Biology Reference
In-Depth Information
beneÝts to the host insects are often clear and evidenced by experimental elimination of the
symbiont. By contrast, it is much less clear how symbionts truly beneÝt from the association. These
mycetocyte symbionts seem to be domesticated by the host insects. In this respect, their relation
to insects somewhat resembles that of livestock to us.
In a similar analogy, the other type of intracellular symbionts of insects may be compared to
cockroaches, house rats, and mice, which sometimes share habitats with us and unilaterally exploit
us. These symbionts are collectively called Ñguest microbesÒ (Ebbert, 1993). Guest microbes also
differ from mycetocyte symbionts in that they are not restricted to a particular cell type, such as
the mycetocyte, but are present in almost all cell types of the host insect. In this respect, again,
they resemble
cockroaches rather than livestock.
MUTUALISTIC SYMBIONTS IN INSECTS
F. Blochmann Ýrst discovered the rod-shaped and bacteria-like symbionts in insect cells. These
symbionts were later named Blochmann bodies in honor of the discoverer (Lanham, 1968). The
Blochmann body corresponds to the mycetocyte symbiont in todayÔs terminology. Guest microbes
are a very different type of inhabitant of the insect cell and were discovered comparatively recently.
In view of their important inÞuence on the insect, these microbes, despite the analogy mentioned
above, may mean much more to insects than do cockroaches or house rats to human beings. In
this section and the next, these two symbiont types will be described in greater detail.
M
S
YCETOCYTE
YMBIONTS
Mycetocyte symbionts are especially characteristic of three insect groups: the order Blattaria
(cockroaches), the order Homoptera (cicadas, leafhoppers, psyllids, aphids, coccids, etc.), and the
family Curculionidae (weevils) of the order Coleoptera (Dasch et al., 1984). These symbionts are
housed in the highly specialized somatic cells of the host insect, termed mycetocytes. The term
was coined based on an early observation that the symbionts were fungi. The insect cell that harbors
bacterial symbionts is more accurately called Ñbacteriocyte.Ò Frequently, however, ÑmycetocyteÒ is
still used regardless of which symbionts are harbored by the cell.
Mycetocytes, or bacteriocytes, are sometimes assembled into a discrete organ called a myce-
tome, or bacteriome (Buchner, 1965). The location of mycetocytes and mycetomes varies depending
on the host species. Mycetomes of weevils and other insects form part of the cellular lining of the
midgut so that the symbionts have ready access to the lumen of the alimentary canal and often are
found there extracellularly (Brooks, 1963). When insects have mycetomes of this type, the associ-
ation between host and symbiont is generally not very intimate, and these symbionts are possibly
cultivable extracellularly on common culture media. In cockroaches and homopterans, mycetocytes
are separated from the digestive tract, lying either in the body cavity or embedded in the fat body.
In the insect with mycetocytes of this type, host and symbiont are, as a rule, closely interdependent,
and thus the symbionts are not generally cultivable
extracellularly. In proportion to the large cell
size, mycetocyte nuclei are generally enlarged, lobed, and polyploid. It has been estimated that
mycetocytes of cockroaches range from tetraploid in the embryo to 512-ploid for some of the
largest cells in adults (Richards and Brooks, 1958).
In most cases, mycetocyte symbionts demonstrate morphological features characteristic of
prokaryotes, usually bacteria, although a great variation in shape is observed from rods to lobed
vesicular bodies. In many cases, the symbionts are surrounded by two endogenous membranes and
an outer, third membrane derived from the host. Whereas the innermost membrane, which encloses
the symbiontÔs cytoplasm, shows little specialization, the second membrane generally forms the
lipopolysaccharideÏlipoprotein layer, which is characteristic of Gram-negative bacteria, or proteo-
bacteria. The structure is thinner and more elastic than in free-living bacteria, showing considerable
adaptation to intracellular life (Houk and GrifÝths, 1980).
