Functional Olfactory Sensory Neurons Housed in Olfactory Sensilla on the Ovipositor of the Hawkmoth Manduca sexta

Abstract

Olfactory systems evolved to detect and identify volatile chemical cues, in many cases across great distances. However, the precision of copulatory and oviposition behaviors suggest that they may be guided by olfactory cues detected by sensory systems located on or near the ovipositor. Here we present evidence of a small number of functional olfactory sensilla on the ovipositor of the hawkmoth Manduca sexta. Gene expression analysis of isolated ovipositor tissue indicated active transcription of gustatory and both classes of olfactory receptor genes. Expression of the olfactory co-receptor ORCo and the antennal ionotropic co-receptors IR8a and IR25a suggests that functional olfactory proteins may be present in the sensory structures located on the ovipositor. Scanning electron microscopy identified five to nine porous sensilla on each of the anal papillae of the ovipositor. Furthermore, HRP immunostaining indicated that these sensilla are innervated by the dendrite-like structures from multiple neurons. Finally, we functionally characterized neural responses in these sensilla using single sensillum recordings. Stimulation with a panel of 142 monomolecular odorants revealed that these sensilla indeed house functional olfactory sensory neurons (OSNs). While it remains to be determined what role these chemosensory sensilla play in odor and gustatory guided behaviors, our data clearly demonstrate an olfactory function for neurons present in M. sexta ovipositor sensilla.

Transcriptome Surveys in Silverfish Suggest a Multistep Origin of the Insect Odorant Receptor Gene Family

“…Consistent with an olfactory function and unlike identified Grs, the Or transcripts were predominantly detected in chemosensory tissues such as antennae and maxillary palps, and some of them were also expressed in other tissues. Expression of Ors outside the classical chemosensory tissues has also been reported in other insect species (Bray and Amrein, 2003;Haverkamp et al, 2016;Klinner et al, 2016;Pitts et al, 2014;Raad et al, 2016). Given the generally low expression levels of Ors and that many of the firebrat Ors identified from the genome form clades with silverfish Ors identified from our RNAseq experiments in our phylogenetic analysis, it is likely that many of the firebrat genes described from the genome by Brand and co-workers (2018) are indeed also expressed in the firebrat antennae and that they were missed in previous RNAseq experiments because of insufficient sequencing depth.…”

Mutagenesis oforcoimpairs foraging but not oviposition in the hawkmothManduca sexta

“…Host localization by a gravid female is the initial step in the antagonistic interaction between M. sexta and D. wrightii. After arriving at a plant, the female hawkmoth holds on to the leaf with her forelegs, curls her abdomen and holds this position in a characteristic motion (27), possibly to assess plant quality through chemoreceptors on the ovipositor (72). When a suitable oviposition site has been identified, the hawkmoth places a single egg and moves to another position on the plant.…”

A short guide to insect oviposition: when, where and how to lay an egg

“…Although the role of some gustatory neurons located on the proboscis or the legs is well established (see below), the contribution of the ovipositor in oviposition site choice remains obscure in most species. The presence of chemosensory sensillae on the ovipositor of various species is indicated by the expression of chemoreceptor genes (odorant and gustatory receptors) (Glaser et al, 2013;Klinner et al, 2016) as well as the electrophysiological responses of some of these sensilla to various volatile or non-volatile molecules ((Klinner et al, 2016;Seada et al, 2016) and references therein; (Yadav and Borges, 2017)). For instance, four gustatory neurons types housed in sensilla on the ovipositor of the noctuid moth Spodoptera littoralis detect salt, caffeine, sugar, and water (Seada et al, 2016).…”