There is a general under-appreciation of the ubiquity of microorganisms and the varied roles they play in the biology and life history of multicellular organisms. Microbes can affect their hosts and associates in unexpected ways, often with profound ecological and evolutionary consequences (McFall-Ngai, 2002; Moran, 2002). If this is true for organisms that are not habitually affiliated with rotting organic matter, shouldn't microbial influence be exponentially higher in cockroaches, insects that seek out habitats saturated with these denizens of the unseen world? Our focus so far has been primarily on the role of microbes in the nutritional ecology of cockroaches. The diverse biosynthetic capabilities of microbes, however, allow for wide-ranging influences in cockroach biology.
Microbes may alter or dictate the thermal tolerance of their host. Hamilton et al. (1985) demonstrated that the sugar alcohol ribitol acts as an antifreeze for C. punctula-tus in transitional weather, and as part of a quick freeze system when temperatures drop. Because microbes produce significantly more five-carbon sugars than animals and because ribitol had not been previously reported in an insect, the authors suggested that microbial symbionts might be responsible for producing the alcohol or its precursors. Cleveland et al. (1934) indicated that the effects of temperature on the cellulolytic gut protozoans of Cryptocercus confine these insects to regions free from climatic extremes. These effects differ between the eastern and western North American species. If the insects are held at 20-23oC, the protozoans of C. clevelandi die within a month, whereas those of C. punctulatus live indefinitely.
Microbial products may act like pheromones. Because cockroach aggregation behavior is in part mediated by fecal attractants in several species, it is possible that gut microbes may be the source of at least some of the components. Such is the case in the aggregation pheromone of locusts (Dillon et al., 2000) and in the chemical cues that mediate nestmate recognition in the termite Reticuli-termes speratus (Matsuura, 2001).
Microbes may influence somatic development. There is a "constant conversation"between host tissues and their symbiotic bacteria during development, with the immune system of the host acting as a key player (McFall-Ngai, 2002). Aside from their profound effect on cockroach development via various nutritional pathways, bacterial mutualists may directly influence cockroach morphogenesis. It is known that gut bacteria are required for the proper postembryonic development of the gut in P. americana (Bracke et al., 1978; Zurek and Keddie, 1996); normal intestinal function may depend on the induction of host genes by the microbes (Gilbert and
Bolker, 2003). The highly complex and tightly coordinated interactions of Blattabacterium endosymbionts with their hosts during transovarial transmission and embryogenesis (Sacchi et al., 1988, 1996, 1998b) suggest that these symbionts may influence the earliest stages of cockroach development.
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