Genetic caste determination in Pogonomyrmex harvester ants

Division of labour between reproductive queens and sterile workers is a hallmark of insect societies and an important component of their ecological success (Hölldobler & Wilson, 1990). In ants and other social Hymenoptera forming large and complex societies, reproductive division of labour is generally associated with marked morphological differences between the queen and worker castes. These differences usually result from the combined effects of environmental and social factors such as diet, temperature and colony size inducing female larvae to engage into the queen or worker developmental pathway (Brian, 1957; Wheeler, 1994). However, genetically determined caste differentiation has been recently discovered in two populations (‘Hidalgo’ and ‘Junction’) of Pogonomyrmex harvester ants (Helms Cahan et al., 2002, 2004; Julian et al., 2002; Volny & Gordon, 2002b; Helms Cahan & Keller, 2003). Each population consists of two genetically distinct and interbreeding lineages (H1 and H2 at Hidalgo and J1 and J2 at Junction). Queens mate multiply (Hölldobler, 1976; Gadau et al., 2003) with males of their own lineage and males of the alternate, co-occurring lineage. Pure-lineage offspring develop into queens while inter-lineage females virtually always develop into workers (Helms Cahan et al., 2002; Julian et al., 2002; Volny & Gordon, 2002b; Helms Cahan & Keller, 2003). In both the Hidalgo and Junction populations, this mode of caste determination is obligate as pure-lineage females almost invariably fail to develop into workers (Helms Cahan Correspondence: Tanja Schwander, Department of Ecology and Evolution, Biology Building, University of Lausanne, 1015 Lausanne, Switzerland. Tel.: 0041 21 692 41 81; fax: 0041 21 692 41 65; e-mail: [email protected]