Nancy A. Moran's study explores accelerated evolution and Muller's ratchet in endosymbiotic bacteria, focusing on Buchnera, a bacterial endosymbiont of aphids. Endosymbiotic bacteria, such as Buchnera, live within host cells and are transmitted maternally, leading to small population sizes and no recombination. These factors result in the accumulation of mildly deleterious mutations, which can drive faster sequence evolution and shifts in base composition due to mutational bias. Analysis of 16S rDNA from five endosymbiotic lineages shows that endosymbionts evolve faster than free-living relatives. In Buchnera, coding genes exhibit accelerated evolution and unusually low ratios of synonymous to nonsynonymous substitutions, consistent with the hypothesis of increased fixation of deleterious mutations. Polypeptides in Buchnera show amino acid substitutions favoring A+T-rich codons, suggesting these substitutions are deleterious for polypeptide function. The study supports the hypothesis that Muller's ratchet, combined with mutational bias, explains the accelerated evolution in endosymbiotic bacteria. This is further supported by observations such as the apparent loss of a repair gene and overproduction of a chaperonin in Buchnera, which may reflect compensatory evolution. The findings contradict the hypothesis that genomic base composition is influenced by selection, as the speedup is concentrated at nonsynonymous sites. The study also highlights that endosymbiotic bacteria, like Buchnera, have small populations and lack recombination, leading to increased fixation of deleterious mutations. This results in faster sequence evolution and a shift in base composition. The findings support Ohta's nearly neutral theory, which posits that a large proportion of mutations are mildly deleterious. The study suggests that the rate increase in Buchnera is not due to selection on genomic base composition but rather due to Muller's ratchet and mutational bias. The results indicate that endosymbionts may undergo long-term deterioration due to mutation accumulation, potentially limiting host fitness. The study provides evidence that endosymbiotic bacteria, such as Buchnera, exhibit accelerated evolution due to Muller's ratchet and mutational bias, with implications for understanding evolutionary processes in endosymbiotic systems.Nancy A. Moran's study explores accelerated evolution and Muller's ratchet in endosymbiotic bacteria, focusing on Buchnera, a bacterial endosymbiont of aphids. Endosymbiotic bacteria, such as Buchnera, live within host cells and are transmitted maternally, leading to small population sizes and no recombination. These factors result in the accumulation of mildly deleterious mutations, which can drive faster sequence evolution and shifts in base composition due to mutational bias. Analysis of 16S rDNA from five endosymbiotic lineages shows that endosymbionts evolve faster than free-living relatives. In Buchnera, coding genes exhibit accelerated evolution and unusually low ratios of synonymous to nonsynonymous substitutions, consistent with the hypothesis of increased fixation of deleterious mutations. Polypeptides in Buchnera show amino acid substitutions favoring A+T-rich codons, suggesting these substitutions are deleterious for polypeptide function. The study supports the hypothesis that Muller's ratchet, combined with mutational bias, explains the accelerated evolution in endosymbiotic bacteria. This is further supported by observations such as the apparent loss of a repair gene and overproduction of a chaperonin in Buchnera, which may reflect compensatory evolution. The findings contradict the hypothesis that genomic base composition is influenced by selection, as the speedup is concentrated at nonsynonymous sites. The study also highlights that endosymbiotic bacteria, like Buchnera, have small populations and lack recombination, leading to increased fixation of deleterious mutations. This results in faster sequence evolution and a shift in base composition. The findings support Ohta's nearly neutral theory, which posits that a large proportion of mutations are mildly deleterious. The study suggests that the rate increase in Buchnera is not due to selection on genomic base composition but rather due to Muller's ratchet and mutational bias. The results indicate that endosymbionts may undergo long-term deterioration due to mutation accumulation, potentially limiting host fitness. The study provides evidence that endosymbiotic bacteria, such as Buchnera, exhibit accelerated evolution due to Muller's ratchet and mutational bias, with implications for understanding evolutionary processes in endosymbiotic systems.