FASTER EVOLUTIONARY RATES IN ENDOSYMBIOTIC BACTERIA THAN IN COSPECIATING INSECT HOSTS

The hypothesis of a universal molecular clock holds that divergent lin eages exhibit approximately constant rates of nucleotide substitution over evolutionary time for a particular macromolecule. We compare dive rgences of ribosomal DNA for aphids (Insecta) and Buchnera, the matern ally transmitted, endosymbiotic bacteria that have cospeciated with ap hids since initially infecting them over 100 million years ago. Substi tution rates average 36 times greater for Buchnera than for their aphi d hosts for regions of small-subunit rDNA that are homologous for prok aryotes and eukaryotes. Aphids exhibit 18S rDNA substitution rates tha t are within the range observed in related insects. In contrast, 16S r DNA evolves about twice as fast in Buchnera as in related free-living bacterial lineages. Nonetheless, the difference between Buchnera and a phids is much greater, suggesting that rates may be generally higher i n bacteria. This finding adds to evidence that molecular clocks are on ly locally rather than universally valid among taxonomic groups. It is consistent with the hypothesis that rates of sequence evolution depen d on generation time.