Biogenic Amine Levels, Reproduction and Social Dominance in the Queenless Ant Streblognathus Peetersi
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Naturwissenschaften (2006) 93: 149–153 DOI 10.1007/s00114-006-0086-1 SHORT COMMUNICATION Virginie Cuvillier-Hot . Alain Lenoir Biogenic amine levels, reproduction and social dominance in the queenless ant Streblognathus peetersi Received: 9 May 2005 / Accepted: 6 January 2006 / Published online: 3 March 2006 # Springer-Verlag 2006 Abstract Social harmony often relies on ritualised dom- Introduction inance interactions between society members, particularly in queenless ant societies, where colony members do not Social dominance refers to an asymmetry of relationship have developmentally predetermined castes but have to among individuals, as defined by the outcomes of dyadic fight for their status in the reproductive and work encounters of a competitive or agonistic nature. Recent hierarchy. In this behavioural plasticity, their social studies on vertebrate and invertebrate species suggest that organisation resembles more that of vertebrates than differences in social status may reflect variations in mono- that of the “classic” social insects. The present study aminergic activity in the central nervous system (Bloch et investigates the neurochemistry of the queenless ant al. 2000; Huber et al. 2001; Kaplan et al. 2002; Reisner et species, Streblognathus peetersi, to better understand the al. 1996). The neurophysiological basis of social status, neural basis of the high behavioural plasticity observed in already extensively examined in vertebrates, has recently queenless ants. We report measurements of brain biogenic begun to be investigated in social insects. Some studies amines [octopamine, dopamine, serotonin] of S. peetersi have shown that biogenic amine levels are linked to the ants; they reveal a new set of biogenic amine influences social role of infertile Apis workers: foragers have higher on social organisation with no common features with levels of octopamine (OA) compared to nurses of the same other “primitively organised societies” (bumble bees) and age (Schulz et al. 2002; Wagener-Hulme et al. 1999), and some common features with “highly eusocial” species dancers have higher levels of dopamine than other foragers (honey bees). This similarity to honey bees may either (Bozic and Woodring 1998). Bloch and colleagues (Bloch confirm the heritage of queenless species from their et al. 2000) investigated the brain biogenic amine levels probably highly eusocial ancestors or highlight indepen- according to reproductive dominance in bumble bee dent patterns of biogenic amine influences on the social workers. They found that, in queen-right colonies, workers organisation of these highly derived species. with higher levels of octopamine had more developed ovaries and were more dominant than sisters of the same age, and that in queenless colonies, high dopamine levels . V. Cuvillier-Hot A. Lenoir were associated with the final stages of ovarian develop- Institut de Recherche sur la Biologie de l’Insecte, Université François Rabelais, CNRS UMR 6035, ment in workers. Faculté des Sciences, Most of the highly social insect species considered so Parc de Grandmont, far, have morphological castes that are determined by a 37200 Tours France specific developmental pathway that fixes the reproductive V. Cuvillier-Hot role and associated behavioural repertoire of each individ- Laboratoire d’Écologie, CNRS UMR 7625, ual in the colony. Queenless ant species, on the other hand, Université Pierre et Marie Curie, have colonies composed exclusively of monomorphic 75252 Paris Cedex 05, France workers, each one capable of mating and laying eggs, Present address: forming societies with a more plastic mode of organisation V. Cuvillier-Hot (*) (Peters 1993). In these colonies, the division of labour Laboratoire de Neuroimmunologie des Annélides, UMR 8017, depends on interindividual behavioural interactions that Université des Sciences et Techniques de Lille, determine not only gathering tasks but also more import- bâtiment SN3, antly, reproductive roles. As in most vertebrate societies, 59655 Villeneuve d’Ascq France e-mail: [email protected] the potential reproducers in a queenless ant colony form a Tel.: +33-32033-7099 dominance hierarchy through confrontations. Because Fax: +33-32043-4054 castes are determined through behavioural interactions, 150 the social position of individual workers changes fre- careful behavioural observation as already described in quently and rapidly. At each change in status, the ant must Cuvillier-Hot et al. (2004). Alpha ants, workers of high quickly adapt its behaviour to its new position. Such rank (beta to delta), and clearly subordinate workers behavioural plasticity is unique among social insects. (foragers) were