Endosymbionts facilitate rapid evolution in a polyphagous herbivore.

Area of Science:

• Insect-microbe interactions
• Evolutionary biology
• Symbiosis

Background:

• Maternally transmitted bacterial symbionts influence insect herbivore-plant interactions.
• Facultative symbionts cause heritable phenotypic variation, driving selection.
• Arsenophonus symbionts benefit cowpea aphids on black locust but not fava.

Purpose of the Study:

• To investigate if fitness differences of symbiont-infected aphids on different host plants lead to rapid evolution.
• To determine if host plant selection drives changes in symbiont infection frequencies.
• To explore the role of symbionts in aphid adaptation to novel host plants.

Main Methods:

• Laboratory experiment over 16 weeks with cowpea aphid populations on black locust and fava.
• Monitoring the frequency of Arsenophonus infection within aphid populations over time.
• Comparing infection frequencies at the end of the experiment with initial frequencies and field data.

Main Results:

• Arsenophonus infection frequency significantly increased in aphid populations on black locust.
• Arsenophonus infection frequency significantly declined in aphid populations on fava.
• Post-experiment infection frequency was over 3x higher on black locust than fava, matching field observations.

Conclusions:

• Aphid populations with facultative symbionts rapidly evolve in response to host plant selective pressures.
• Host plant-driven selection is a sufficient explanation for the association between Arsenophonus-infected aphids and black locust.
• Arsenophonus infection may have pre-adapted aphids for specialization on novel host plants, facilitating global spread.