Functional specialisation and polyphenism in aphid olfactory sensilla

Electroantennogram (EAG) responses to the aphid sex pheromone components, (1R,4aS,7S,7aR)-nepetalactol and (4aS,7S,7aR)-nepetalactone, and a plant volatile, (E)-2-hexenal, were investigated at three different positions (5/6th, 4/5th and 3/4th inter-segmental regions) along the antennae of four different morphs in two host-alternating aphid species, Aphis fabae Scopoli and Rhopalosiphum padi (L.). Position-dependent and morph-specific EAG responses were elicited in both species. The nepetalactol and nepetalactone isomers elicited large EAG responses at all three recording positions in males of both species, such that primary rhinaria as well as secondary rhinaria appeared to respond. Asexual female morphs showed relatively smaller EAG responses to these compounds. The secondary rhinaria, which have been reported as sex pheromone receptors in males, were not very different in their number and distribution between gynoparae and alate virginoparae, but the gynoparae showed significantly larger EAG responses to nepetalactol and nepetalactone. The alate virginoparae showed EAG responses that were similar to those of apterous virginoparae, which lack secondary rhinaria. Taking the EAG response profiles together with the distribution of the secondary rhinaria, it is suggested that the function of secondary rhinaria differs between the morphs. Secondary rhinaria appear to detect sex pheromone components in males and gynoparae but not in the alate virginoparae. If they are functional in the latter morph, they are likely to play a role in detecting specific, but as yet unknown, volatile compounds. Some 30 plant volatiles were tested but none evoked an EAG response that could be allocated to the secondary rhinaria. In contrast to the very different EAG response profiles to the pheromone compounds between morphs, EAG responses to (E)-2-hexenal were similar in all forms and both species. These findings suggest that this plant volatile was detected only by the two primary rhinaria, which are common to all morphs. The present study showed that EAG responses were not a simple summation of receptor potentials between recording and reference electrodes in aphids. The localised distribution pattern of olfactory receptor neurones around the recording electrode was also likely to contribute to the EAG responses.