Pheromone gland development and monoterpenoid synthesis specific to oviparous females in the pea aphid

Secondary Rhinaria Contribute to Major Sexual Dimorphism of Antennae in the Aphid Semiaphis heraclei (Takahashi)

Sexual generation is an important generation in the life cycle of host-alternating aphids, and its population size determines the intensity of the peak in the next spring. Although male trapping techniques based on olfactory stimuli have been successfully established in the field, the biological basis of olfactory perception in males is unclear. In this study, we compared the morphology of antennae and the types, sizes, numbers, and distribution of sensilla between males and sexual females in the host-alternating aphid Semiaphis heraclei (Hemiptera: Aphididae). We found that flagellum length differentiation contributed to the majority of the sexual dimorphism of antennae. Most sensillum types or subtypes, including trichoid sensilla subtype I, campaniform sensilla, and primary rhinaria subtypes I and II, were enlarged in males. In addition, males bore more trichoid sensilla subtype I than sexual females. In particular, secondary rhinaria were present in males only and could not be detected in sexual females. These results revealed the structural basis of male olfactory perception. Our findings provide insight into the mechanism underlying chemical communication between sexual aphids and could thus be useful for pest control.

Biosynthesis of iridoid sex pheromones in aphids

Plants, animals, and microbes produce a plethora of natural products that are important for defense and communication. Most of these compounds show a phylogenetically restricted occurrence, but, in rare instances, the same natural product is biosynthesized by organisms in two different kingdoms. The monoterpene-derived iridoids, for example, have been found in more than 50 plant families but are also observed in several insect orders. The discovery of the aphid iridoid pathway, one of the longest and most chemically complex insect-derived natural product biosynthetic pathways reported to date, highlights the mechanisms underlying the convergent evolution of metabolic enzymes in insects and plants, including the recruitment of different enzyme classes to catalyze the same chemical processes.

Iridoid monoterpenes, widely distributed in plants and insects, have many ecological functions. While the biosynthesis of iridoids has been extensively studied in plants, little is known about how insects synthesize these natural products. Here, we elucidated the biosynthesis of the iridoids cis-trans-nepetalactol and cis-trans-nepetalactone in the pea aphid Acyrthosiphon pisum (Harris), where they act as sex pheromones. The exclusive production of iridoids in hind legs of sexual female aphids allowed us to identify iridoid genes by searching for genes specifically expressed in this tissue. Biochemical characterization of candidate enzymes revealed that the iridoid pathway in aphids proceeds through the same sequence of intermediates as described for plants. The six identified aphid enzymes are unrelated to their counterparts in plants, conclusively demonstrating an independent evolution of the entire iridoid pathway in plants and insects. In contrast to the plant pathway, at least three of the aphid iridoid enzymes are likely membrane bound. We demonstrated that a lipid environment facilitates the cyclization of a reactive enol intermediate to the iridoid cyclopentanoid-pyran scaffold in vitro, suggesting that membranes are an essential component of the aphid iridoid pathway. Altogether, our discovery of this complex insect metabolic pathway establishes the genetic and biochemical basis for the formation of iridoid sex pheromones in aphids, and this discovery also serves as a foundation for understanding the convergent evolution of complex metabolic pathways between kingdoms.