Impact of host demography and evolutionary history on endosymbiont molecular evolution: a test in carpenter ants (Genus Camponotus) and their Blochmannia 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 Camponotus; subgenera Camponotus
and Tanaemyrmex) and their Blochmannia endosymbionts as our study system
to address whether Camponotus demography shapes Blochmannia molecular
evolution. Using whole-genome phylogenomics, we confirmed previous work
identifying codiversification between carpenter ants and their Blochmannia
endosymbionts. We found that Blochmannia 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 Blochmannia genes, we found signatures of
positive selection and shifts in selection strength across the phylogeny.
Host demography was associated with Blochmannia shifts toward increased
selection strengths, but not associated with Blochmannia selection
relaxation, positive selection, genetic drift rates, or genome size
evolution. Mixed support for relationships between host effective
population sizes and Blochmannia molecular evolution suggests weak or
uncoupled relationships between host demography and Blochmannia population
genomic processes. Finally, we found that Blochmannia genome size
evolution was associated with genome-wide estimates of genetic drift and
number of genes with relaxed selection pressures. --