Nvestigations Into the Meaning of a Fruit Fly Pheromone

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( ) 2 ,, ' Contents Introduction.....................................................................................................7 The three levels of mate choice..................................................................7 The meaning of pheromones ......................................................................9 Study species and Methods...........................................................................14 Natural history..........................................................................................14 Culturing ..................................................................................................15 Assessment of pheromone streaks............................................................16 Results and discussion ..................................................................................18 Individual variation ..................................................................................18 Heritability ...............................................................................................23 Cost ..........................................................................................................24 Conclusions...................................................................................................30 Sammanfattning ............................................................................................33 Partnervalets tre nivåer.............................................................................33 Feromoners innebörd................................................................................35 Studieart och metoder...............................................................................36 Resultat och diskussion ............................................................................36 Acknowledgments.........................................................................................39 References.....................................................................................................41 Introduction Chemical signaling is generally regarded as the most ancient and widespread form of communication (Bradbury & Vehrencamp, 1998; Wyatt, 2003). Throughout the living world, chemicals are exchanged between mating part- ners at each step in the complex suite of behaviours leading to reproduction. This has been evident almost since the word ‘pheromone’ was coined in 1959 (Karlson & Lüscher, 1959), and indeed the first identified pheromone was the chemical released by female silk moths, Bombyx mori, to attract males (Butenandt et al., 1959). Since then, the use of pheromones during reproduction has been reported from taxa ranging from bacteria to mammals (see paper I for a review). The three levels of mate choice Pheromones are ubiquitous in sexual communication, but in general there is little knowledge about what these signals actually communicate. Broadly speaking, sexual communication aims at influencing mate choice, defined as ‘any behaviour that restricts the set of potential mates’ (Wiley & Poston, 1996). Such behaviours should be beneficial whenever they increase the number and/or quality of offspring for choosy individuals as compared to those mating randomly. However, this wide definition of mate choice neces- sitates some sub-categories. First, we might distinguish between ‘direct’ and ‘indirect’ mate choice. Direct mate choice, in the words of Wiley and Poston, ‘requires discrimination between attributes of individuals of the op- posite sex’, whereas indirect mate choice is the result of all other behaviours that restrict the set of potential mates. Second, the set of potential mates might be restricted to the right species, the right mate recognition system (sensu Paterson, 1985) or the right individual qualities. These three levels of mate choice are defined both by the different cognitive abilities needed to perform them and by different evolutionary processes. Thus, species recognition evolves to avoid the costs of mating with the wrong species, mate recognition to coordinate the sexual behaviour between the sexes, and individual mate assessment to increase success in intraspecific competition for mates. From an evolutionary point of view, the results of selection for species and mate recognition are much the same. Both are driven by non-intraspecific processes, i.e. interspecific com- 7 petition for communication channels (Cardé & Baker, 1984) and environ- mental change (Paterson, 1985) respectively; both are essentially adaptive (Löfstedt, 1993) and both could be subject to stabilising selection (Cardé & Baker, 1984; Paterson, 1985). In contrast, individual mate assessment evolves by the benefits of choosing a mate that is in some way superior to its conspecifics, insofar as this superiority translates into more or better quality offspring. Moreover, a failure to recognise the right species or mate recognition system might result in no offspring at all, whereas misjudgement of mate quality rarely has such dire consequences. Sexual signals have evolved to influence mate choice at each of these three levels and we can consequently distinguish between species recognition signals, mate recognition signals and mate assessment signals. Each of these three types of signals has its own characteristic design, evolved through different processes and advertising different kinds of information. Thus, species and mate recognition signals could be expected to be relatively uniform within each species and sex respectively. In contrast, mate assessment signals must advertise the individual identity of the sender and its quality as a mate, and moreover do this in a way that cannot be faked. Mate assessment signals are thus likely to be costly and to vary qualitatively with the condition of the sender in order to provide honest information in a stable signalling system (Grafen, 1990; Rowe & Houle, 1996); they should fur- thermore be highly variable between individuals and are prone to a kind of exaggeration that often seems quite detrimental to the survival of the sender. Although species recognition, mate recognition and mate assessment are not mutually exclusive functions of a signal, it might be helpful to keep these distinctions in mind when investigating the role of a specific sexual signal. In the study of chemical signals in particular, where signal function is often summarily described by the word ‘sex pheromone’, additional insights may be gained by investigating at which level a specific pheromone acts. Studies on the roles of pheromones in sexual behaviour have tended to focus attention on long-range mate attraction (Svensson, 1996) or the impor- tance of chemical signals in species and mate recognition (Löfstedt, 1993; Ptacek, 2000). Indeed, in his seminal review of sexual selection Malte Andersson (1994) cites just nine studies that had explored pheromone-based mating preferences; in striking contrast, several hundred studies are cited that had explored the consequences of visual or acoustic signals. Moreover, while a vast body of research has investigated the molecular composition of pheromones (for reviews, see e.g. Stevens, 1998; Arn et al., 1986), com- paratively little work has been carried out on their behavioural significance at the level of individual mate assessment. However, several lines of evidence suggest that the role pheromones may play in mate choice is far Note that the term ‘mate recognition signal’ used here is not identical to the term ‘sex recognition signal’ used in paper I, although the former includes the latter. 8 greater than previously expected. First, there are a number of recent studies indicating individual mate choice by chemical cues (see table 1). Further, other studies have revealed considerable individual variation in pheromone production (e.g. Schlyter & Birgersson, 1989; Buesching et al., 2002), and researchers have also begun exploring the cost of pheromone production (e.g. Bushmann & Atema, 2000; Nahon et al., 1995). The meaning of pheromones In paper I, we review the evidence for pheromones as indicators of mate quality and their use in individual mate assessment. For a sexual signal to be adaptive, it must convey a benefit to the

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