What Remains of the Controversy 25 Years After Keller and Nonacs’ Seminal Paper?

What Remains of the Controversy 25 Years After Keller and Nonacs’ Seminal Paper?

Journal of Chemical Ecology (2018) 44:805–817 https://doi.org/10.1007/s10886-018-0974-9 REVIEW ARTICLE Queen Control or Queen Signal in Ants: What Remains of the Controversy 25 Years After Keller and Nonacs’ Seminal Paper? Irene Villalta1,2 & Silvia Abril3 & Xim Cerdá 1 & Raphael Boulay2,4 Received: 30 January 2018 /Revised: 5 April 2018 /Accepted: 25 May 2018 /Published online: 2 June 2018 # Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract Ant queen pheromones (QPs) have long been known to affect colony functioning. In many species, QPs affect important reproductive functions such as diploid larvae sexualization and egg-laying by workers, unmated queens (gynes), or other queens. Until the 1990s, these effects were generally viewed to be the result of queen manipulation through the use of coercive or dishonest signals. However, in their seminal 1993 paper, Keller and Nonacs challenged this idea, suggesting that QPs had evolved as honest signals that informed workers and other colony members of the queen’s presence and reproductive state. This paper has greatly influenced the study of ant QPs and inspired numerous attempts to identify fertility-related compounds and test their physiological and behavioral effects. In the present article, we review the literature on ant QPs in various contexts and pay special attention to the role of cuticular hydrocarbons (CHCs). Although the controversy generated by Keller and Nonacs’ (Anim Behav 45:787–794, 1993) paper is currently less intensively debated, there is still no clear evidence which allows the rejection of the queen control hypothesis in favor of the queen signal hypothesis. We argue that important questions remain regarding the mode of action of QPs, and their targets which may help understanding their evolution. Keywords Cuticular hydrocarbons . Fertility signal . Signal honesty . Signal perception . Social insects . Sociobiology Introduction care for the brood, forage, and defend the colony. In most species, queens attract workers, who form a retinue The Formicidae are characterized by great diversity in around them. In addition, a queen’s presence has multiple social organization. Colonies can contain one or multiple consequences for a colony’s reproductive output: it in- reproductive females and tens to millions of workers who hibits egg-laying by workers and gynes (winged virgin queens), reduces the fertility of nestmate queens, and reg- This review is dedicated to Prof. Luc Passera for his enormous ulates diploid brood sexualization. Myrmecologists have contribution to the knowledge of ant biology that inspired generations long recognized the role of queen pheromones (QPs) in of myrmecologists and sociobiologists. mediating these effects. However, until the 1990s, ant colonies were generally viewed as top-down regulated * Raphael Boulay systems in which queens had full control over the workers [email protected] and the brood. It was believed that queens used phero- 1 ’ Estación Biologica de Doñana, CSIC, Seville, Spain mones to manipulate nestmates behavior and physiology 2 ’ against their own interest (Röseler 1991). Institut de Recherche sur la Biologie de l Insecte, Université de B Tours, Tours, France In 1993, Keller and Nonacs published Theroleof 3 queen pheromones in social insects: queen control or Departament de Ciències Ambientals, Universitat de Girona, ^ Girona, Spain queen signal? . In this paper, they questioned the existing hypotheses regarding the evolution of QP-mediated effects 4 IRBI CNRS UMR 7261, Faculté des Sciences et Techniques, Avenue Monge, Parc de Grandmont, Université de Tours, in social insects. They argued that, although it was possible 37200 Tours, France that queens manipulate workers and brood through 806 J Chem Ecol (2018) 44:805–817 behavioral interactions, chemical control is much less ants or other social insects. As argued by Keller (2009) likely. More than a decade earlier Seeley (1979, 1985) and Grüter and Keller (2016), important gaps in our had already challenged the idea that honeybee queens used knowledge of the chemical nature and physiological func- their pheromones to manipulate the hive. Keller and tioning of QPs hamper a good understanding of their evo- Nonacs (1993) expanded this concept to other social in- lution. Here, we review the literature on QP-mediated ef- sects and proposed a general framework to study the evo- fects in ants. We first present the major contexts in which lution of QPs. They reasoned that queen chemical control QPs affect colony reproduction. Second, we review stud- could have evolved either because QPs were coercive or ies on the sources of QPs and evaluate current evidence because they dishonestly signaled a