Autofluorescence-Based Identification and Functional Validation of Antennal Gustatory Sensilla in a Specialist Leaf Beetle.

Stefan Pentzold,Veit Grabe,Antje Burse,Frédéric Marion-Poll

doi:10.3389/fphys.2019.00343

Abstract

Herbivorous insects mainly rely on their sense of taste to decode the chemical composition of potential hosts in close range. Beetles for example contact and scan leaves with their tarsi, mouthparts and antennal tips, i.e., appendages equipped with gustatory sensilla, among other sensillum types. Gustatory neurons residing in such uniporous sensilla detect mainly non-volatile compounds that contribute to the behavioral distinction between edible and toxic plants. However, the identification of gustatory sensilla is challenging, because an appendage often possesses many sensilla of distinct morphological and physiological types. Using the specialized poplar leaf beetle (Chrysomela populi, Chrysomelidae), here we show that cuticular autofluorescence scanning combined with electron microscopy facilitates the identification of antennal gustatory sensilla and their differentiation into two subtypes. The gustatory function of sensilla chaetica was confirmed by single sensillum tip-recordings using sucrose, salicin and salt. Sucrose and salicin were found at higher concentrations in methanolic leaf extracts of poplar (Populus nigra) as host plant compared to willow (Salix viminalis) as control, and were found to stimulate feeding in feeding choice assays. These compounds may thus contribute to the observed preference for poplar over willow leaves. Moreover, these gustatory cues benefited the beetle’s performance since weight gain was significantly higher when C. populi were reared on leaves of poplar compared to willow. Overall, our approach facilitates the identification of insect gustatory sensilla by taking advantage of their distinct fluorescent properties. This study also shows that a specialist beetle selects the plant species that provides optimal development, which is partly by sensing some of its characteristic non-volatile metabolites via antennal gustatory sensilla.

Highlights

Plant-feeding insects often judge the suitability of potential hosts as food source or oviposition site by sensing various phytochemicals (Van Naters and Carlson, 2006; Agnihotri et al, 2016)
Leaves of P. nigra are usually considered as main host of C. populi, whereas S. viminalis leaves are preferred by other leaf beetle species such as Chrysomela saliceti, Weise and Plagiodera versicolora (Laicharting) (Gök and Çilbiroglu, 2005; La Spina et al, 2010)
After centrifugation at 15,000 g for 5 min at 4◦C to pellet debris, the supernatant was filtered through a 0,22 μm polyvinylidene fluoride membrane (Durapore R, GVWP04700, Merck) by an additional centrifugation step and used for analysis via high pressure liquid chromatography coupled to mass spectrometry (LC-MS) and gas chromatography coupled to mass spectrometry (GC-MS)

Summary

INTRODUCTION

Plant-feeding insects often judge the suitability of potential hosts as food source or oviposition site by sensing various phytochemicals (Van Naters and Carlson, 2006; Agnihotri et al, 2016). The same secondary metabolites often stimulate feeding of specialists that have overcome and may even exploit host chemical defense (Bernays et al, 2000; Robert et al, 2012; Gray et al, 2015; Müller et al, 2015; Sollai et al, 2017). This pattern has been found for different insect species in case of salicinoids, which are phenolic glucosides typically found in Salicaceae plants (Fernandez and Hilker, 2007; Boeckler et al, 2011; Julkunen-Tiitto and Virjamo, 2016). We show that adult C. populi select a certain plant species which serves optimal weight gain and is partly due to tasting some of the host’s characteristic metabolites

MATERIALS AND METHODS

RESULTS

Findings

DISCUSSION