Besides Buchnera, aphids often harbor facultative heritable
symbiotic bacteria known as secondary symbionts, of which
different strains have been shown to protect pea aphid hosts from
heat stress, fungal pathogens, and parasitoid wasps [6]. As part of
the pea aphid genome project, the genomic sequence of the
secondary symbiontRegiella insecticolawas obtained [8]. Along with
the recently completed sequence for the secondary symbiont
Hamiltonella defensa[9], these data contrast with the genomes of
Buchneraand other obligate symbionts, illustrating the genomic
underpinnings of two very different symbiotic lifestyles.Buchnera
possesses a highly reduced genome largely comprised of genes
essential for basic cellular processes and aphid nutrition. Its
chromosome is unusually stable and completely lacks mobile
elements, bacteriophage, or genes for toxin production. In
contrast, H. defensaandR. insecticolapossess phage genes, many
mobile elements, and numerous genes predicted to encode toxins
[6,8,50]. For example, about 12% of all R. insecticolagenes are
homologous to transposases of mobile elements, and 5% of genes
are phage-related, suggesting a highly dynamic genome especially
as compared toBuchneraand other small genome symbionts.
Lateral gene transfer from bacteria to the host. The pea
aphid genome provides a first opportunity for an exhaustive search
for genes of bacterial origin in the genome of a eukaryotic host
showing persistent associations with heritable bacterial symbionts