A central paradigm in the field of plant-herbivore interactions is that the diversity and complexity of secondary compounds in plants have intensified over evolutionary time, resulting in the great variety of secondary products that currently exists. Unfortunately, testing of this proposal has been very limited. We analyzed the volatile chemistry of 70 species of the tropical plant genus Bursera and used a molecular phylogeny to test whether the species' chemical diversity or complexity have escalated. The results confirm that as new species diverged over time they tended to be armed not only with more compounds/species, but also with compounds that could potentially be more difficult for herbivores to adapt to because they belong to an increasing variety of chemical pathways. Overall chemical diversity in the genus also increased, but not as fast as species diversity, possibly because of allopatric species gaining improved defense with compounds that are new locally, but already in existence elsewhere.Bursera ͉ chemical complexity ͉ chemical diversity ͉ evolution of secondary compounds ͉ coevolution
Background Recent findings indicate that several insect lineages receive protection against particular natural enemies through infection with heritable symbionts, but little is yet known about whether enemies are able to discriminate and respond to symbiont-based defense. The pea aphid, Acyrthosiphon pisum , receives protection against the parasitic wasp, Aphidius ervi , when infected with the bacterial symbiont Hamiltonella defensa and its associated bacteriophage APSE ( Acyrthosiphon pisum s econdary e ndosymbiont). Internally developing parasitoid wasps, such as A. ervi , use maternal and embryonic factors to create an environment suitable for developing wasps. If more than one parasitoid egg is deposited into a single aphid host (superparasitism), then additional complements of these factors may contribute to the successful development of the single parasitoid that emerges. Results We performed experiments to determine if superparasitism is a tactic allowing wasps to overcome symbiont-mediated defense. We found that the deposition of two eggs into symbiont-protected aphids significantly increased rates of successful parasitism relative to singly parasitized aphids. We then conducted behavioral assays to determine whether A. ervi selectively superparasitizes H. defensa -infected aphids. In choice tests, we found that A. ervi tends to deposit a single egg in uninfected aphids, but two or more eggs in H. defensa -infected aphids, indicating that oviposition choices may be largely determined by infection status. Finally, we identified differences in the quantity of the trans-β-farnesene, the major component of aphid alarm pheromone, between H. defensa -infected and uninfected aphids, which may form the basis for discrimination. Conclusions Here we show that the parasitic wasp A. ervi discriminates among symbiont-infected and uninfected aphids, and changes its oviposition behavior in a way that increases the likelihood of overcoming symbiont-based defense. More generally, our results indicate that natural enemies are not passive victims of defensive symbionts, and that an evolutionary arms race between A. pisum and the parasitoid A. ervi may be mediated by a bacterial symbiosis.
Heteropteran insects often protect themselves from predators with noxious or toxic compounds, especially when these insects occur in aggregations. The predators of heteropteran insects change from small insect predators to large avian predators over time. Thus, a chemical that is deterrent to one type of predator at one point in time may not be deterrent to another type of predator at another point in time. Additionally, these predator deterrent compounds may be used for other functions such as alarm signaling to other conspecifics. Defensive secretion compounds from the adult and the nymph giant mesquite bug (Thasus neocalifornicus: Coreidae) were isolated and identified by gas chromatography-mass spectrometry and NMR. The predominant compounds isolated from the nymph mesquite bugs during a simulated predator encounter were (E)-2-hexenal and 4-oxo-(E)-2-hexenal. In adults, the major compounds released during a simulated predator encounter were hexyl acetate, hexanal, and hexanol. Results from predator bioassays suggest the nymph compounds are more effective at deterring an insect predator than the adult compounds. By using behavioral bioassays, we determined the role of each individual compound in signaling to other mesquite bugs. The presence of the nymph secretion near a usually compact nymph aggregation caused nymph mesquite bugs to disperse but did not affect adults. Conversely, the presence of the adult secretion caused the usually loose adult aggregation to disperse, but it did not affect nymph aggregation. The compounds that elicited nymph behavioral responses were (E)-2-hexenal and 4-oxo-(E)-2-hexenal, while those that elicited adult behavioral responses were hexyl acetate and hexanal. The differences between the chemical composition of nymph and adult defensive secretions and alarm behavior are possibly due to differences in predator guilds.
The coreid bug Thasus neocalifornicus Brailovsky and Barrera, commonly known as the giant mesquite bug, is a ubiquitous insect of the southwestern United States. Both nymphs and adults are often found aggregated on mesquite trees (Prosopis spp.: Fabaceae) feeding on seedpods and plant sap. We characterized the indigenous bacterial populations of nymphs and adults of this species by using molecular and phylogenetic techniques and culturing methods. Results show that this insect's bacterial gut community has a limited diversity dominated by Burkholderia associates. Phylogenetic analysis by using 16s rRNA sequences suggests that these β-Proteobacteria are closely related to those symbionts obtained from other heteropteran midgut microbial communities but not to Burkholderia symbionts associated with other insect orders. These bacteria were absent from the eggs and were not found in all younger nymphs, suggesting that they are acquired after the insects have hatched. Rearing experiments of nymphs with potentially Burkholderia contaminated soil suggested that if this symbiont is not acquired, giant mesquite bugs experience higher mortality. Egg, whole-body DNA extractions of younger nymphs, and midgut DNA extractions of fifth-instar nymphs and adults also revealed the presence of α-Proteobacteria from the Wolbachia genus. However, this bacterium was also present in reproductive organs of adults, indicating that this symbiont is not specific to the gut.
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