asobara species, and the ramifications for the phylogenetic distribution of sex determination mechanisms in the Hymenoptera.

Genetic incompatibility drives mate choice in a parasitic wasp

IntroductionAllelic incompatibility between individuals of the same species should select for mate choice based on the genetic make-up of both partners at loci that influence offspring fitness. As a consequence, mate choice may be an important driver of allelic diversity. A complementary sex determination (CSD) system is responsible for intraspecific allelic incompatibility in many species of ants, bees, and wasps. CSD may thus favour disassortative mating and in this, resembles the MHC of the vertebrate immune system, or the self-incompatibility (SI) system of higher plants.ResultsHere we show that in the monogamous parasitic wasp Bracon brevicornis (Wesmael), females are able to reject partners with incompatible alleles. Forcing females to accept initially rejected partners resulted in sex ratio distortion and partial infertility of offspring.ConclusionsCSD-disassortative mating occurred independent of kin recognition and inbreeding avoidance in our experiment. The fitness consequences of mate choice are directly observable, not influenced by environmental effects, and more severe than in comparable systems (SI or MHC), on individuals as well as at the population level. Our results thus demonstrate the strong potential of female mate choice for maintaining high offspring fitness in this species.