Ecology of a novel defensive symbiont of Drosophila: Spiroplasma-mediated protection against parasitic nematodes




Cockburn, Sarah

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Recently, there has been growing awareness that many animals and plants harbour bacterial symbionts that help protect them against natural enemies. The mushroom-breeding fly Drosophila neotestacea is commonly infected with a virulent parasitic nematode, Howardula aoronymphium. Infections are severe, reducing adult survival and mating success, and until recently virtually all females were rendered sterile. We have discovered that D. neotestacea harbours a strain of the bacterial symbiont Spiroplasma that restores fertility to nematode-parasitized female flies. Spiroplasma appears to be both increasing in frequency and spreading westward across N. America. My thesis examines associations between flies, nematodes and Spiroplasma in British Columbia, which appears to lie at the edge of the range of advancing Spiroplasma infections. I identified Spiroplasma-infected flies in British Columbia for the first time. Sequencing a number of Spiroplasma genes, as well as fly mitochondrial DNA, strongly suggests that the defensive symbiont is spreading westward. Furthermore, high nematode infection rates in BC, as well as laboratory experiments demonstrating the ability of Spiroplasma to restore fertility to nematode-parasitized BC flies, suggest that there is a strong selective pressure for Spiroplasma to continue to spread in BC. I also examined the generality of Spiroplasma-mediated defense by exposing flies to a gram-negative bacterial pathogen, Pectobacterium carotovorum. Exposure dramatically reduced survival regardless of Spiroplasma infection, suggesting that Spiroplasma does not defend against gram-negative bacteria.



bacterial symbionts, flies, spiroplasma