Female sex pheromone and male behavioral responses of the bombycid moth Trilocha varians: comparison with those of the domesticated silkmoth Bombyx mori

Interactive parallel sex pheromone circuits that promote and suppress courtship behaviors in the cockroach

Many animals use multicomponent sex pheromones for mating, but the specific function and neural processing of each pheromone component remain unclear. The cockroach Periplaneta americana is a model for studying sex pheromone communication, and an adult female emits major and minor sex pheromone components, periplanone-B and -A (PB and PA), respectively. Attraction and courtship behaviors (wing-raising and abdominal extension) are strongly expressed when adult males are exposed to PB but weakly expressed when they are exposed to PA. When major PB is presented together with minor PA, behaviors elicited by PB were impaired, indicating that PA can both promote and suppress courtship behaviors depending on the pheromonal context. In this study, we identified the receptor genes for PA and PB and investigated the effects of knocking down each receptor gene on the activities of PA- and PB-responsive sensory neurons (PA- and PB-SNs), and their postsynaptic interneurons, and as well as effects on courtship behaviors in males. We found that PB strongly and PA weakly activate PB-SNs and their postsynaptic neurons, and activation of the PB-processing pathway is critical for the expression of courtship behaviors. PA also activates PA-SNs and the PA-processing pathway. When PA and PB are simultaneously presented, the PB-processing pathway undergoes inhibitory control by the PA-processing pathway, which weakens the expression of courtship behaviors. Our data indicate that physiological interactions between the PA- and PB-processing pathways positively and negatively mediate the attraction and courtship behaviors elicited by sex pheromones.

A volatile sex attractant of tsetse flies

Tsetse flies transmit trypanosomes-parasites that cause devastating diseases in humans and livestock-across much of sub-Saharan Africa. Chemical communication through volatile pheromones is common among insects; however, it remains unknown if and how such chemical communication occurs in tsetse flies. We identified methyl palmitoleate (MPO), methyl oleate, and methyl palmitate as compounds that are produced by the tsetse fly Glossina morsitans and elicit strong behavioral responses. MPO evoked a behavioral response in male-but not virgin female-G. morsitans. G. morsitans males mounted females of another species, Glossina fuscipes, when they were treated with MPO. We further identified a subpopulation of olfactory neurons in G. morsitans that increase their firing rate in response to MPO and showed that infecting flies with African trypanosomes alters the flies' chemical profile and mating behavior. The identification of volatile attractants in tsetse flies may be useful for reducing disease spread.

Lipid droplets in the pheromone glands of bombycids: Effects of larval diet on their size and pheromone titer

In addition to the blend ratio, the quantity of sex pheromone components secreted by female moths may affect the efficient attraction of conspecific males. The present study using the silkmoth Bombyx mori, which has bombykol as its pheromone component, demonstrated that pheromone titer, body weight, and lipid droplet (LD) diameter in the pheromone gland were affected by the larval diet. Although the artificial diet contained approximately 11-fold more total fatty acids than mulberry leaf, the pheromone titer in the group fed the artificial diet (group AD) was approximately 2-fold higher than that of the group fed mulberry (group M). The diameter of LDs, which store the pheromone-precursor fatty acyl, E10,Z12-16:Acyl, was also larger in the AD group. The relatively small increase in sex pheromone titer by feeding on a fatty-acid-rich diet may be partly attributable to the storage of excess precursors in the LDs. We detected LDs in the pheromone glands of Trilocha varians, the closest non-congener of B. mori available in Bombycidae. T. varians uses bombykal and bombykyl acetate as sex pheromone components, which are biosynthesized via the same precursor fatty acyl as that of B. mori. The presence of LDs in T. varians suggests that the storage and mobilization mechanisms of the pheromone precursor fatty acyl via LDs may be conserved in bombycids.

Hybrid Sex Pheromones of the Hibiscus Flower-bud Borer, Rehimena surusalis

The sex pheromone of the hibiscus flower borer Rehimena surusalis (Walker) (Lepidoptera: Crambidae) was analyzed by gas chromatography with electroantennographic detection (GC-EAD) and GC-mass spectrometry (GC/MS). Three EAD-active components were found in crude pheromone gland extracts of calling females. GC/MS and GC analyses using synthetic chemicals and derivatization of the extracts identified three components as (10E,12Z)-hexadeca-10,12-dienal (E10,Z12-16:Ald,), (10E,12E)-hexadeca-10,12-dienyl acetate (E10,Z12-16:OAc), and (3Z,6Z,9Z)-tricosa-3,6,9-triene (Z3,Z6,Z9-23:HC). In field tests, male moths were strongly attracted to a ternary blend of E10,Z12-16:Ald, E10,Z12-16:OAc, and Z3,Z6,Z9-23:HC at a ratio of 1:5:14, but single and binary blends showed only weak or no attraction.

