Cockroache; Ecology, behavior & history - W.J. Bell

phae, and plant tissue (Lepschi, 1989), and gut content
analyses have clearly established that many species in
tropical rainforest consume fungal hyphae and spores
(WJB, unpubl. obs.). Australian Ellipsidion spp. are often
associated with sooty mold, although it is not known if
they eat it (Rentz, 1996). No known cockroach specializes
on fungi, although species that live in the nests of fungusgrowing
ants and termites may be candidates.
All types of decaying plant tissues, whether foliage,
wood, roots, seeds, or fruits, are thoroughly permeated by
filamentous fungi (Kukor and Martin, 1986). The fungal
contribution to the nutrient budget of cockroaches, however,
is unknown. Chitin is the major cell wall component
of most fungi and constitutes an average of 10% of fungal
dry weight (range 2.6–26.2) (Blumenthal and Roseman,
1957). Although chitinases are apparently rare in
the digestive processes of most detritus-feeding insects
(Martin and Kukor, 1984), it is distributed throughout
the digestive tract of P. americana. The enzyme is related
to cannibalism and the consumption of exuvia (Waterhouse
and McKellar, 1961), but may also play a role in
breaking down fungal polysaccharides.
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 mycetocytes
or bacteriocytes, with each symbiont individually
enclosed in a cytoplasmic vacuole (Fig. 5.6A,C). They are
transmitted between generations vertically, via transovarial
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
m. (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 bacteroid,
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.
MICROBES: THE UNSEEN INFLUENCE 83