Biology Reference
In-Depth Information
Some insect species contain several different types of symbionts in different
tissues, including the gut, Malpighian tubules, fat body, or gonads. Bacteroids,
spiroplasmas, rickettsia, mycoplasmas, or virus-like symbionts are found in dip-
teran testes, ovaries, pole cells, nurse cells, and gut wall cells. Symbionts in scale
insects are particularly diverse, with almost 20 different types of associations
described so far. In the leafhopper
Euscelidium variegatus
, specific bacteria are
essential for normal growth and development, breaking down uric acid in the
host cells and synthesizing amino acids and vitamins. Symbionts are involved in
normal egg development of
E. variegatus
; embryos artificially lacking symbionts
fail to develop normal abdomens. It is hypothesized that some genes from this
microorganism have been transferred to the nuclear genome of
E. variegatus
in
a manner parallel to that of mitochondria.
Some insects lacking their symbionts are apparently completely normal. For
example, in the beetle family Cerambycidae, all of which live in wood, some spe-
cies have symbionts, whereas others lack them. The hypothesis that symbionts
supply a nutrient deficiency in the insect's diet thus seems to be simplistic; some
insects feeding on a well-balanced diet have symbionts.
In some cases, symbionts increase the likelihood that an insect vector can trans-
mit (vector) a disease. For example, rickettsia-like organisms in the tsetse fly
Glossina morsitans morsitans
affect infection by the sleeping sickness trypano-
somes (
Welburn et al. 1993
). The rickettsia-like organisms produce endochitinases
in the tsetse gut that inhibit lectins in newly emerged adults. Tsetse flies lacking
the rickettsia-like organisms are less susceptible to trypanosomes (refractory).
In some cases, symbionts may prevent vector insects from transmitting patho-
gens (
Teixeira et al. 2008, Hughes et al. 2011, Pan et al. 2012
). For example, a
strain of
Wolbachia
induces cytoplasmic incompatibility and blocks transmission
of dengue in the mosquito
Aedes albopictus
(
Blagrove et al. 2012
).
Microbial symbionts are common in insects, but a full understanding of their
genetic and evolutionary role is an exciting area of research that is expanding at a
rapid rate (
Schwemmler and Gassner 1989, Moran and Baumann 2000, Zchori-Fein
and Bourtzis 2011
). In the cases that have been well studied, a genetic interplay
between insect host and primary (obligate) symbiont occurs; each supplies factors
to the other, and the microorganism has specific means of movement and relo-
cation within the insect. Insects must recognize a symbiont as “self” rather than
as foreign or the microorganism would be subject to the insect's immune system.
Our understanding of how microorganisms have become incorporated into insect
organ tissues and cells is advancing with the use of molecular tools.
