Biology Reference
In-Depth Information
The overwhelming majority of endosymbionts of insects indisputably have been acquired with
food through the mouthparts. However, evidence suggests that there may be additional routes of
acquisition. The tracheae of insects often contain bacteria generally related to those of the milieu
.
Also, the hemolymph may contain microbes without injury to the insect body. Interestingly,
however, these two Þora are not related to the intestinal Þora of the same insect, suggesting that
the spiracle can provide a route of acquisition. Honeydew, the sweet excrement of
Homoptera
, may
also have led occasionally to anal acquisition of microbes. Since
Rickettsia
species are known to
use this route at times, it is worth considering that
, a group of so-called guest microbes,
may use the same route in their horizontal transmission (see below).
Wolbachia
E
NDOPARASITISM
Parasitism is a one-sided symbiosis in which one of the associants beneÝts largely at the expense
of the other. As noted by de Bary (1879) and Starr (1975), however, in some cases it is difÝcult to
draw a clear line between parasitism and symbiosis. For the sake of convenience, I distinguish
between the two based on the mode of transmission. If an organism is transmitted from one host
to another only horizontally, I regard it here as a parasite.
It is worth noting that mutualistic symbionts of insects are always unicellular, whether
prokaryotic or eukaryotic, whereas parasites may be unicellular or multicellular. Generally
speaking, insect hosts suffer more from multicellular endoparasites. Endoparasitic associations
involving insects can be categorized into four types. The Ýrst type is one in which insects serve
as vectors of pathogenic protozoans harmful to vertebrates. A noticeable characteristic of this
association is that insect hosts suffer very little, if at all, from their unicellular parasites, from
which vertebrate hosts usually suffer disastrously. As far as insects are concerned, it may be
more accurate to refer to these pathogenic protozoans as commensals rather than parasites. One
typical example of these parasites is hemoÞagellates, which inhabit the gut of some insects. The
hemoÞagellates that invade the reticuloendothelial tissue of vertebrates include
Trypanosoma
brucei,
, which cause African sleeping sickness, ChagasÔ
disease, and Oriental sore, respectively. Little is known about how insect hosts are affected by
these parasites. Another well-known example of parasites of this type is the
T. cruzi
, and
Leishmania donovani
Plasmodium
species,
malaria parasites that are transmitted by female
Anopheles
. Although mosquito genes that are
responsible for allowing the infection of
have been identiÝed, little is known about
how the insects are affected by these parasites (Ito et al., 2002).
The second type of insect endoparasitism involves nematodes. Among more than 3000 nema-
todeÏinsect associations, a remarkable example involves the nematode
Plasmodium
. The nematode
carries bacterial symbionts and releases them when inside the insectÔs alimentary canal. The
symbionts invade the insectÔs body cavity, rapidly multiply there, and cause the death of the insect
host. Subsequently, nematodes develop rapidly in the dead host and become sexually mature
(Nickle, 1984). Although endoparasitisms with nematodes that modify the insectÔs morphology,
physiology, and behavior are considered to be a potentially important tool of biological control
over noxious insects, their biochemical and molecular mechanisms are largely unknown.
The third type of endoparasitism is characterized by the fact that both the host and parasite are
insects. In this association, parasitic insects are usually called parasitoids, which are either
hymenopterous or dipterous insects, because their behavior is somewhat similar to that of predators,
and the association necessarily results in the hostÔs death. Although the interaction between hosts
and parasitoids includes many interesting aspects from physiological and biochemical points of
view, a detailed description of this association is beyond the scope of this paper.
In addition to the parasitoids mentioned above, probably the most aggravating parasites to insects
are fungi. These organisms comprise the fourth type of endoparasitic association with insects. In
almost every conceivable ecological niche of insects, fungi have developed associations with insects
that range from casual to intimate (Anderson et al., 1984). This is, at least, partly because the two
Neoaplectana
