Molecular evolution of sexual and parthenogenetic lineages of the armored scale insect aspidiotus nerii (Hemiptera: Diaspididae) and Its primary bacterial endosymbiont, Uzinura diaspidicola

Here we compare rates of molecular evolution in sexual and parthenogenetic lineages of Aspdiotus nerii Bouché (Hemiptera: Diaspididae) using the genealogies of three protein-coding loci from A. nerii (one mitochondrial and two nuclear), and two protein-coding loci from the primary endosymbiont Uzinura diaspidicola Gruwell (Proteobacteria: Gammaproteobacteria). To our knowledge, this is the first study to examine how the loss of sex affects DNA sequence substitution rates across nuclear, mitochondrial, and endosymbiont genomes. We find no differences between ratios of nonsynonymous to synonymous substitutions (dN/dS) in sexual and parthenogenetic lineages for nuclear loci (CAD and EFl) and endosymbiont loci (rspB and GroEL). We do find, however, for a fragment spanning portions of the mitochondrial genes cytochrome oxidase 1 and 2 (CO1-CO2) that a model including separate dN/dS ratios for the sexual and parthenogenetic lineages is a significantly better fit for the data (P = 0.003) than a model that includes a single dN/dS ratio for both lineages. We find this result striking because for asexual lineages nuclear, mictochondrial, and endosymbiont genomes share a similar mode of transmission (i.e., vertical from mother to offspring), yet our results show that elevated dN/dS ratios were only observed in the mitochondrial genome. This result supports a recent hypothesis that interactions between the endosymbiont and nuclear genomes may limit the accumulation of deleterious mutations in the endosymbiont genome, and suggests that these same interactions may influence mutation rates in the nuclear genomes of asexual organisms as well.