Bacteroids

Bacteroids are symbiotic gram-negative bacteria of the genus Blattabacterium living in the fat body of all cockroaches and of the termite Mastotermes darwiniensis. The endosymbionts reside in specialized cells, called myceto-cytes or bacteriocytes, with each symbiont individually enclosed in a cytoplasmic vacuole (Fig. 5.6A,C). They are transmitted between generations vertically, via transo-varial transmission, a complex, co-evolved, and highly coordinated process (Sacchi et al., 1988; Wren et al., 1989; Lambiase et al., 1997; Sacchi et al., 2000). DNA sequence analyses indicate that the phyletic relationships of the bacteroids closely mirror those of their hosts, with nearly equivalent phylogenies of host and symbiont (Bandi et al., 1994,1995; Lo et al., 2003a) (Fig. 5.7). Bacteroids synthesize vitamins, amino acids, and proteins (Richards and Brooks, 1958; Garthe and Elliot, 1971) but the symbiotic relationship appears grounded on their ability to recycle nitrogenous waste products and return usable molecules to the host (Cochran and Mullins, 1982; Cochran, 1985; Mullins and Cochran, 1987). The establishment of the urate-bacteroid system in the cockroach-termite lineage occurred at least 140 mya (Lo et al., 2003a), and was an elaborate, multi-step process. It involved the regulation or elimination of urate excretion, the intracellular integration of the bacteroids, the evolution of urate and mycetocyte cells in the fat body, and the coordination of the intricate interplay between host and symbionts during transovarial transmission (Cochran, 1985).

Fig. 5.6 Transmission electron micrographs of the fat body of Cryptocercus punctulatus. (A) Bacteriocyte with cytoplasm filled by symbiotic bacteria (g = glycogen granules; m = mitochondria; arrows = vacuolar membrane). Scale bar = 2.2 jxm. (B) Urocyte of C.punctulatus. Note the crystalloid subunit arranged concentrically around dark cores of urate structural units. Scale bar = 0.8 ^m. (C) Detail of a bacteriocyte showing glycogen particles (arrows) both enclosed in a vacuolar vesicle and within the vacuolar space surrounding the bac-teroid, suggesting exchange of material between host cell cytoplasm and the endosymbiont. Scale bar = 0.5 ^m. From Sacchi et al. (1998a); photos courtesy of Luciano Sacchi.

Fig. 5.6 Transmission electron micrographs of the fat body of Cryptocercus punctulatus. (A) Bacteriocyte with cytoplasm filled by symbiotic bacteria (g = glycogen granules; m = mitochondria; arrows = vacuolar membrane). Scale bar = 2.2 jxm. (B) Urocyte of C.punctulatus. Note the crystalloid subunit arranged concentrically around dark cores of urate structural units. Scale bar = 0.8 ^m. (C) Detail of a bacteriocyte showing glycogen particles (arrows) both enclosed in a vacuolar vesicle and within the vacuolar space surrounding the bac-teroid, suggesting exchange of material between host cell cytoplasm and the endosymbiont. Scale bar = 0.5 ^m. From Sacchi et al. (1998a); photos courtesy of Luciano Sacchi.

Fig. 5.7 Phylogeny of dictyopteran species and a comparison with the phylogeny of endosymbiotic Blattabacterium spp. The host phylogeny was based on a combined analysis of 18S rDNA and mito-chondrial COII, 12S rDNA, and 16S rDNA sequences. Tree length: 2901, consistency index: 0.55. Bold lines indicate those dictyopteran taxa that harbor Blattabacterium spp., and that were examined in host endosymbiont congruence tests. The asterisk indicates the only node in the topology that was in disagreement with that based on host phylogeny. From Lo et al. (2003a), reprinted with permission from Nathan Lo and the journal Molecular Biology and Evolution.

Fig. 5.7 Phylogeny of dictyopteran species and a comparison with the phylogeny of endosymbiotic Blattabacterium spp. The host phylogeny was based on a combined analysis of 18S rDNA and mito-chondrial COII, 12S rDNA, and 16S rDNA sequences. Tree length: 2901, consistency index: 0.55. Bold lines indicate those dictyopteran taxa that harbor Blattabacterium spp., and that were examined in host endosymbiont congruence tests. The asterisk indicates the only node in the topology that was in disagreement with that based on host phylogeny. From Lo et al. (2003a), reprinted with permission from Nathan Lo and the journal Molecular Biology and Evolution.

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