Since the origin of life on Earth approximately 4.6 billion years ago, microscopic organisms have evolved into the most abundant and diverse life forms on the planet. They have played key roles in biogeochemical processes, including helping convert the Earth’s atmosphere into an oxygen rich biosphere and recycling organic compounds in every ecosystem (Pace, 1997; Hoehler et al., 2001; Dawson and Pace, 2002; Baldauf, 2003). However, it is the capacity microbes have to form symbiotic associations with other organisms that has led to many major evolutionary transitions that shape our planet. Examples include the formation of the eukaryotic cell and the colonization of land by plants (Margulis, 1970; Selosse et al., 2004). In addition to these mutualistic host-microbe associations, parasitic microbes shape the ecology and evolution of every living organism, incur a strong selection pressure on their hosts, and are recognized as a major force driving host evolution and biological diversi cation (Jaenike, 1978; Hamilton, 1980; Price et al., 1986; Ewald, 1994). Thus, symbiotic microbes, both as parasites and mutualists, in uence the biology of every living organism (Boucher et al., 1982; Ewald, 1994; Lutzoni et al., 2001; Jaenike and Perlman, 2002; Sanders, 2002).