1997). The most well known symbionts in this category belong to the clade referred to as Wolbachia, an ancient group that contains members with a variety of kinds of interactions with hosts. In arthropods, Wolbachia is primarily exploitative, undergoes transfer among host lineages, and has a plastic genome with ongoing recombination and containing phage-derived elements (Masui et al., 2000; Wu et al., 2004). In contrast, Wolbachia in filarial nematodes appear to have been strictly vertically transmitted during host diversification, are required by hosts for normal development, and have a smaller and more static genome, lacking phage (Foster et al., 2005). Population studies indicate that exploitative symbionts can act as a force for reproductive isolation of populations with different infections (e.g., Jaenike et al., 2006). Thus, symbionts likely contribute to the species richness of hyperdiverse taxa such as the insects, not only by enabling expansion of lineages into novel ecological niches through augmentation of metabolic capabilities but also by affecting mating systems and reproductive compatibility of populations. As in the case of symbionts such as Buchnera that have evolved as beneficial symbionts, exploitative symbionts can become essential for host reproduction because of coadaptation of host genomes (e.g., Dedeine et al., 2005). Thus, complex development dependence on symbiotic partners is possible even when the original association was not beneficial for the host.

The literature on symbiosis is vast and growing quickly, largely because of the insights based in genomics. Although symbiosis was once discounted as an important evolutionary phenomenon (e.g., Sapp, 2004), the evidence is now overwhelming that obligate associations among microorganisms and between microorganisms and multicellular hosts have been crucial in many landmark events in evolution, in the generation of phenotypic diversity, and in the origin of complex phenotypes able to colonize new environments. Such evidence is abundant for the symbiotic systems found in insects, which are far better understood than in the recent past, largely because of molecular and genomic studies. Examples from insects show that symbioses can result in specialized organs with unique development, innovations in metabolic capabilities that allow new lifestyles, defenses against natural enemies and other environmental challenges, constraints on evolutionary range, and ongoing acquisition of novel genes and capabilities.