Biology Reference
In-Depth Information
are maintained at high density, while infections in G. austeni and G. brevipalpis are signiÝcantly
less, as observed by both PCR (Cheng and Aksoy, 1999) and microscopic analysis (Moloo and
Shaw, 1989). The factors that control the tissue tropism and density of symbionts in different host
species are not known but may be attributable to both symbiont- and host-controlled functions.
Although Wolbachia infections were initially thought to be largely restricted to the germ-line
tissue of their insect hosts, they recently have been observed in a variety of somatic tissues from
several different insects (Min and Benzer, 1997; Dobson et al., 1999). Analysis of tissue tropism
of Wolbachia in tsetse indicates that while in G. morsitans and G. brevipalpis the infections are
restricted to gonads, in G. austeni they are detected from various somatic tissues, even in 2-d-old
teneral Þies. The extensive tissue infections observed in G. austeni may reÞect the particular strain
harbored in this species, similar to what has been reported with the W. popcorn strain characterized
from D. melanogaster (Min and Benzer, 1997).
Given that most insects have robust immune mechanisms to combat foreign pathogens, it is of
interest how the commensal symbionts are maintained in their hosts. Recent studies with the tsetse
immunity genes showed in midgut and fat body tissues constitutive expression of diptericin , an
antimicrobial peptide gene speciÝc for Gram-negative bacteria (Hao et al., 2001). Constitutive
expression of diptericin in insects is unprecedented and might reÞect natural tsetse immune
responses to the presence of its symbionts. Our analysis with synthetic diptericin has shown that
Sodalis is more resistant to its antibacterial action than the related bacterium E. coli in vitro (Hao
et al., 2001) and hence may escape its intended harmful effects. In the presence of trypanosome
infections in tsetse, expression of several immunity genes (attacin, diptericin, defensin ) has been
found to be induced. It remains to be seen if the expression of these antimicrobial products affects
the viability of the tsetse symbionts Ð in particular Wigglesworthia , which might lead to reduced
fecundity of parasite-infected Þies.
SYMBIONTS AS GENE-EXPRESSION VEHICLES
From an applied perspective, beneÝcial microbes can be used for drug and vaccine delivery (Hooper
and Gordon, 2001) or for the expression of foreign genes designed to block the development of
pathogens (Aksoy et al., 2001). Since transgenic approaches involving egg manipulation are difÝcult
due to the viviparous reproductive biology of tsetse, transgenic symbionts provide an alternative
method for examining gene functions in vivo .
The availability of an in vitro culture of Sodalis has allowed for the development of a genetic
transformation system to introduce and express foreign gene products in these cells (Welburn et al.,
1987; Beard et al., 1993). It has been found that the in vitro manipulated recombinant Sodalis cells
are successfully acquired by the intrauterine progeny when microinjected into the female parent
hemolymph. The adults that have acquired the recombinant symbionts as larvae have been shown
to continue to express the marker gene product green Þuorescent protein (GFP) (Cheng and Aksoy,
1999). Using a similar microinjection approach, the facultative S-symbiont (PASS) in aphids has
also been successfully introduced from Acyrthosiphon pisum into A. kondoi Shinji (blue alphalfa
aphid) as well as into A. pisum symbiont-free clones, where it has been found to be maintained in
the offspring of the injected mother aphids with a high rate of maternal transmission (Chen and
Purcell, 1997).
Since the symbionts live in close proximity to the developing trypanosomes in midgut,
antipathogenic products expressed and secreted from these cells could adversely affect parasite
transmission. The identiÝcation of monoclonal antibodies (mABs) with parasite-transmission-
blocking characteristics and their expression as single-chain antibody gene fragments in the sym-
bionts provides a vast array of potential antipathogenic products. To this end, several transmission-
blocking antibodies targeting the major surface protein of the insect-stage procyclic trypanosomes
have already been reported (Nantulya and Moloo, 1988). Recently it has been possible to express
and secrete a single-chain antibody gene product in the transformed symbionts of reduviid bugs in
 
Search WWH ::




Custom Search