In 1865 Louis Pasteur discovered that microsporidia cause pebrine disease in silkworms, a condition characterized by melanization in the silkworm, and a significant economic concern (Brey 1998). In the following decades Carlos Finlay showed that mosquitoes were vectors for yellow fever (Chaves-Carballo 2005). This led to a great deal of interest in the early twentieth century on characterizing the microbial flora associated with insects (Steinhaus 1940).
By the end of 1960s, it was already known that pathogens such as fungi, protozoa, viruses, and bacteria could infect insects. In response to these infections, insects activated cellular and humoral immune defenses, including phagocytosis and the production of antimicrobial substances (Heimpel and Harshbarger 1965). A milestone in the insect immunity field was the study by Hans Boman and colleagues on the inducible antibacterial defense mechanisms of Drosophila (Boman et al. 1972). In subsequent years, a number of studies were done to characterize specific antimicrobial peptides (AMP) and the genes encoding them in various insects, including Drosophila (Hultmark et al. 1983; Kylsten et al. 1990; Samakovlis et al. 1990; Steiner et al. 1981; Sun et al. 1991). These antimicrobial peptides are small, cationic molecules that are effective against specific classes of pathogens. For example, in Drosophila, defensin acts against Gram-positive bacteria (Dimarcq et al. 1994), and Diptericin, Drosocin and Attacin are effective against Gram-negative bacteria (Asling et al. 1995; Michaut et al. 1996; Wicker et al. 1990), whereas drosomycin shows anti-fungal activity. Metchnikowin and Cecropin are both antibacterial and antifungal (Ekengren and Hultmark 1999; Levashina et al. 1995;
Samakovlis et al. 1992). These antimicrobial peptides are critical for resistance to infection, such that transgenic expression of a single antimicrobial peptide can protect immunodeficient flies (Tzou et al. 2002).
Since then, two major questions have shaped the field of insect immunology. How are microbes recognized? And, how is antimicrobial peptide gene expression regulated? Having the most powerful genetic and molecular tools available, Drosophila became the preferred experimental system to address these issues.
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