House fly larvae face several challenges during their development to adulthood. They must: (1) condition their nutritional resources while avoiding intraspecific competition; (2) avoid competitive fungi and/or inhibit fungal growth; and (3) obtain sufficient bacteria as food supplements. In my thesis, I show that house fly eggs are provisioned with bacterial symbionts that play a major role in addressing all of these challenges, and that these bacterial symbionts can be vertically transmitted by house flies from one generation to the next. Specifically, I have shown that: (1) gravid female house flies deposit, and respond to, a time-dependent bacterial cue, Klebsiella oxytoca, that proliferates over time on the surfaces of deposited eggs, inhibiting further oviposition when a threshold bacterial density is reached. This affords female house flies the resource-conditioning benefits of aggregated oviposition while decreasing the risks of cannibalism by older conspecifics; (2) house fly eggs are associated with several bacterial strains, each with a different spectrum of anti-fungal properties that aid in inhibiting the growth of competitive fungi. Volatile semiochemical cues produced by these fungi inhibit oviposition by gravid female house flies, helping them avoid detrimental competition with them; (3) gravid female house flies deposit bacteria that significantly increase larval survival in resources lacking in appropriate bacterial food, likely through supplementation of larval nutrition. Using pEGFP-transformed K. oxytoca, I demonstrated that K. oxytoca introduced onto the surface of house fly eggs is maintained on and in house flies throughout larval, pupal, and adult stages.
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