Wolbachia, normally a symbiont of Drosophila, can be virulent, causing degeneration and early death

Abstract

Wolbachia, a maternally transmitted microorganism of the Rickettsial family, is known to cause cytoplasmic incompatibility, parthenogenesis, or feminization in various insect species. The bacterium-host relationship is usually symbiotic: incompatibility between infected males and uninfected females can enhance reproductive isolation and evolution, whereas the other mechanisms enhance progeny production. We have discovered a variant Wolbachia carried by Drosophila melanogaster in which this cozy relationship is abrogated. Although quiescent during the fly's development, it begins massive proliferation in the adult, causing widespread degeneration of tissues, including brain, retina, and muscle, culminating in early death. Tetracycline treatment of carrier flies eliminates both the bacteria and the degeneration, restoring normal life-span. The 16s rDNA sequence is over 98% identical to Wolbachia known from other insects. Examination of laboratory strains of D. melanogaster commonly used in genetic experiments reveals that a large proportion actually carry Wolbachia in a nonvirulent form, which might affect their longevity and behavior.

Figure 1

Proliferation of the Wolbachia popcorn strain in the adult Drosophila brain. (A) One-day-old fly carrying popcorn. Note bacteria in the cytoplasm of a neuronal cell body in the brain cortex. (B) In a fly at age 8 days, extreme proliferation of the bacteria is seen. (C) Fusion of two infected brain cells. The bacteria in one of the cells have assumed an electron-dense and elongated form, resembling endospores. (D) In a still older fly (11 days), the dense form predominates. (Bar = 2 μm.)

Figure 2

Proliferation of popcorn in muscle, retina, and ovary. (A) Thoracic indirect flight muscle, seen in oblique section, showing invasion of the bacteria. The large, dark bodies are mitochondria, which deteriorate as the infection progresses. (B) Retina. Masses of the bacteria appear in both photoreceptor cells and surrounding pigment cells. The round, dark bodies are sections of the photoreceptor cell rhabdomeres. (C) Ovarian oocytes. (D) Higher magnification shows the bacterial cell wall and plasma membrane; the morphology is typical of Wolbachia (20). (Bars = 1 μm.)

Figure 4

Reduction of life-span by popcorn infection, and recovery by tetracycline treatment. (A) Survival curve of uninfected control flies (w¹¹¹⁸). (B) w¹¹¹⁸ flies after infection with popcorn. (C) Cured by tetracycline treatment. Each initial population was ≈100. These curves were obtained by culture at 29°C. The effects at 25°C are similar, with all lifetimes longer by a factor of 2.

Figure 3

Detection of popcorn in ovaries by PCR and immunostaining with monoclonal antibody. (A) Detection by PCR of Wolbachia 16s rDNA in ovaries of flies carrying popcorn, and elimination of the bacteria by tetracycline treatment. Lanes: 1, DNA size markers; 2, uninfected flies (w¹¹¹⁸); 3, Wolbachia-infected flies (wild-type Canton-S); 4, Canton-S, after tetracycline treatment; 5, w¹¹¹⁸ flies after infection with popcorn by maternal transmission; 6, same strain as lane 5, after tetracycline treatment. (B–D) Visualization of popcorn in cryostat section of ovary by monoclonal antibody. (B) As seen by Nomarski optics. (C) Uninfected strain (w¹¹¹⁸) stained with Wolbachia-specific monoclonal antibody of Kose and Karr (18). (D) Strain carrying popcorn.