Abstract

Facultative bacterial endosymbionts are key components of aphid biology, influencing host performance and ecological interactions. We investigated how facultative endosymbionts affect competitive interactions between aphid clones of the cereal aphid Sitobion avenae (Fabricius) in Chilean wheat fields. We assessed the population development and life-history traits of the two dominant genotypes (G1 and G2) under single-clone rearing, intra- and inter-genotypic competition scenarios, with and without the facultative endosymbiont Regiella insecticola shaping these interactions. Our findings reveal that R. insecticola does not significantly alter inter-genotypic competition outcomes: clone G2 consistently outcompeted G1 regardless of infection status. However, under intra-genotypic competition, R. insecticola-infected G1 aphids outperformed their uninfected counterparts, while the opposite trend was observed in G2. R. insecticola influenced winged morph production and aphid weight in a genotype-dependent manner. Aphids harbouring R. insecticola induced less physiological stress on host plants, as evidenced by lower root/shoot ratios. These results suggest that facultative endosymbionts may confer clone-specific advantages, shaping aphid performance and aphid clone-endosymbiont-plant dynamics in multifaceted ways.