queen’s quality. In regarding the specific role of cuticular hydrocarbons both cases, the result would be a modification of the be- (CHCs). Finally, we analyze current knowledge on the havior and/or physiology of a queen’s nestmates against physiological and molecular effects of QPs in ants and their own interest. However, in the first case, the produc- finish by identifying questions that should be addressed tion of a coercive pheromone would inevitably result in in the future. Throughout this article we will refer to QPs selection for resistance, so that, to retain control, queens using the definition of Holman et al. (2010a): a chemical would have to continuously increase their pheromone pro- or chemical blend that^i) characterizes the reproduc- duction or diversify its chemical composition. Eventually, tive(s) and (ii) induces behavioral and/or physiological the cost for queens of this chemical arms race would prob- responses in other colony members^. ably outweigh the benefit of their reproductive monopoly. Moreover, queens would have to be immune to their own QPs. In the second case, the evolution of a dishonest signal Pheromone-Mediated Effects could be counteracted fairly easily if workers had alterna- on Reproduction and Development tive reliable sources of information, such as egg number, to evaluate queen quality or presence. Regulation of Worker Egg-Laying Keller and Nonacs proposed an alternative hypothesis inspired by signaling theory and deeply rooted in evolu- It has been known for many years that ant QPs influence the tionary biology. They argued that QPs are honest signals regulation of worker egg-laying (Bier 1954;Hölldoblerand directed to nestmates which enable them to behave in a Wilson 1983; Passera 1969, 1980a). In many species, workers way that maximizes their own inclusive fitness. At that have functional ovaries but generally refrain from laying via- time, a QP had only been identified in the honeybee ble eggs when they perceive the presence of a queen—in some (Apis mellifera). This paper provoked intense controversy species, queenright workers lay trophic eggs that are con- and stimulated many attempts to identify QPs in other sumed by the queen or the brood (Dietemann and Peeters social insects and to determine the precise mechanisms 2000; Hölldobler and Carlin 1989;). In contrast, when no by which they affect colony reproductive functions. In queen is present, workers readily lay arrhenotokous haploid particular, the diversity in the social organization of ants eggs that yield males or, more rarely, thelytokous diploid eggs provided an excellent system with which to initiate com- that yield workers or queens. Two hypotheses explain why parative analyses and test predictions related to the hon- workers collectively refrain from laying eggs when a queen esty of QPs in various contexts. This research was further is present in the colony. First, in polyandrous colonies, facilitated by the development of novel and powerful workers are genetically more related to the queen’ssonsthan technologies, like capillary gas chromatography and the to other workers’ sons and should therefore favor the former analysis of microsatellite markers, which allowed joint over the latter (Ratnieks 1988; Wenseleers and Ratnieks 2006; studies of ant chemistry and social systems. Woyciechowski and Łomnicki 1987). However, worker polic- In this controversy, proximate effects of QPs on re- ing is not restricted to polyandrous species and occurs even if ceivers’ physiology and behavior have not always been the genetic relatedness of workers to their sisters’ sons is high well differentiated from ultimate control of receivers’ fit- [see studies of Platythyrea punctata (Hartmann et al. 2003) ness by queens (Peso et al. 2015). Although recent model- and Aphaenogaster japonica (Iwanishi et al. 2003)]. A sec- ing suggests that queen control may theoretically evolve ond, more general explanation may be that worker egg-laying under certain conditions (Olejarz et al. 2017), there seems reduces colony-level productivity, which negatively affects all to be a large consensus in current literature that QPs are individuals (Hammond and Keller 2004;Heinze2004; honest, rather than coercive or dishonest, signals (Grüter Ratnieks 1988). Colony level costs of worker reproduction and Keller 2016;Heinzeandd’Ettorre 2009;Smithand have rarely been accurately measured and could potentially Liebig 2017). However, most conclusions are based on include a reduction in the efficiency of other tasks perfor- inferences and, to our knowledge, no experimental evi- mance (like foraging, colony defense etc.) or an increase of dence clearly rejects the hypothesis of queen control in overt aggression among workers. To our knowledge, the best J Chem Ecol (2018) 44:805–817 807 estimation of

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