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Accelerated evolution and Muller's rachet in endosymbiotic bacteria
1Department of Ecology and Evolutionary Biology, University of Arizona, Tuscon 85721, USA.
Summary
Endosymbiotic bacteria accumulate deleterious mutations due to small population size and lack of recombination. This leads to faster sequence evolution and altered base composition, supporting Muller's ratchet theory in these unique bacterial lineages.
Area of Science:
- Microbial genomics
- Evolutionary biology
- Bacterial endosymbiosis
Background:
- Endosymbiotic bacteria inhabit animal cells and are inherited maternally.
- These bacteria possess small population sizes and limited recombination, leading to mutation accumulation.
Purpose of the Study:
- To investigate the evolutionary rates and base composition changes in endosymbiotic bacteria.
- To test the hypothesis of deleterious mutation accumulation and Muller's ratchet in endosymbionts.
Main Methods:
- Analysis of 16S ribosomal DNA (rDNA) from five endosymbiont clades.
- Comparative analysis of coding genes in aphid endosymbionts (Buchnera) and free-living relatives (enterics).
Main Results:
- Endosymbionts exhibit faster sequence evolution compared to free-living bacteria.
- Buchnera coding genes show accelerated evolution, with a higher rate of nonsynonymous substitutions.
- Accumulation of amino acids from A+T-rich codon families in Buchnera polypeptides suggests deleterious mutations.
Conclusions:
- Observations support Muller's ratchet in small, asexual endosymbiotic populations, driving accumulation of deleterious mutations.
- Faster evolution and base composition shifts in endosymbionts are linked to mutational bias and relaxed selection.
- Compensatory evolution, such as loss of repair genes or chaperonin overproduction, may occur in response to mutation accumulation.