ACCELERATED EVOLUTION AND MULLERS RACHET IN ENDOSYMBIOTIC BACTERIA

Many bacteria live only within animal cells and infect hosts through c ytoplasmic inheritance. These endosymbiotic lineages show distinctive population structure, with small population size and effectively no re combination, As a result, endosymbionts are expected to accumulate mil dly deleterious mutations, If these constitute a substantial proportio n of new mutations, endosymbionts will show (i) faster sequence evolut ion and (ii) a possible shift in base composition reflecting mutationa l bias, Analyses of 16S rDNA of five independently derived endosymbion t clades show, in every case, faster evolution in endosymbionts than i n free-living relatives. For aphid endosymbionts (genus Bucknera), cod ing genes exhibit accelerated evolution and unusually low ratios of sy nonymous to nonsynonymous substitutions compared to ratios for the sam e genes for enterics, This concentration of the rate increase in nonsy nonymous substitutions is expected under the hypothesis of increased f ixation of deleterious mutations, Polypeptides for all Buchnera genes analyzed have accumulated amino acids with codon families rich in A+T, supporting the hypothesis that substitutions are deleterious in terms of polypeptide function. These observations are best explained as the result of Muller's ratchet within small asexual populations, combined with mutational bias, In light of this explanation, two observations reported earlier for Buchnera, the apparent loss of a repair gene and the overproduction of a chaperonin, may reflect compensatory evolution , An alternative hypothesis, involving selection on genomic base compo sition, is contradicted by the observation that the speedup is concent rated at nonsynonymous sites.