Vibrio cholerae Infection of Drosophila melanogaster Mimics the Human Disease Cholera

Cholera, the pandemic diarrheal disease caused by the gram-negative bacterium Vibrio cholerae, continues to be a major public health challenge in the developing world. Cholera toxin, which is responsible for the voluminous stools of cholera, causes constitutive activation of adenylyl cyclase, resulting in the export of ions into the intestinal lumen. Environmental studies have demonstrated a close association between V. cholerae and many species of arthropods including insects. Here we report the susceptibility of the fruit fly, Drosophila melanogaster, to oral V. cholerae infection through a process that exhibits many of the hallmarks of human disease: (i) death of the fly is dependent on the presence of cholera toxin and is preceded by rapid weight loss; (ii) flies harboring mutant alleles of either adenylyl cyclase, Gsα, or the Gardos K(+) channel homolog SK are resistant to V. cholerae infection; and (iii) ingestion of a K(+) channel blocker along with V. cholerae protects wild-type flies against death. In mammals, ingestion of as little as 25 μg of cholera toxin results in massive diarrhea. In contrast, we found that ingestion of cholera toxin was not lethal to the fly. However, when cholera toxin was co-administered with a pathogenic strain of V. cholerae carrying a chromosomal deletion of the genes encoding cholera toxin, death of the fly ensued. These findings suggest that additional virulence factors are required for intoxication of the fly that may not be essential for intoxication of mammals. Furthermore, we demonstrate for the first time the mechanism of action of cholera toxin in a whole organism and the utility of D. melanogaster as an accurate, inexpensive model for elucidation of host susceptibility to cholera

Molecular evolution of visual pigments of the Tokay gecko and bluefin killifish

Visual pigments are proteins which absorb photons and convert light into neuronal signals which are interpreted by the visual cortex of the brain. Functionally, the visual pigment has two major roles: absorb a photon within a specific spectral range and initiate a G-protein signaling cascade. In nature, visual pigments absorb light of many different wavelengths, from 360 nm to 635 nm. Previous studies have shown that changing specific amino acid residues in visual pigments alters maximal light absorbance (λmax), and that this may enhance vision under various environmental lighting conditions. The goal of this dissertation was to study visual pigment function and evolution in two species, the Tokay gecko and the Bluefin killifish, from two distinct lighting environments, a nocturnal habitat and an aquatic habitat. First, in Chapter 2, the vision of the nocturnal Tokay gecko was studied. It was determined that this species relies on only three cone visual pigments for vision (SWS1, RH2, and MWS) and two of these cone visual pigments are dramatically blue-shifted in λmax (RH2 and MWS). Secondly, in Chapters 3 and 4, the molecular evolution of visual pigments in the bluefin killifish were examined. Eight opsin cDNA\u27s were identified and characterized for this species. It was demonstrated that for short wavelength vision, improved chromatic discrimination arose in the bluefin killifish as the result of a gene duplication of the SWS2 class opsin and amino acid changes at sites 44, 94, 118, and 265 to blue-shift the function of one copy, changing the function from blue light absorbance to violet light absorbance