Biology Reference
In-Depth Information
organisms share an important substance. Both contain chitin as a structural component in the cell
wall of fungi and in the exoskeleton of insects. Species of
Beauveria
,
Cordyceps
,
Coelomomyces
,
Entomorphthora
are well-known examples of fungi that are pathogens of many
insects. These parasitic fungi infect insect hosts, assimilate their hemolymph, digest soft tissue, and
produce mycotoxins, which kill the host. A typical example of this association is the parasitism by
, and
Metarrhizium
Cordyceps
attack many different types of insects and spiders. The infected hosts shrivel
and dry after death but resist decay because of an antibiotic compound, cordycepin, produced by
the fungus. After the host dies, the fungus produces a cylindrical structure that grows out from the
host and produces many fruiting bodies, which contain spores. Dried bodies of insects with the
outgrowth of fruiting bodies of
.
Cordyceps
are called Ñplant wormÒ and are believed to contain
physiologically active substances good for our health (Anderson et al., 1984).
Not all parasitic fungi are harmful to insects. There are many fungusÏinsect associations in
which the hosts are not harmed by or even beneÝt from fungi. A typical example of these associations
is fungus gardens in which some species of termites and ants cultivate fungus for food. Termites
use fungi as a supplemental source of vitamins, whereas ants rely entirely on the fungi for their
nutrients. The fungi are grown in specially prepared beds that contain a mixture of plant materials
and insect excrement. Unlike pathogenic fungi, all these beneÝcial fungi stay outside the hostÔs
body. Once taken inside, these fungi may
Cordyceps
become mutualistic symbionts that will be vertically
transmitted through the hostÔs generations (Ahmadjian and Paracer, 1986). Indeed, molecular
phylogenetic evidence suggests that intercellular symbionts found in planthoppers and some aphid
species are close relatives of an entomogenous fungus (see below).
E
S
XTRACELLULAR
YMBIOSIS
All mutualistic endosymbionts that are vertically transmitted through generations of host insects
are invariably unicellular organisms, either prokaryotic or eukaryotic, and either extracellular or
intracellular. Although most of the gut microbes mentioned above are extracellular symbionts in a
broader sense, their association with insects must be renewed whenever the hostÔs generation is
renewed. Other microbes penetrate the insect body more deeply to maintain a somewhat more
prolonged and intimate association with the host. However, the location of these microbes is quite
variable, and the same species are quite often found in the gut lumen and even inside certain cells,
probably reÞecting the fact that these microbes are at transient stages from mutualistic gut inhab-
itants to intracellular symbionts. Some homopteran insects, including scale insects, cicadas, and
aphids, harbor yeast-like microbes in the space between hypodermis and the intestinal tract. Some
aphid species belonging to the tribe Cerataphidini have been shown to harbor yeast-like extracellular
symbionts in the hemocoel and fat body, instead of
, which are intracellular bacterial
symbionts common to the rest of the family Aphididae (see below).
Buchnera
I
S
NTRACELLULAR
YMBIOSIS
Many insect species harbor intracellular symbionts that are vertically transmitted through their
generations. If the transient stages mentioned above are included, the percentage of insect species
that contain intracellular symbionts would be as high as 70% or more. Intracellular symbiosis is
the most intimate association between two different organisms, and it is generally reasoned that
the association is maintained through the hostÔs generations because the host and symbiont equally
beneÝt from the association. In reality, however, it does not seem to apply in intracellular symbiosis
between insects and microorganisms.
Intracellular symbioses of insects with microorganisms are categorized into two types, which
are (or at least appear to be) quite different from each other. One is the so-called mycetocyte
symbiosis, in which symbiotic microbes are harbored by the host mycetocyte, or bacteriocyte, a
special cell differentiated for this purpose (Buchner, 1965; Ishikawa, 1989). In this association, the
