DELETERIOUS MUTATIONS DESTABILIZE RIBOSOMAL-RNA IN ENDOSYMBIOTIC BACTERIA

In populations that are small and asexual, mutations with slight negat ive effects on fitness will drift to fixation more often than in large or sexual populations in which they will be eliminated by selection. If such mutations occur in substantial numbers, the combined effects o f longterm asexuality and small population size may result in substant ial accumulation of mildly deleterious substitutions. Prokaryotic endo symbionts of animals that are transmitted maternally for very long per iods are effectively asexual and experience smaller effective populati on size than their free-living relatives. The contrast between such en dosymbionts and related free-living bacteria allows us to test whether a population structure imposing frequent bottlenecks and asexuality d oes lead to an accumulation of slightly deleterious substitutions. Her e we show that several independently derived insect endosymbionts, eac h with a long history of maternal transmission, have accumulated desta bilizing, base substitutions in the highly conserved 16S rRNA. Stabili ties of Domain I of this subunit are 15-25% lower in endosymbionts tha n in closely related free-living bacteria. By mapping destabilizing su bstitutions onto a reconstructed phylogeny, we show that decreased rib osomal stability has evolved separately in each endosymbiont lineage. Our phylogenetic approach allows us to demonstrate statistical signifi cance for this pattern: becoming endosymbiotic predictably results in decreased stability of rRNA secondary structure.