Drosophila melanogaster A Fruitful Model

The fruit fly Drosophila melanogaster is one of the most extensively used species in genetics and development for almost a century. Its short life cycle of just 2 weeks and its small size make it easy to keep in large numbers. The sequencing of the entire genome revealed that a set of around 14 000 genes is required for the generation of a multicellular organism of this kind that is able to perform complex actions. Several thousand mutant fly strains with a defect in a gene are available for the genetic dissection of traits (http://flybase.net).

Insects have long been known for their ability to ward off invaders with a variety of immune responses. Since there is a good correlation between immune responses of mammals and of insects, these systems can be good models of host-pathogen interactions [43]. Drosophila is able to react to different kinds of infections caused by gram-positive or gram-negative bacteria, fungi, or parasitic protozoa [44-47]. Most studies use injection methods to directly stimulate the immune system, since only few pathogens are capable of naturally infecting D. melanogaster [48], and the genetic tractability of Drosophila has made it possible to define many of the genes involved in innate immunity. Upon infection a signaling cascade is activated which leads to the production of antimicrobial peptides in the fat body of the fly [49]. It turned out that the activation of peptide production depends mainly on two distinct signal transduction pathways, the Toll and IMD (immune deficiency) pathways. Both pathways share striking similarities with the immune response pathways of mammals [50, 51]. This innate immune system is activated differently depending on the nature of the attacker, thus discriminating between different classes of pathogens: fungi and gram-positive bacteria induce the Toll pathway, whereas gram-negative bacteria are sensed by the IMD pathway. Other signaling pathways such as the JAK-STAT and JNK pathways maybe also involved in the immune response, although the exact nature of these pathways is not yet clear [52].

The most prominent pathogen studied in D. melanogaster is P. aeruginosa. For this pathogen it could be shown that the virulence factors required for full infec-tivity in Drosophila are the same as for mammals [53]. Moreover, the verification of the activation of the type III secretion system upon entry of the bacteria into the animal opens up the prospect of Drosophila as a real in vivo model for infection with this pathogen [54]. Further pathogens currently under investigation are Ser-ratica marcescens, Listeria monocytogenes, and Mycobacterium marinum. Infections with fungi can also be monitored. In one study the authors infected immune-deficient flies with different Candida albicans mutants. They found that virulence patterns against Drosophila in these strains reproduced those in a murine model.

Importantly, using this insect model they found additional virulence properties undetectable in the murine model system [55].

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