Identification of Cryptic Hosts for Two Inquiline Parasites of the Seed-Harvester Ant Pogonomyrmex and New Localities for P. Anergismus (Hymenoptera: Formicidae) (Report‪)‬

Members of the genus Pogonomyrmex are primarily seed-harvester ants, but the rarely encountered species P. anergismus and P. colei are permanent social parasites (inquilines) that live and reproduce in nests of related congeneric hosts (Parker and Rissing, 2002). These two inquilines apparently have lost the ability to produce workers, such that their continued survival relies on the worker caste of the hosts P. barbatus or P. rugosus. Inquiline queens must gain access to the host colony, coexist with its queen and workers, and then lay eggs that are reared as sexuals by the host workers. Because host colonies possess a highly evolved self-recognition system, social parasites possess a variety of strategies to gain access to the colonies and to exploit resources of hosts (Holldobler and Wilson, 1990). Hosts often are closely related species emphasizing the importance of host-parasite relatedness for successful invasion of colonies (Huang and Dornhaus, 2008). In this paper, we determine the identity of hosts for two inquiline species, report new localities for P. anergismus, and discuss factors that may influence dispersal of parasitic ants and the diversity of hosts that are parasitized. Historically, researchers recognized two distinct species of hosts according to morphology: P. barbatus and P. rugosus (Cole, 1954; Rissing, 1983; MacKay and Van Vactor, 1985). However, recent molecular evidence demonstrated that the morphology of this group was misleading and that the two morphologically defined hosts actually were composed of at least six cryptic and reproductively isolated lineages; three that resemble P. rugosus and three that resemble P. barbatus (Anderson et al., 2006). All six lineages are highly distinct according to multiple classes of molecular markers (Helms Cahan and Keller, 2003; Anderson et al., 2006; Schwander et al., 2007), suggesting that successful invasion of colonies by the inquiline parasite may be influenced by host-parasite relatedness. Additionally, four lineages of hosts have a unique structure of colonies and populations that are associated with an inability to produce workers within the lineage. In the social Hymenoptera, whether an individual develops as a worker or queen (reproductive-caste determination) typically is determined by environmental factors such as the type or amount of nutrition provided to developing larvae by worker ants (Wheeler, 1986). Under this type of developmental programming, eggs of identical genotype can become either a worker or queen phenotype. However, in many populations of hosts, the reproductive caste is determined strictly according to genotype, a phenomenon referred to as genetic-caste determination (for a review, see Anderson et al., 2008). Each population exhibiting genetic-caste determination is composed of two morphologically cryptic and reproductively isolated lineages. Each lineage lacks genetic components that produce the worker phenotype from within-lineage matings, so queens must outcross with a male of the alternate-lineage to produce workers (Helms Cahan et al., 2002; Julian et al., 2002; Volny and Gordon, 2002). Thus, all workers in colonies of both lineages are F1 inter-lineage hybrids, but this has no influence on reproductive isolation because such workers are functionally sterile. Queens mate with multiple males from their own and from the alternate lineage, and lineages remain genetically isolated because new reproductive females (future queens) result from within-lineage matings, and new males result from unfertilized (haploid) eggs produced by the pure-lineage queen. These populations are called dependent-lineage systems (Anderson et al., 2006) because production of workers is necessary to initiate a colony and to attain reproductive maturity. Thus, a dependent-lineage system is essentially an ant-ant obligate mutualism (Anderson et al., 2009), wherein each of the interacting lineages must acquire from its partner lineage the genes that permit exp