Endosymbiont effects on host plant usage in Aphis craccivora

Facultative bacterial endosymbionts are microbes that infect insects but are not required for host survival.  These endosymbionts are often maternally inherited and can confer beneficial traits to the host, such as providing defense against natural enemies. Recent studies have shown that populations of polyphagous insects found on different host plants can be infected with different facultative endosymbionts. For example, when the cowpea aphid, Aphis craccivora, is found on Medicago sativa (alfalfa) it is predominantly infected with the facultative endosymbiont Hamiltonella, whereas when the aphid is found on Robinia pseudoacacia (locust) it is usually infected with the endosymbiont Arsenophonus. This distribution suggests that either of these symbionts may be facilitating host plant usage. My objectives are to assess the relative fitness of differentially-infected A. craccivora across three of its host plants, and to test whether either endosymbiont affects aphid fitness as a function of host plant use.  I have compared population growth of 3 aphid lines (Arsenophonus-infected locust-origin aphids, Hamiltonella-infected alfalfa-origin aphids, and uninfected alfalfa-origin aphids that descended from antibiotic-cured ancestors) on the two origin plants as well as the generally permissive host, Vicia faba (fava). When all three lines were tested on alfalfa or locust, each performed well only on their plant of origin. When Hamiltonella and cured lines were compared, a fitness cost was observed regardless of host plant. On locust, this fitness cost was the difference between slowly growing populations and declining populations. Thus, Hamiltonella may influence host plant usage by restricting population growth on some plant species.