Research article Population- and sociogenetic structure of the leaf-cutter ant Atta colombica (Formicidae, Myrmicinae) M. Helmkampf 1 , J. Gadau 2 and H. Feldhaar 3, * 1 Zoological Museum of the University of Hamburg, Martin-Luther-King-Platz 3, D-20146 Hamburg, Germany, e-mail: helmkampf@bni-hamburg.de 2 School of Life Sciences, Arizona State University, PO Box 874501, Tempe, AZ 85287–4501, USA, e-mail: Juergen.Gadau@asu.edu 3 Behavioral Physiology and Sociobiology, Biocenter of the University of Würzburg, Am Hubland, D-97074, Würzburg, Germany, e-mail: feldhaar@biozentrum.uni-wuerzburg.de Received 18 March 2008; revised 14 July 2008; accepted 18 July 2008. Published Online First 11 August 2008 Abstract. Relatedness and genetic variability in colonies of social insects are strongly influenced by the number of queens present and the number of matings per queen, but also by the genetic variability in the population. Thus, multiple paternity will enhance within-colony genetic variability more strongly when the males a queen mates with are unrelated. To study the kin-structure within colonies of the leaf-cutter ant Atta colombica and the population structure of this species around Barro Colo- rado Island, Panama, we developed five polymorphic microsatellite loci with a range of three to 17 alleles in At. colombica, all of which cross-amplify in other higher attines as well. The average effective mating frequency calculated from four-locus microsatellite genotypes was 1.89 Æ 0.12 (harmonic mean Æ SE) and thus slightly lower than the average observed mating frequency of 2.50 Æ 0.11 (arithmetic mean Æ SE) over the 55 colonies studied, confirming former studies that utilized fewer loci. The discrepancy between observed mating frequency and effective mating frequency is most probably due to paternity skew within colonies. The study population proved to be genetically diverse and in Hardy-Weinberg equilibrium, suggesting random mating within the study area. No population substructure was observed, neither considering nuclear (global F ST = 0.011 Æ 0.003 SE) nor mitochondrial markers (mean F ST = 0.008). Consequent- ly, gene flow is obviously promoted by both sexes across the range investigated here. Thus, multiple mating and long-distance dispersal appear to be two interconnected behavioural mechanisms to create and maintain genetic diversity in At. colombica. The advantages of this system are partly offset by paternity skew and the non-zero relatedness among colony fathers found in the study population. Keywords: Ants, Attini, multiple mating, microsatellites, polyandry, population genetics. Introduction High degrees of multiple mating by queens occur in only a few genera of social insects. In ants, monandry, or single mating by queens, is probably the ancestral state (Ham- ilton, 1964; Boomsma, 2007), and high mating frequencies in ants have been found only in a few genera. Ants that mate multiply typically have large colonies with polymor- phic workers and a high degree of division of labor, as seen in army ants of the genera Eciton, Dorylus , Aenictus and Neivamyrmex (Denny et al., 2004; Kronauer et al., 2004; JaffØ et al., 2007), seed-harvester ants of the genus Pogonomyrmex (Gadau et al., 2003; Wiernasz et al., 2004; Pol et al., 2008), and leaf-cutter ants of the genera Acromyrmex and Atta (Fjerdingstad et al., 1998; Bekke- vold et al., 1999; Boomsma et al., 1999; Villesen et al., 1999; Fjerdingstad and Boomsma, 2000; Murakami et al., 2000; Sumner et al., 2004). Although the lower relatedness within such colonies may pose a threat to eusociality (Hamilton, 1964), it has been shown that enhanced genetic variability within colonies as a consequence of multiple paternity is advantageous because it may enhance resist- ance towards parasites (Baer and Schmid-Hempel, 1999, 2001; Hughes and Boomsma, 2004, 2006; Tarpy and Seeley, 2006; Seeley and Tarpy, 2007), lead to higher colony * Author for correspondence. Insect. Soc. 55 (2008) 434 – 442 0020-1812/08/040434-9 DOI 10.1007/s00040-008-1024-3  Birkhäuser Verlag, Basel, 2008 Insectes Sociaux