Muller's ratchet decreases fitness of a DNA-based microbe

Area of Science:

• Evolutionary Biology
• Microbiology
• Genetics

Background:

• Muller's ratchet theory posits that asexual reproduction leads to irreversible accumulation of deleterious mutations.
• Experimental evidence for Muller's ratchet is primarily from RNA viruses with exceptionally high mutation rates.
• The operation of Muller's ratchet in DNA-based organisms with typical mutation rates remains less understood.

Purpose of the Study:

• To investigate the operation of Muller's ratchet in Salmonella typhimurium, a DNA-based bacterium with a standard mutation rate.
• To determine if genetic drift in asexual populations of S. typhimurium leads to increased mutational load.
• To calculate the spontaneous mutation rate in S. typhimurium under conditions favoring genetic drift.

Main Methods:

• Asexual cultivation of Salmonella typhimurium under conditions promoting high genetic drift.
• Monitoring of fitness changes, specifically growth rate, across multiple lineages over 1700 generations.
• Measurement of auxotroph appearance to estimate the spontaneous mutation rate across approximately 200 genes.

Main Results:

• Salmonella typhimurium accumulated deleterious mutations over 1700 generations of asexual growth.
• One percent of tested lineages exhibited a significant loss of fitness due to accumulated mutations.
• The calculated spontaneous mutation rate was approximately 0.3-1.5 x 10^-9 mutations per base pair per generation.

Conclusions:

• Muller's ratchet operates in DNA-based organisms like S. typhimurium under conditions of asexual reproduction and high genetic drift.
• Compensatory or back mutations are insufficient to counteract the accumulation of deleterious mutations in these conditions.
• The study provides experimental validation of Muller's ratchet in a model organism with a typical mutation rate.