ORIGINAL PAPER Colony fusion causes within-colony variation in a parthenogenetic ant Katrin Kellner & Benjamin Barth & Juergen Heinze Received: 21 November 2009 / Revised: 26 November 2009 / Accepted: 28 November 2009 / Published online: 17 December 2009 # Springer-Verlag 2009 Abstract The evolutionary stability of cooperation and altruism in colonies of social insects requires that nestmates be to some extent related. An efficient system of discrimination against non-nestmates protects the nest against unrelated conspecifics, which might exploit or parasitize the colony. The co-occurrence of unrelated individuals in mature colonies therefore is a rare event that deserves more attention. Here, we report on the relatively common incidence of colony fusion in the ant Platythyrea punctata. Workers of this ant can produce genetically identical female offspring from unfertilized eggs through thelytokous parthenogenesis. Consequently, the majority of colonies has a “clonal structure” and consists of individuals with identical multilocus genotypes. Nevertheless, field observations indicate that a surprisingly large percentage of colonies contain workers belonging to two or more different genetic lineages. Much of this genetic heterogeneity is incompatible with eventual recombination or mutation events, but instead appears to result from colony fusion or the adoption of unrelated individuals. Indeed, colonies of P. punctata from the Dominican Republic and Barbados readily merged in the laboratory and, after elimination of one of the two reproductive workers, formed stable, genetically heterogeneous colonies. We discuss the possible causes and benefits of colony fusion in natural populations. Keywords Agonistic behavior . Colony takeover . Replacement . Thelytoky . Within colony relatedness . Platythyrea punctata Introduction The maintenance of cooperation and altruism in animal societies requires that they protect themselves against parasites and exploitation by unrelated individuals. The colonies of social insects are generally closed systems, which maintain their integrity through a colony odor shared by all nestmates and an efficient discrimination against non-nestmates (Wilson 1971; Crozier and Dix 1979; Hölldobler and Wilson 1990, 2009). Even in the so-called “unicolonial ” species, which have been thought to completely lack colony borders (Tsutsui and Suarez 2003; Chapuisat et al. 2005), aggression between supercolonies has recently been found (Vogel et al. 2009). The adoption of alien individuals or even the complete merger of two unrelated colonies is usually a rare event (Foitzik and Heinze 1998; van Wilgenburg et al. 2006; Johns et al. 2009). The occurrence of colony fusion, its causes and consequences are therefore of considerable interest for our understanding of the stability and dynamics of insect societies. Colony fusion may be difficult to detect when colonies have a complex genetic architecture, e.g., when queens mate multiply or when multiple queens produce offspring. It is more clearly visible when individual colonies are genetically homogeneous, as in Platythyrea punctata (F. Smith 1858). In Communicated by L. Keller K. Kellner (*) : B. Barth : J. Heinze Biologie I, University of Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany e-mail: katrin.kellner@biologie.uni-regensburg.de Present Address: B. Barth Institut für Biologie, Martin-Luther-Universität Halle-Wittenberg, Hoher Weg 4, 06099 Halle/Saale, Germany Behav Ecol Sociobiol (2010) 64:737–746 DOI 10.1007/s00265-009-0891-6