Biology Reference
In-Depth Information
FUNCTIONAL ROLE OF TSETSE SYMBIONTS
It has been difÝcult to study the individual functions of the multiple symbionts in tsetse. Attempts
to eliminate the symbionts by administration of antibiotics, lysozyme, and speciÝc antibodies have
resulted in retarded growth of the insect and a decrease in egg production, preventing the ability
of the aposymbiotic host to reproduce (Nogge, 1976, 1978, 1980). The ability to reproduce, however,
can be partially restored when the aposymbiotic tsetse Þies receive a blood meal that is supplemented
with B-complex vitamins (thiamine, pantothenic acid, pyridoxine, folic acid, and biotin), suggesting
that the endosymbionts probably play a role in metabolism that involves these compounds (Nogge,
1981). The sequenced genome of
Wigglesworthia,
which has extensive vitamin-coding capabilities,
indeed supports this role, as described earlier.
The functional signiÝcance of
Sodalis
for tsetse biology is unknown at present. Its genomic
aspects, such as its size, A+T bias, and genome contents, are reminiscent of free-living organisms
rather than mutualists. While all tsetse species harbor bacteriome-associated
Wigglesworthia
, the
prevalence of
Sodalis
infections in the various tsetse species studied has been shown to vary widely
(Cheng and Aksoy, 1999). In at least one study, it has been possible to preferentially eliminate
Sodalis
by using the sugar analog and antibiotic streptozotocin without drastically reducing Ýtness
of the Þies, implying that its presence may be dispensable (Dale and Welburn, 2000). The presence
of
Sodalis
in tsetse has also been implicated in enhancing the susceptibility trait of tsetse for
trypanosome transmission (Welburn and Maudlin, 1991). It has been shown that
Sodalis
produces
at least one type of chitinase enzyme that may be responsible for the increased trypanosome
susceptibility of its tsetse host (Welburn et al., 1993; Welburn and Maudlin, 1999).
The
W. pipientis
-like symbiont detected from several tsetse species has been found to infect a
wide range of invertebrate hosts. In one survey in the neotropics, more than 15% of the analyzed
taxa were reported to carry this group of microorganisms (Werren et al., 1995). The functional
presence of
Wolbachia
has been shown to result in a variety of reproductive abnormalities in the
various hosts they infect. One of these abnormalities is termed cytoplasmic incompatibility (CI)
and when expressed commonly results in embryonic death due to disruptions in early fertilization
events (Hoffmann and Turelli, 1997). In an incompatible cross, the sperm enters the egg but does
not successfully contribute its genetic material to the potential zygote. In most species, this results
in very few hatching eggs. The infected females have a reproductive advantage over their uninfected
counterparts as they can produce successful progeny with both the imprinted and normal sperm.
This reproductive advantage allows the infected insects to spread into populations. Most functional
studies have involved curing insects of their
Wolbachia
infections by administering antibiotics in
their diet. This approach, however, has not been feasible in tsetse because the antibiotic treatment
of Þies results in the clearing of all bacterial symbionts, including
Wigglesworthia
, which results
in Þy sterility. The analysis of tsetse laboratory colonies has shown that 100% of sampled individuals
carry
Wolbachia
infections, making the analysis of
Wolbachia-
mediated effects impossible by
traditional mating experiments. The infection prevalence in Ýeld populations, however, has shown
signiÝcant polymorphism (Cheng et al., 2000). Perhaps
Wolbachia-
infected and uninfected lines
can be developed from these polymorphic Ýeld populations and used to elucidate the functional
role of this organism in tsetse biology.
SYMBIONT-HOST INTERACTIONS
In addition to differences in genomic characteristics and evolutionary history with the tsetse host,
symbionts also display differences in tissue tropism as shown by a PCR-based assay. While
Wigglesworthia
appears to be strictly associated with the bacteriome,
Sodalis
has been detected in
midgut, muscle, fat body, hemolymph, milk gland, and salivary glands of certain tsetse species
(Cheng and Aksoy, 1999). Furthermore, the density of infections with
Sodalis
appears to vary
widely in the different species analyzed. Infections in
G. morsitans
and
G. palpalis
midgut tissues
