BACKGROUND: Primary bacterial endosymbionts of insects (p-endosymbionts) are 
thought to be undergoing the process of Muller's ratchet where they accrue 
slightly deleterious mutations due to genetic drift in small populations with 
negligible recombination rates. If this process were to go unchecked over time, 
theory predicts mutational meltdown and eventual extinction. Although genome 
degradation is common among p-endosymbionts, we do not observe widespread 
p-endosymbiont extinction, suggesting that Muller's ratchet may be slowed or 
even stopped over time. For example, selection may act to slow the effects of 
Muller's ratchet by removing slightly deleterious mutations before they go to 
fixation thereby causing a decrease in nucleotide substitutions rates in older 
p-endosymbiont lineages.
METHODOLOGY/PRINCIPAL FINDINGS: To determine whether selection is slowing the 
effects of Muller's ratchet, we determined the age of the Candidatus 
Riesia/sucking louse assemblage and analyzed the nucleotide substitution rates 
of several p-endosymbiont lineages that differ in the length of time that they 
have been associated with their insect hosts. We find that Riesia is the 
youngest p-endosymbiont known to date, and has been associated with its louse 
hosts for only 13-25 My. Further, it is the fastest evolving p-endosymbiont with 
substitution rates of 19-34% per 50 My. When comparing Riesia to other insect 
p-endosymbionts, we find that nucleotide substitution rates decrease 
dramatically as the age of endosymbiosis increases.
CONCLUSIONS/SIGNIFICANCE: A decrease in nucleotide substitution rates over time 
suggests that selection may be limiting the effects of Muller's ratchet by 
removing individuals with the highest mutational loads and decreasing the rate 
at which new mutations become fixed. This countering effect of selection could 
slow the overall rate of endosymbiont extinction.