Cracking the olfactory code of a butterfly: the scent of ageing

Abstract

Although olfaction is a primary mode of communication, its importance in sexual selection remains understudied. Here, using the butterfly Bicyclus anynana, we address all the parameters of importance to sexual selection for a male olfactory signal. We show that variation in the male sex pheromone composition indicates male identity and male age. Courting males of different ages display small absolute (c. 200 ng) but large relative (100%) change of one specific pheromone component (hexadecanal) which, unlike the other components, showed no heritability. Females prefer to mate with mid-aged over younger males and the pheromone composition is sufficient to determine this preference. Surprisingly refined information is thus present in the male olfactory signal and is used for sexual selection. Our data also reveal that there may be no 'lek paradox' to resolve once the precise signal of importance to females is identified, as hexadecanal is, as expected, depleted in additive genetic variation.

Selection on male sex pheromone composition contributes to butterfly reproductive isolation

“…We based our selection method on criteria established in the field of sexual selection and widely supported across taxa and across phenotypic traits: we picked male-specific, repeatable and most abundant compounds as these are most likely under directional selection for largest possible amounts by females. These characteristics are indeed the ones of the male sex pheromone identified using electrophysiological and behavioural experiments in the model species B. anynana [25][26][27]. The use of a selection method also has the advantage to avoid circular reasoning, as we show that the compounds chosen on one basis (male sex specificity, repeatability and amount in males) show a biological pattern of species specificity and RCD.…”

“…This was expected as, notwithstanding our selection method, the majority of male major compounds are simply absent in related species. The traces of pMSP components found in other Bicyclus species are either present: (i) at low amount, in that case we expect the female to reject the male, as shown in B. anynana in which mate choice is based on maximal amounts of MSP [26,27]; or (ii) in a female, in that case, the chemical cue is not going to be a problem as females rarely court other females in B. anynana. Thus, in Bicyclus butterflies, the presence or absence of pMSP components probably allows species recognition.…”

“…Altogether, our criteria maximized the probability of selecting compounds acting as male secondary sexual traits under sexual selection by females, and the inclusion of compounds with other functions in our pMSP selection, such as anti-aphrodisiacs, would have blurred the RCD pattern. Future behavioural studies using synthetic sex pheromone perfumes as in Nieberding et al [26] are needed to confirm the role of the selected compounds in sexual communication and reproductive isolation in field-caught populations of Bicyclus sharing some pMSP components.…”

“…Interestingly, the key taxonomic trait used to distinguish Bicyclus species is the number and position of androconia [24], the wing structures thought to be involved in the production and emission of male sex pheromone. Furthermore, it has been demonstrated that male sex pheromone (MSP) plays a key role in mate choice in the model species Bicyclus anynana [25][26][27]. After landing behind a female, B. anynana males flicker their wings and erect their androconial hair, probably favouring the dissemination of MSP at short range [25].…”

“…Therefore, we postulated that selection on differentiation in the sex pheromone composition has produced recurrent RCD in this butterfly genus. Building on knowledge from the model species B. anynana [25][26][27], we first developed a methodology to select from complex chemical profiles the components most likely to form the male sex pheromone for over 30 Bicyclus species. Accounting for the potential effect of the environment, we then assessed the presence of a recurrent pattern of character displacement in the predicted sex pheromone composition across the genus.…”