Untangling nature’s experiment with lice and endosymbiotic bacteria

Highlight

• Over one million insect species are host to heritable endosymbiotic microorganisms. Many of these endosymbionts provide insects with essential nutrients, allowing insects to exploit nutritionally poor diets.
• Lice are obligate ectoparasites of birds and mammals, and many rely on endosymbiotic bacteria to provide vitamins lacking in their diet.
• Phylogenomic studies suggest that some parasitic lice repeatedly acquired new endosymbiotic partners, recruiting them from widely distributed and free-living progenitor species. Once established, lice and their endosymbionts coevolved in relative isolation and under very similar conditions.
• Recent studies harnessed the natural replay experiment between feather lice and endosymbiotic bacteria, examining the predictability of genome evolution in host-associated bacteria.

Abstract

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.