Fitness costs of the aphid endosymbiont, <i>Hamiltonella defensa</i>

Abstract

Black sage aphids, Obtusicauda frigidae, are infected by the bacterial endosymbiont Hamiltonella defensa. The bacteria confers a resistance to parasitism by the solitary endoparastic wasp Aphidius ervi. The bacteria also confers a number of other advantages to the aphids. Despite the number of advantages conferred by bacterial infection, H. defensa is only seen at intermediate frequencies in the aphid population. Our experiment was designed to test whether or not there is a fitness cost to the aphid by infection with H. defensa which would explain the intermediate frequency of infection. I placed infected and uninfected aphids on plants in bags. I dyed the initial population of aphids in order to track the mortality of only that initial population. Then I surveyed each population at weekly intervals in order to gather data on the effect of infection on the fitness initial and subsequent aphids. I found that uninfected aphids had lower mortality and that their offspring had higher survivorship than those of infected aphids.

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