Single-Component Pheromone Consisting of Bombykal in a Diurnal Hawk Moth, Neogurelca himachala sangaica

Recent work has suggested that hawk moths share pheromone components but are sexually separated by qualitative and quantitative differences in their pheromone blends. During field assays on the sex pheromones of other species, a diurnal hawk moth, Neogurelca himachala sangaica (Lepidoptera: Sphingidae), was frequently captured, but the composition of the sex pheromone of this species was not known. Analysis of hexane extracts of the pheromone glands of calling female by gas chromatography (GC) using an electroantennographic detector (EAD) revealed two components that elicited EAD responses from male moth antennae. These components were identified by their mass spectra and retention indices on two GC columns as (10E,12Z)-10,12-hexadecadienal (E10,Z12-16:Ald) and a trace of its (10E,12E)-isomer (E10,E12-16:Ald) in 98:2 ratio. In field experiments, E10,Z12-16:Ald alone attracted male moths, and addition of E10,E12-16:Ald significantly reduced the attractiveness, even at the naturally-occurring ratio. Analysis of the data using a generalized linear mixed model showed that E10,Z12-16:Ald positively contributed to attractiveness, whereas E10,E12-16:Ald did so negatively, and it was concluded that the sex pheromone of N. himachala sangaica consists solely of E10,Z12-16:Ald, bombykal. The negative effect of E10,E12-16:Ald on attractiveness could promote the species-specificity of this single-component pheromone system.

Antennal lobe organization and pheromone usage in bombycid moths

We investigated the neuroanatomy of the macroglomerular complex (MGC), which is involved in sex pheromone processing, in five species in the subfamily Bombycinae, including Ernolatia moorei, Trilocha varians, Rondotia menciana, Bombyx mandarina and Bombyx mori. The glomerulus located at the dorsal-most part of the olfactory centre shows the largest volume in moth species examined to date. Such normal glomerular organization has been observed in E. moorei and T. varians, which use a two-component mixture and includes the compound bombykal as a mating signal. By contrast, the other three species, which use another component as a single attractant, exhibited a modified arrangement of the MGC. This correlation between pheromone usage and neural organization may be useful for understanding the process of speciation.

Molecular and neural mechanisms of sex pheromone reception and processing in the silkmoth Bombyx mori

Male moths locate their mates using species-specific sex pheromones emitted by conspecific females. One striking feature of sex pheromone recognition in males is the high degree of specificity and sensitivity at all levels, from the primary sensory processes to behavior. The silkmoth Bombyx mori is an excellent model insect in which to decipher the underlying mechanisms of sex pheromone recognition due to its simple sex pheromone communication system, where a single pheromone component, bombykol, elicits the full sexual behavior of male moths. Various technical advancements that cover all levels of analysis from molecular to behavioral also allow the systematic analysis of pheromone recognition mechanisms. Sex pheromone signals are detected by pheromone receptors expressed in olfactory receptor neurons in the pheromone-sensitive sensilla trichodea on male antennae. The signals are transmitted to the first olfactory processing center, the antennal lobe (AL), and then are processed further in the higher centers (mushroom body and lateral protocerebrum) to elicit orientation behavior toward females. In recent years, significant progress has been made elucidating the molecular mechanisms underlying the detection of sex pheromones. In addition, extensive studies of the AL and higher centers have provided insights into the neural basis of pheromone processing in the silkmoth brain. This review describes these latest advances, and discusses what these advances have revealed about the mechanisms underlying the specific and sensitive recognition of sex pheromones in the silkmoth.

Concentric zones for pheromone components in the mushroom body calyx of the moth brain

The spatial distribution of input and output neurons in the mushroom body (MB) calyx was investigated in the silkmoth Bombyx mori. In Lepidoptera, the brain has a specialized system for processing sex pheromones. How individual pheromone components are represented in the MB has not yet been elucidated. Toward this end, we first compared the distribution of the presynaptic boutons of antennal lobe projection neurons (PNs), which transfer odor information from the antennal lobe to the MB calyx. The axons of PNs that innervate pheromonal glomeruli were confined to a relatively small area within the calyx. In contrast, the axons of PNs that innervate nonpheromonal glomeruli were more widely distributed. PN axons for the minor pheromone component covered a larger area than those for the major pheromone component and partially overlapped with those innervating nonpheromonal glomeruli, suggesting the integration of the minor pheromone component with plant odors. Overall, we found that PN axons innervating pheromonal and nonpheromonal glomeruli were organized into concentric zones. We then analyzed the dendritic fields of Kenyon cells (KCs), which receive inputs from PNs. Despite the strong regional localization of axons of different PN classes, the dendrites of KCs were less well classified. Finally, we estimated the connectivity between PNs and KCs and suggest that the dendritic field may be organized to receive different amounts of pheromonal and nonpheromonal inputs. PNs for multiple pheromone components and plant odors enter the calyx in a concentric fashion, and they are read out by the elaborate dendritic field of KCs.

Reinvestigation of the sex pheromone of the wild silkmoth Bombyx mandarina: the effects of bombykal and bombykyl acetate

Sex pheromone investigations of the domesticated silkmoth, Bombyx mori (Lepidoptera: Bombycidae), helped elucidate the molecular and physiological fundamentals of chemical communication in moths, yet little is known about pheromone evolution in bombycid species. Therefore, we reexamined the sex pheromone communication in the wild silkmoth, Bombyx mandarina, which is considered ancestral to B. mori. Our investigations revealed that (a) B. mandarina females produce (E,Z)-10,12-hexadecadienol (bombykol), but not (E,Z)-10,12-hexadecadienal (bombykal) or (E,Z)-10,12-hexadecadienyl acetate (bombykyl acetate), which are pheromone components in other bombycid moths; (b) antennae of male B. mandarina respond strongly to bombykol as well as to bombykal and bombykyl acetate; and (c) bombykal and bombykyl acetate strongly inhibit attraction of B. mandarina males to bombykol in the field. The present study clarifies the evolution of pheromone communication in bombycid moths.