Impact of host demography and evolutionary history on endosymbiont molecular evolution: A test in carpenter ants (genus ) and their endosymbionts.

Obligate endosymbioses are tight associations between symbionts and the hosts they live inside. Hosts and their associated obligate endosymbionts generally exhibit codiversification, which has been documented in taxonomically diverse insect lineages. Host demography (e.g., effective population sizes) may impact the demography of endosymbionts, which may lead to an association between host demography and the patterns and processes of endosymbiont molecular evolution. Here, we used whole-genome sequencing data for carpenter ants (Genus subgenera and ) and their endosymbionts as our study system to address whether demography shapes molecular evolution. Using whole-genome phylogenomics, we confirmed previous work identifying codiversification between carpenter ants and their endosymbionts. We found that genes have evolved at a pace ~30× faster than that of their hosts' molecular evolution and that these rates are positively associated with host rates of molecular evolution. Using multiple tests for selection in genes, we found signatures of positive selection and shifts in selection strength across the phylogeny. Host demography was associated with shifts toward increased selection strengths, but not associated with selection relaxation, positive selection, genetic drift rates, or genome size evolution. Mixed support for relationships between host effective population sizes and molecular evolution suggests weak or uncoupled relationships between host demography and population genomic processes. Finally, we found that genome size evolution was associated with genome-wide estimates of genetic drift and number of genes with relaxed selection pressures.