Hormonal Correlates of Dominance Ranking in Colonies of the Queenless Ant

1. Hormonal Correlates of Dominance Ranking in Colonies of the Queenless Ant Streblognathus peetersi (Hymenoptera: Ponerinae) Colin S. Brent, School of Life Sciences, Arizona State University INTRODUCTION Streblognathus peetersi is a queenless ant with morphologically similar workers that can all potentially reproduce1. Agonistic interactions among nestmates establish a dominance hierarchy to regulate reproductive capability2-4. Three different social categories are established; alpha, high rank, and low rank. High ranking workers show frequent aggression and have better developed ovaries than low rankers. Juvenile hormone (JH) and ecdysteroids, primary regulators of insect behavior and oogenesis5-7, likely underlie the observed status differences among S. peetersi workers. Topically applied JH analogue decreases fertility in alpha females and causes a loss of their reproductive status. To better understand the role of JH and ecdysteroids in S. peetersi, hormonal profiles of differently ranked workers were examined. RESULTS The behavioral and endocrinological shift from a subordinate high ranker to a replacement alpha is rapid; high ranking females showed alpha specific behaviors within 24 hrs of removing the alpha. After 48 hrs, the replacement alphas had JH production rates equivalent to that of older alphas (0.189 ± 0.077 pmol/hr, n=5; ANOVA on ranks, Q=0.805, p>0.05). Low rankers had the fastest rate of JH production, 4X faster than high rankers, and 240X faster than alphas. Ecdysteroid content also varied between ranks, increasing as rank decreased. CONCLUSIONS The results suggest that JH suppresses ovarian activity and aggression, while ecdysteroids may promote vitellogenesis. The loss of the queen phenotype in S. peetersi would have necessitated the development of a novel means for controlling reproductive activity, evolving within the constraints created by the worker phenotype. One potential explanation for our current findings is that the worker endocrine system retains some characteristics found in immature stages, for which high concentrations of circulating JH can inhibit the development of adult features, such as functional ovaries. METHODS Colonies were collected near Magoebaskloof, Limpopo province, South Africa. Colonies were reared in plaster nests, at 25°C, under a 12:12 light:dark cycle and were provided meal worms, 20% sugar water and tap water ad libitum. Workers were individually marked with enamel paint, and rank was determined by recording behaviors during two 10-min periods each day, over six days. High rankers show aggression; alphas display stereotypical gaster curling; low rankers are non-aggressive.3The in vitro biosynthetic rates of JH production in individual workers of low, high and alpha rank were measured by a rapid-partition radiochemical assay8. JH production was also measured in 48hr old replacement alphas. Ecdysteroid titer of hemolymph pooled from pairs of workers was determined by radioimmunoassay.9 • REFERENCES • Robertson HG (2002) Zootaxa 97: 1-16 • Monnin T, Peeters C (1999) Behav. Ecol. 10: 323-332 • Cuvillier-Hot V, Lenoir A, Crewe R, Malosse C, Peeters C (2004) An. Behav. 68: 1209–1219 • Cuvillier-Hot V, Lenoir A, Peeters C (2004) Behav Ecol 15: 970–975 • Goodman WG, Granger NA (2005) Comp. Mol. Insect Sci., Vol 3, pp 320-408 • Raikhel AS, Brown MR, Belles X (2005) Comp. Mol. Insect Sci., Vol 3, pp 433-491 • Swever BL, Raikhel AS, Sappington TW, Shirk P, Iatrou K (2005) Comp. Mol. Insect Sci., Vol 1, pp 87-155 • Brent CS, Vargo EL (2003) J Insect Physiol 49: 967–974 • Brent CS, Schal C,Vargo EL (2005) J Insect Physiol, in press Figure 1. Mean (± s.e.) in vitro rate of juvenile hormone (JH) release (red bars) and ecdysteroid titer (black bars) for low, high or alpha rankers. Sample sizes are indicated above each bar. Significant (p<0.05) pairwise differences between groups are indicated with different letters.