Mobile elements create strain-level variation in the services conferred by an aphid symbiont

Area of Science:

• Microbial Ecology
• Evolutionary Biology
• Insect-Microbe Interactions

Background:

• Heritable, facultative symbionts are prevalent in arthropods, often conferring host defense capabilities.
• Mobile genetic elements (MGEs) are key drivers of genomic variation and phenotypic diversity in symbionts, despite their reduced genomes.
• Strain-level variation in the aphid symbiont Hamiltonella defensa, specifically its bacteriophages (APSEs), is linked to differences in aphid defense against parasitoids.

Purpose of the Study:

• To disentangle the contributions of Hamiltonella defensa genotype and APSE type to aphid anti-parasitoid defense.
• To investigate the evolutionary dynamics of APSEs within H. defensa and their role in rapid adaptation.
• To explore the genomic basis of coinfection with Fukatsuia symbiotica and its potential impact on H. defensa.

Main Methods:

• Comparative genomics of H. defensa isolates with varying APSE types.
• Phenotypic analysis of aphid defense levels correlated with specific APSE virulence modules.
• Genomic analysis to identify genetic elements associated with coinfection by F. symbiotica.

Main Results:

• Aphid protection levels were directly correlated with APSE virulence module type, independent of H. defensa genotype.
• Evidence suggests APSEs undergo repeated horizontal transfer within H. defensa, facilitating rapid evolution of defense mechanisms.
• Genomic data did not support nutritional interactions as the primary driver for coinfection with F. symbiotica, but bacteriocin plasmids were unique to co-infecting strains.

Conclusions:

• Strain diversity within symbionts, coupled with horizontal gene transfer of MGEs and symbionts, significantly contributes to the rapid evolutionary adaptation of arthropods.
• APSEs are crucial in mediating strain-level variation in symbiont-mediated defense.
• The co-evolutionary dynamics of symbionts and their hosts are shaped by mobile genetic elements and horizontal transfer events.