selected for brain biogenic amine level Queenless ants could prove useful in investigating the measurements. neural bases of behavioural plasticity. Unlike wasps, workers in queenless ant species are restricted by ecological constraints from forming new, independent Sample preparation colonies; their access to reproduction is entirely dependent on their rank in the dominance hierarchy. The strict Ants were quickly frozen in liquid N2; the heads were behavioural castes created by this social system make dissected in dried ice and the brains without the optic lobes queenless ants a particularly suitable model for studying were kept at −80°C until amine analysis. Each brain was the interactions between social environment and individual treated separately. The brain was placed in an Eppendorf behaviour in a context of high relatedness and develop- tube on ice with 100 μl of cold perchloric acid (0.1 M) mental similarity among colony members. Indeed, unlike mixed with an internal standard (DHBA, 50 pg/μl). queen-right ants, queenless ants all experience the same Samples were sonicated (Ultra Turax T25, Bioblock developmental program—that of a worker. For this reason, Scientific, Vernon Hills, IL), chilled for 20 min, and then we will compare our results mainly to bibliographic data centrifuged at 13.000 rpm for 10 min (4°C). The superna- that concern workers, rather than queens of other social tant was collected and filtered through a syringe-connected species. filter (Durapore membrane, 0.45 μm, SIGMA), and 20-μl The clear behavioural interactions displayed in queen- volume of this solution was injected in the chromatography less societies allow one to determine the “behavioural circuit. state” of each worker through behavioural observation. As emphasized by Page and Erber (2002), this is an important prerequisite in studies on the effect of the biogenic amine HPLC dosage of brain octopamine, dopamine levels on social behaviour. We chose to investigate the and serotonin relationship between social dominance and brain levels of octopamine, dopamine and serotonin in a queenless ant Biogenic amines were dosed by high-pressure liquid whose behavioural castes are well-defined. Streblognathus chromatography (HPLC) coupled with electrochemical peetersi is a monogynous queenless ant with colonies detection (Antec, electrode potential fixed at 800 mV). organised in three distinct groups: the alpha ant is the most Samples were injected using a Rheodyne 7725i injector dominant worker and the only one to mate and lay eggs; valve with a 20-μl injection loop. Brain extracts were workers of high social ranks are behaviourally dominant separated on a reversed phase C18 column (3.2×100 mm, (while submissive to the alpha), are rather young and do 5-μm particle size, LC-22C, BAS, West Lafayette, IN). not mate or lay eggs; workers of low social ranks are The mobile phase (pH 3) was composed of 3% methanol, subordinate to the other two groups, can be of any age and 7% acetonitrile and 90% citric acid 20 mM, monobasic also remain infertile. This species is particularly useful for phosphate sodium 10 mM, octanesulfonic acid 3 mM, two reasons: alpha and high-ranked workers (ranks beta to heptanesulfonic acid 3.25 mM, EDTA 0.1 mM, KCl 2 mM, delta) each show clear behavioural displays that allow their o-phosphoric acid 6%, and diethylamine 2%. The flow immediate and unequivocal identification (Cuvillier-Hot et rate was adjusted at 0.3 ml/min with a Gold 118 system al. 2004). In addition, the individuals are very large (∼2cm (Beckman, Fullerton, CA), and temperature was set at long), making behavioural observation easier and allowing 35°C (technical set-up from (Kodas et al. 2002)). External amine dosage at the individual level. standards (octopamine, dopamine, serotonin) were run before and after each set of biological samples. Materials and methods Statistical analysis Ant collection and housing Kruskal–Wallis and Mann–Whitney tests were used to test Entire colonies of S. peetersi were collected near the significance of the observed differences (XLStatistics). Magoebaskloof, Limpopo province, South Africa: three Differences were considered significant at α≤0.05. in January 2000, two in February 2001 and seven in April 2001. The ants were kept in plaster nests in the laboratory, at 25°C and under 12 h light : 12 h dark cycles. Colonies Results were fed daily with cockroach nymphs and crickets. Each ant was labelled with a plastic number glued onto the Figure 1 compares the levels of octopamine, dopamine and thorax. The dominance hierarchy was established by serotonin for each social category. Octopamine levels were 151 a Octopamine level per brain Discussion 1,6 * Individual dosage of brain monoamine levels revealed that 1,4 fertile alpha workers are clearly characterised by higher 1,2 levels of octopamine (Fig.