Many eukaryotic organisms have symbiotic associations
with obligate intracellular bacteria. The clonal transmission of endosymbionts between host generations
should lead to the irreversible fixation of slightly
deleterious mutations in their non-recombinant genome
by genetic drift. However, the stability of endosymbiosis
indicates that some mechanism is involved in the
amelioration of the effects of these mutations. We
propose that the chaperone GroEL was involved in the
acquisition of an endosymbiotic lifestyle not only by
means of its over-production, as proposed by Moran,
but also by its adaptive evolution mediated by positive
selection to improve the interaction with the unstable
endosymbiont proteome