Untangling nature's experiment with lice and endosymbiotic bacteria

Insects have formed close relationships with endosymbiotic microorganisms, enabling adaptation and promoting diversification. In this review, we examined studies of endosymbiotic bacteria in parasitic lice (Psocodea: Phthiraptera). Lice and their endosymbionts lead fairly secluded lives, with each louse-host and louse-endosymbiont pair evolving in relative isolation. Consequently, each louse lineage and its associated endosymbiont represents natural replicates, useful for understanding how endosymbiosis arises and evolves under similar ecological conditions. While louse endosymbionts are vertically transmitted, they show surprisingly low levels of cospeciation with their louse hosts. Instead, phylogenomic evidence indicates repeated, independent acquisitions of endosymbionts from free-living progenitors. Following each acquisition, endosymbiont lineages experienced elevated evolutionary rates and genomic reduction, losing functionally redundant pathways while retaining functions necessary to maintain the symbiosis.