ACCELERATED EVOLUTIONARY RATE IN SULFUR-OXIDIZING ENDOSYMBIOTIC BACTERIA ASSOCIATED WITH THE MODE OF SYMBIONT TRANSMISSION

The nearly neutral theory of molecular evolution predicts that the rat e of nucleotide substitution should accelerate in small populations at sites under low selective constraint. We examined these predictions w ith respect to the relative population sizes for three bacterial life histories within chemolithoautotrophic sulfur-oxidizing bacteria: (1) free-living bacteria, (2) environmentally captured symbionts, and (3) maternally transmitted symbionts. Both relative rates of nucleotide su bstitution and relative ratios of loop, stem, and domain substitutions from 1,165 nt of the small-subunit 16S rDNA were consistent with expe ctations of the nearly neutral theory. Relative to free-living sulfur- oxidizing autotrophic bacteria, the maternally transmitted symbionts h ave faster substitution rates overall and also in low-constraint domai ns of 16S rDNA. Nucleotide substitition rates also differ between loop and stem positions. All of these findings are consistent with the pre dictions that these symbionts have relatively small effective populati on sizes, in contrast, the rates of nucleotide substitution in environ mentally captured symbionts are slower, particularly in high-constrain t domains, than in free-living bacteria.