Chemosensory detection of glucosinolates as token stimuli for specialist insects on brassicaceous plants: discovery and impact.

In this paper, we take a historical perspective by going back to Verschaffelt's landmark study published in 1910, in which he found that glucosinolates were used as token stimuli by larvae of Pieris butterflies, specialist feeders on plants in the family Brassicaceae. This classic discovery provided key evidence for Fraenkel (Science 129:1466-1470, 1959) to elaborate on the function of secondary plant substances and for Ehrlich and Raven (Evolution 18:586-608, 1964) to put forward the hypothesis of insect-plant coevolution. The discovery by Schoonhoven (Kon Nederl Akad Wetensch Amsterdam Proc Ser C70:556-568, 1967) of taste neurons highly sensitive to glucosinolates in Pieris brassicae was an important milestone in elucidating the chemosensory basis of host-plant specialization. The molecular basis of glucosinolate sensitivity was elucidated recently (Yang et al., PLoS Genet 17, 2021) paving the way to unravel the evolution of gustatory receptors tuned to glucosinolates that are crucial for host-plant selection of Pieris butterflies. We propose a hypothetical model for the evolution of labeled-line neurons tuned to token stimuli.