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

ADDITIONAL MICROBIAL INFLUENCES
Fig. 5.9 Urate pellet excretion by adult female Parcoblatta fulvescens
in relation to the reproductive cycle and level of dietary
nitrogen. Filled triangles, 4.0% nitrogen diet; filled circles,
5.4% nitrogen diet; filled squares, 6.7% nitrogen diet. EC, egg
case formation; ECD, egg case deposition. From Cochran
(1986b), courtesy of Donald G. Cochran, with permission
from Elsevier Press.
gregation of the cockroaches that excrete urate pellets
(like Parcoblatta) potentially benefits when just one of
them exceeds its nitrogen threshold (Lembke and Cochran,
1990). In cockroach species in which the male transfers
urates to the female during or after mating (Mullins
and Keil, 1980; Schal and Bell, 1982), it would not be surprising
to discover that female mate or sperm choice decisions
are based on the size or quality of the nuptial gift
(Chapter 6). The diversity of modes of post-ovulation
provisioning of offspring observed in cockroaches (brood
milk, gut fluids, exudates) is likely to be rooted in the ability
of a parent to mobilize and transfer stored reserves of
nitrogen (Nalepa and Bell, 1997). Finally, cockroaches are
able to use the uric acid scavenged from the feces of birds,
reptiles, and non-blattarian insects, adding to the list of
advantages of a generalized coprophagous lifestyle (Schal
and Bell, 1982).
Bacteroids as Food
There is some evidence that fat body endosymbionts in
cockroaches and in the termite Mastotermes may be a direct
source of nutrients to developing embryos. During
embryogenesis a portion of the bacterial population degenerates,
with a concomitant increase in glycogen granules
in the cytoplasm as the symbionts degrade (Sacchi et
al., 1996, 1998b). Bacteroids are also reported to shrivel in
size, then disappear when a postembryonic cockroach is
starved (Steinhaus, 1946; Walker, 1965).
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. punctulatus
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 20–23 o C, 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 Reticulitermes
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
86 COCKROACHES