Symbiosis in Tsetse - Insect Symbiosis - page 53

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Symbiosis in Tsetse

Serap Aksoy

CONTENTS

Introduction ......................................................................................................................................53
Characterization of Tsetse Symbionts .............................................................................................53
Evolutionary Histories of Symbionts ..............................................................................................54
Genomics of the Obligate Symbiont Wigglesworthia .....................................................................56

Genomics of Genus .............................................................................................................57

Functional Role of Tsetse Symbionts..............................................................................................59
SymbiontÏHost Interactions.............................................................................................................59
Symbionts as Gene-Expression Vehicles.........................................................................................60
Trypanosomiasis Control and the Impact of Parasite Refractory Tsetse
on Disease Transmission..................................................................................................................61
Acknowledgments ............................................................................................................................62
References ........................................................................................................................................62

Sodalis

INTRODUCTION

Tsetse Þies (Diptera: Glossinidae) are important agricultural and medical vectors that transmit the
protozoan African trypanosomes, the agents of sleeping sickness disease in humans and various
diseases in animals (nagana). In addition to the parasite they vector, tsetses provide a suitable niche
for multiple bacterial symbionts. The evolutionary histories of the symbionts with their tsetse host
vary, and their functional associations range from obligate mutuals to facultative parasites. As a
well-deÝned microhabitat, tsetses provide an ideal system for observing the dynamics of interactions
among the multiple symbionts and investigating host responses to the different associations. From
a practical perspective, symbiotic organisms stand to beneÝt from therapeutic approaches to con-
trolling plant and animal diseases transmitted by their arthropod hosts.

CHARACTERIZATION OF TSETSE SYMBIONTS

Insects with a single diet throughout their entire developmental cycle (such as blood, plant sap, or
wood) rely on microbial symbionts for additional nutrients that are not found in their restricted
diet and that they are unable to synthesize (Stuhlman, 1907; Roubaud, 1919; Wigglesworth, 1929;
Pinnock and Hess, 1974; Shaw and Moloo, 1991). Microorganisms with different ultrastructural
characteristics have been reported from various tissues of tsetse, which only feeds on vertebrate
blood during all developmental stages (Stuhlman, 1907; Roubaud, 1919; Wigglesworth, 1929;
Pinnock and Hess, 1974; Shaw and Moloo, 1991). Recent phylogenetic analyses have conÝrmed
that these organisms represent three distinct associations (Aksoy, 2000). Two of these symbionts
are members of the Enterobacteriaceae: the primary (P)-symbiont (genus

Wigglesworthia ) (Aksoy,

1995; Aksoy et al., 1995) and the secondary (S)-symbiont (genus Sodalis ) (Aksoy et al., 1995;

Cheng and Aksoy, 1999; Dale and Maudlin, 1999). Wigglesworthia lives intracellularly within the