Biology Reference
In-Depth Information
Insects have one of the most successful lifestyles on earth. One important factor in their success
is that insects have adapted to a wide variety of diets. Such Þexible feeding habits have been brought
to insects, at least partially, by the endosymbionts they harbor. Actually, endosymbionts are fre-
quently observed near an insectÔs digestive tract, and it is widely accepted that endosymbionts play
important roles in the nutrition of host insects. More recently, however, it has become increasingly
apparent that symbiontsÔ contributions to their host are not limited to nutrition. In many cases,
insect endosymbionts, especially intracellular symbionts, are signiÝcant for their hosts not merely
as nutritional supplements but also as DNA-containing genetic elements (Ishikawa, 1989).
Insects display the full scope of endosymbiosis. Some cases seem to be very new in an
evolutionary sense because the association between host insect and symbiont is so loose and casual,
whereas others are so intimate that symbionts seem to be tightly integrated into the physiology of
the host. In the latter cases, endosymbionts appear to be cell organelles, and their signiÝcance for
the host is no less than that of mitochondria for a eukaryotic cell.
One important point to keep in mind is that symbiosis is not necessarily an equal association
in which both partners beneÝt equally. Mutualism, not to mention parasitism, is not always
maintained on a 50Ï50 basis. One associate usually takes more, sometimes much more, than the
other, or at least so it appears in the light of present knowledge. This is observed typically in
the endosymbioses between insects and microorganisms. For example, aphids appear to proÝt
disproportionately from the symbiosis with
symbionts, yet the symbiosis has lasted
for some 200 million years (Moran et al., 1993). On the other hand,
Buchnera
symbionts take
full advantage of insect hosts so as to propagate their progeny, whereas the hosts do not seem
to receive any reward, and yet they have harbored these selÝsh passengers for an evolutionarily
signiÝcant length of time (Werren and OÔNeill, 1997). Does such apparent unfairness really exist
in symbiosis, or is it a one-sided view of symbiosis because of our incomplete understanding of
this biological phenomenon?
This chapter outlines various types of interactions between insects and microorganisms,
both prokaryotic and eukaryotic, mainly from physiological and evolutionary points of view.
Detailed studies and recent advances in each of these interactions will be fully described in
subsequent chapters.
Wolbachia
TYPES OF ASSOCIATIONS BETWEEN INSECTS
AND MICROORGANISMS
Although microbes are ubiquitous both inside and outside insect bodies, the present discussion is
limited to interactions inside insect bodies.
does not necessarily refer to the inside of a cell
but may mean the gut lumen, which, strictly speaking, is the outside, or the intercellular or
intracellular space of the host tissue. Of course, the deeper inside the microbe resides, the more
intimate the interaction with the host insect.
Inside
G
M
UT
ICROBES
Since insects are surrounded by a great variety of microbes, they should harbor some of them in
the gut. It is likely that some of these gut microbes are enclosed by cells of the epithelial tissue
and eventually become mutualistic symbionts. Gut microbes of insects are composed of a wide
variety of species, including bacteria, archaea, and eukaryea, but their habitat in the insect body is
usually restricted to one region, most commonly the hindgut. In many phytophagous insects, this
region forms a large anaerobic chamber for the fermentation of cellulose and other ingested plant
polysaccharides. Well-known examples include termites (Breznak, 1984) and cockroaches (Bracke
et al., 1979). As in vertebrates, gut microbes of most insects are extracellular, either lying free in
the lumen or adhering to the gut wall. In this sense, the yeast
is an exception; it lives in
blind-ended caeca of the anobiid beetles that inhabit timber or other plant products (Jurzitza, 1979).
Taphrina
