Genomes within genomes: nested symbiosis and its implications for plant evolution.

Many important plant traits are products of nested symbiosis: mobile genetic elements (MGEs) are nested within microbes, which in turn, are nested within plants. Plant trait variation is therefore not only determined by the plant's genome, but also by loci within microbes and MGEs. Yet it remains unclear how interactions and coevolution within nested symbiosis impacts the evolution of plant traits. Despite the complexities of nested symbiosis, including nonadditive interactions, understanding the evolution of plant traits is facilitated by combining quantitative genetic and functional genomic approaches that explicitly consider sources of nested genetic variation (from loci in MGEs to microbiomes). Additionally, understanding coevolution within nested symbiosis enables us to design or select for MGEs that promote plant health.