Insects and their endosymbionts: phylogenetics and evolutionary rates

Abstract

Obligate mutualistic intracellular endosymbionts have been acquired multiple times over the last few hundred million years by common ancestors of several major insect taxonomic groups. These ancient associations between hosts and endosymbionts mean that the phylogenies of the symbiotic partners are congruent. This allows the use of endosymbiont molecular phylogenies to resolve host relationships. Bacterial endosymbionts evolve under strong host-driven selection. Factors influencing host evolution might affect symbionts in similar ways, potentially leading to correlations between the molecular evolutionary rates of hosts and symbionts. Investigations of rate correlations between hosts and symbionts are lacking. Here, I generate molecular phylogenies based on complete mitochondrial genomes from cockroach, termite and mantis species and generate the most comprehensive cockroach phylogeny based on 104 Blattabacterium. I also estimate divergence times of cockroaches, based on cockroach mtDNA and Blattabacterium genes. Then, I demonstrate a correlation in molecular rates between the genomes of Blattabacterium cuenoti and the mitochondrial genomes of their cockroach hosts using the B. cuenoti dataset to compare phylogenetic relationships and evolutionary rates for 55 host/symbiont pairs. I find the first evidence of a correlation between evolutionary rates of the two genomes, based on comparisons of root-to-tip distances and the branch lengths of phylogenetically independent species pairs. Finally, I conduct the same analyses using genes from Buchnera aphidicola and Sulcia muelleri, and the mitochondrial genomes of their sap-feeding insect hosts. My results underscore the profound effects that ancient symbiosis can have on the biology of each symbiotic partner. The results here indicate that evolutionary rate correlations between hosts and long-term symbionts may be a widespread phenomenon.