Evolving aphids: one genome-one organism insects or holobionts?

Aphids have obligate mutualistic relationships with microorganisms that provide them with essential substances lacking in their diet, together with symbionts conferring them conditional adaptive advantages related, for instance, to the thermal tolerance and to the resistance to parasitoid wasps. The presence/absence of a secondary symbiont may have a relevant phenotypic effect so that aphid microbial symbionts constitute a sort of second genome with its own genetic inheritance. On the whole, genes important for aphid survival and reproduction are not uniquely present in the aphid nuclear and mitochondrial genomes, but also in the chromosomes of each symbiont. As a consequence, aphids should be viewed as holobionts with an extended genome (the hologenome) including the host and its symbiotic microbiome. In this connection, the true unit of selection in evolution must be considered the aphid holobiont, in place of the single host as individual separated from its symbionts.