Aphids harbour several obligate and facultative bacterial symbionts that have important effects on their life. Several surveys of secondary symbionts clearly show that particular species are strongly associated with aphids feeding on certain food plants. For instance, most pea aphid clones feeding on clover Trifolium sp. harbour Regiella insecticola, while those feeding on Medicago usually have Hamiltonella defensa.
How can we explain such a difference? The most intriguing hypothesis is that these patterns reflect a role of these symbionts in the host plant use. However, they may also be present in view of factors correlated with host plant use or simple historical contingency. So the question is: Can symbiont drive the choice of the plant by aphids or they simply change in view of the plants where aphids live?
Several studies tried to distinguish between these explanations furnishing controversial scenarios. Tsuchida et al. (2004) removed R. insecticola from a clover-associated pea aphid clone using antibiotics and found that performance on Trifolium, but not Vicia, was negatively affected. In the same year, Leonardo repeated the same experimental plan but without finding any fitness effects of removing R. insecticola from two clones of aphid specialized on Trifolium. With a different approach, based on the artificial introduction of R. insecticola into five symbiont-free clones not previously associated with clover, no effect on performance of aphids on Trifolium have been observed by Ferrari et al. (2007). These results, as a whole, suggested that symbionts may be involved in the plant choice but not alone. Probably, interactions between aphids and plants involve the genotype of either the host or symbiont and both can influence host plant use.
Ferrari and Godfray here reported a further set of experiments where they evaluated the fitness consequences of introducing different strains of the symbiont Hamiltonella defensa into three aphid clones (that naturally lack symbionts) collected on Lathyrus pratensis and of removing symbionts from 20 natural aphid–bacterial associations. Ferrari and Godfray reported that: “Infection decreased fitness on Lathyrus but not on Vicia faba, a plant on which most pea aphids readily feed. This may explain the unusually low prevalence of symbionts in aphids collected on Lathyrus. There was no effect of presence of symbiont on performance of the aphids on the host plants of the clones from which the H. defensa strains were isolated. Removing the symbiont from natural aphid–bacterial associations led to an average approximate 20 per cent reduction in fecundity, both on the natural host plant and on V. faba, suggesting general rather than plant-species-specific effects of the symbiont. Throughout, we find significant genetic variation among aphid clones”.
Can you have now a better scenario? As a whole, the results provide no evidence that secondary symbionts have a major direct role in facilitating aphid utilization of particular host plant species, but only the aphid genome seem to have a pivotal role in the plant choice. At present we have a reply, but further experiments on different aphid species are welcome!
McLean, A., van Asch, M., Ferrari, J., & Godfray, H. (2010). Effects of bacterial secondary symbionts on host plant use in pea aphids Proceedings of the Royal Society B: Biological Sciences, 278 (1706), 760-766 DOI: 10.1098/rspb.2010.1654
Tsuchida, T. (2004). Host Plant Specialization Governed by Facultative Symbiont Science, 303 (5666), 1989-1989 DOI: 10.1126/science.1094611
Leonardo, T. (2004). Removal of a specialization-associated symbiont does not affect aphid fitness Ecology Letters, 7 (6), 461-468 DOI: 10.1111/j.1461-0248.2004.00602.x
Ferrari, J., Scarborough, C., & Godfray, H. (2007). Genetic variation in the effect of a facultative symbiont on host-plant use by pea aphids Oecologia, 153 (2), 323-329 DOI: 10.1007/s00442-007-0730-2