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Social evolution research seeks to explain the origin, maintenance, and diversification of both cooperative and competitive social traits. This goal requires understanding the character of social environments that mediate selection on these traits. The distribution of behavioral and genetic diversity within and across groups of social animals has received much attention (Krebs and Davies, 1997; Oxley et al., 2010; Waddington et al., 2010). In contrast, relatively little is known about the structure of diversity among natural groups of social microbes (Fortunato et al., 2003b; Vos and Velicer, 2006, 2008a; Gilbert et al., 2009; Köhler et al., 2009; Wilder et al., 2009; Wollenberg and Ruby, 2009). However, detailed knowledge of group composition is necessary for understanding the roles of mutation, migration, lateral gene transfer, genetic drift, and various forms of selection in shaping the evolution of social microbes in natural habitats.

Microbes engage in a wide range of social behaviors, both cooperative and antagonistic, that affect the evolutionary fitness of others (Velicer, 2003; West et al., 2007a; Nadell et al., 2009). Some of the most biologically complex forms of prokaryotic cooperation occur in the myxobacteria (order Myxococcales, ?-proteobacteria), which are best known for social development of multicellular, spore-bearing fruiting bodies in response to starvation (Shimkets et al., 2006). In particular, the predatory soil bacterium Myxococcus xanthus has become a model organism for the study of microbial sociality, including cooperative motility (Wu and Kaiser, 1995), social predation (Berleman and Kirby, 2009) and fruiting body formation (Shimkets et al., 2006), and its population biology (Velicer and Vos, 2009).

M. xanthus cells swarm in a coordinated manner through soil habitats in cohesive groups using two genetically distinct motility systems, one of which is obligately social [type IV pili-driven “S-motility” (Hodgkin and Kaiser, 1977; Wu and Kaiser, 1995)] and one of which allows individual cell movement (“A-motility”) (Hodgkin, 1979; Sun et al., 2011). Swarms of M. xanthus in the soil kill and lyse prey cells of other micro-bial species with secreted antibiotics and lytic enzymes (Rosenberg and Varon, 1984). Upon starvation, swarming cells aggregate and develop into multicellular fruiting bodies (Shimkets et al., 2006). In these fruiting body aggregates a minority of cells convert to metabolically quiescent spores, whereas many other cells within the fruiting body lyse, possibly to the benefit of sporulating cells (Nariya and Inouye, 2008). The precise advantages of sporulation within fruiting bodies are unknown, although several hypotheses have been proposed, including enhanced dispersal, increased germination and/or growth rates in high-density groups, and protection from predation and/or environmental insults [summarized in greater detail in Velicer and Vos (2009)]. Here “fruiting body group” and



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