Mutational meltdown in primary endosymbionts: selection limits Muller's ratchet

Area of Science:

• Evolutionary biology
• Microbial genomics
• Symbiosis

Background:

• Primary bacterial endosymbionts (p-endosymbionts) accumulate deleterious mutations via Muller's ratchet due to small population sizes and low recombination.
• Theory predicts mutational meltdown and extinction if Muller's ratchet is unchecked.
• Widespread extinction is not observed, suggesting mechanisms that slow or halt Muller's ratchet.

Purpose of the Study:

• To investigate if natural selection counteracts Muller's ratchet in insect p-endosymbionts.
• To analyze the relationship between the age of endosymbiotic association and nucleotide substitution rates.

Main Methods:

• Determined the age of the "Candidatus Riesia"/sucking louse symbiosis.
• Analyzed nucleotide substitution rates across p-endosymbiont lineages with varying host association durations.

Main Results:

• "Candidatus Riesia" is the youngest known p-endosymbiont (13-25 million years association).
• It exhibits the highest substitution rate (19-34% per 50 million years).
• Nucleotide substitution rates significantly decrease with increasing age of endosymbiosis.

Conclusions:

• Decreasing substitution rates over time indicate selection limits Muller's ratchet.
• Selection removes individuals with high mutational loads, slowing mutation fixation.
• This selective pressure may reduce the rate of p-endosymbiont extinction.