Inherited Bacteroidetes symbionts in arthropods

The variety of partnerships between eukaryotes and prokaryotes is astounding in the arthropods, where bacteria live within body cavities, somatic cells, and germ line cells, and perform a vast array of functions. The taxonomic diversity of the prokaryotes inhabiting arthropod guts is extremely broad, and distributed among the Bacteria and Archea (Brauman et al., 2001; Dillon and Dillon, 2004) . This chapter, however, focuses on the more specialized intracellular symbionts, with exclusive or almost exclusive transmission from mother to offspring. Intracellular symbionts must possess adaptations for living within eukaryotic cells, and be able to recognize and invade germ line cells or segregate to the germ region in early embryos. Also, strictly vertically transmitted symbionts will only spread in a population if infected host females produce more infected daughters than uninfected host females produce (uninfected) daughters (Bull, 1983). Thus, inherited symbionts must either contribute directly to the fitness of their hosts or manipulate host reproduction in ways that increase the relative number or fitness of females. Because of the specialization required in living within the cell, as well as in vertical transmission, the number of lineages involved in this lifestyle appears limited. Many inherited symbionts are in the Proteobacteria, where the closest relatives to mitochondria are found. Some arthropod symbionts in the Proteobacteria that are required for host nutrition include Buchnera, Wiggleworthia, Blochmannia, Carsonella, Tremblaya, Nardonella, and Portiera (Munson et al., 1991; Aksoy, 1995; Sauer et al., 2000; Thao et al., 2000; Thao et al., 2002; Lefevre et al., 2004; Thao and Baumann, 2004). Facultatively associated arthropod symbionts in the Proteobacteria include Wolbachia, Rickettsia, Arsenophonus, Sodalis, Hamiltonella, and Regiella (Gherna et al., 1991; Dale and Maudlin, 1999; Moran et al., 2005a). In recent years, inherited arthropod symbionts have been discovered in groups such as Spiroplasma (Mollicutes within the Firmicutes) (Ludwig and Klenk, 2001) and Chlamydia, a small phylum of largely vertebrate pathogens (Corsaro and Venditti, 2004). Nowhere has there been a greater burst of discovery of symbionts, however, than in the Bacteroidetes (= Cytophaga-Flexibacter-Bacteroides, or CFB).