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

Allen JM, Light JE, Perotti MA, Braig HR, Reed DL - PLoS ONE (2009)

Bottom Line: When comparing Riesia to other insect p-endosymbionts, we find that nucleotide substitution rates decrease dramatically as the age of endosymbiosis increases.A decrease in nucleotide substitution rates over time suggests that selection may be limiting the effects of Muller's ratchet by removing individuals with the highest mutational loads and decreasing the rate at which new mutations become fixed.This countering effect of selection could slow the overall rate of endosymbiont extinction.

View Article: PubMed Central - PubMed

Affiliation: Zoology Department and Florida Museum of Natural History, University of Florida, Gainesville, Florida, USA. juliema@ufl.edu

ABSTRACT

Background: Primary bacterial endosymbionts of insects (p-endosymbionts) are thought to be undergoing the process of Muller's ratchet where they accrue slightly deleterious mutations due to genetic drift in small populations with negligible recombination rates. If this process were to go unchecked over time, theory predicts mutational meltdown and eventual extinction. Although genome degradation is common among p-endosymbionts, we do not observe widespread p-endosymbiont extinction, suggesting that Muller's ratchet may be slowed or even stopped over time. For example, selection may act to slow the effects of Muller's ratchet by removing slightly deleterious mutations before they go to fixation thereby causing a decrease in nucleotide substitutions rates in older p-endosymbiont lineages.

Methodology/principal findings: To determine whether selection is slowing the effects of Muller's ratchet, we determined the age of the Candidatus Riesia/sucking louse assemblage and analyzed the nucleotide substitution rates of several p-endosymbiont lineages that differ in the length of time that they have been associated with their insect hosts. We find that Riesia is the youngest p-endosymbiont known to date, and has been associated with its louse hosts for only 13-25 My. Further, it is the fastest evolving p-endosymbiont with substitution rates of 19-34% per 50 My. When comparing Riesia to other insect p-endosymbionts, we find that nucleotide substitution rates decrease dramatically as the age of endosymbiosis increases.

Conclusions/significance: A decrease in nucleotide substitution rates over time suggests that selection may be limiting the effects of Muller's ratchet by removing individuals with the highest mutational loads and decreasing the rate at which new mutations become fixed. This countering effect of selection could slow the overall rate of endosymbiont extinction.

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Related in: MedlinePlus

Maximum likelihood phylogram representing phylogenetic relationships of Riesia p-endosymbionts.Numbers above the node represent divergence dates (in millions of years) whereas numbers below the nodes are bootstrap support values (only numbers greater then 60 are shown). The divergence date calibration point of 9.42–17.38 My is indicated with an asterisk.
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pone-0004969-g002: Maximum likelihood phylogram representing phylogenetic relationships of Riesia p-endosymbionts.Numbers above the node represent divergence dates (in millions of years) whereas numbers below the nodes are bootstrap support values (only numbers greater then 60 are shown). The divergence date calibration point of 9.42–17.38 My is indicated with an asterisk.

Mentions: Using the 9.42–17.38 My split between Pediculus and Pthirus as a calibration date [35], [36], we estimated the divergence time between Riesia pediculicola (human head and body louse p-endosymbionts) and Riesia pediculischaeffi (chimp louse p-endosymbionts) at 5.42 My, which is very close to the ages estimated for these lice and for their vertebrate hosts [36]. We further estimate that the p-endosymbionts of the human head lice originated 0.90 My (Figure 2), which is similar to the estimate of 1.2 My for the lice [36]. The pairwise sequence divergence for Riesia p-endosymbionts of Pediculus and Pthirus is 12.90% (GTR+I model), therefore the absolute rate of evolution of Riesia p-endosymbionts is 0.0037–0.0684 substitutions per site per million years which translates to 18.56–34.24% per 50 My (Table 1).


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

Allen JM, Light JE, Perotti MA, Braig HR, Reed DL - PLoS ONE (2009)

Maximum likelihood phylogram representing phylogenetic relationships of Riesia p-endosymbionts.Numbers above the node represent divergence dates (in millions of years) whereas numbers below the nodes are bootstrap support values (only numbers greater then 60 are shown). The divergence date calibration point of 9.42–17.38 My is indicated with an asterisk.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2654755&req=5

pone-0004969-g002: Maximum likelihood phylogram representing phylogenetic relationships of Riesia p-endosymbionts.Numbers above the node represent divergence dates (in millions of years) whereas numbers below the nodes are bootstrap support values (only numbers greater then 60 are shown). The divergence date calibration point of 9.42–17.38 My is indicated with an asterisk.
Mentions: Using the 9.42–17.38 My split between Pediculus and Pthirus as a calibration date [35], [36], we estimated the divergence time between Riesia pediculicola (human head and body louse p-endosymbionts) and Riesia pediculischaeffi (chimp louse p-endosymbionts) at 5.42 My, which is very close to the ages estimated for these lice and for their vertebrate hosts [36]. We further estimate that the p-endosymbionts of the human head lice originated 0.90 My (Figure 2), which is similar to the estimate of 1.2 My for the lice [36]. The pairwise sequence divergence for Riesia p-endosymbionts of Pediculus and Pthirus is 12.90% (GTR+I model), therefore the absolute rate of evolution of Riesia p-endosymbionts is 0.0037–0.0684 substitutions per site per million years which translates to 18.56–34.24% per 50 My (Table 1).

Bottom Line: When comparing Riesia to other insect p-endosymbionts, we find that nucleotide substitution rates decrease dramatically as the age of endosymbiosis increases.A decrease in nucleotide substitution rates over time suggests that selection may be limiting the effects of Muller's ratchet by removing individuals with the highest mutational loads and decreasing the rate at which new mutations become fixed.This countering effect of selection could slow the overall rate of endosymbiont extinction.

View Article: PubMed Central - PubMed

Affiliation: Zoology Department and Florida Museum of Natural History, University of Florida, Gainesville, Florida, USA. juliema@ufl.edu

ABSTRACT

Background: Primary bacterial endosymbionts of insects (p-endosymbionts) are thought to be undergoing the process of Muller's ratchet where they accrue slightly deleterious mutations due to genetic drift in small populations with negligible recombination rates. If this process were to go unchecked over time, theory predicts mutational meltdown and eventual extinction. Although genome degradation is common among p-endosymbionts, we do not observe widespread p-endosymbiont extinction, suggesting that Muller's ratchet may be slowed or even stopped over time. For example, selection may act to slow the effects of Muller's ratchet by removing slightly deleterious mutations before they go to fixation thereby causing a decrease in nucleotide substitutions rates in older p-endosymbiont lineages.

Methodology/principal findings: To determine whether selection is slowing the effects of Muller's ratchet, we determined the age of the Candidatus Riesia/sucking louse assemblage and analyzed the nucleotide substitution rates of several p-endosymbiont lineages that differ in the length of time that they have been associated with their insect hosts. We find that Riesia is the youngest p-endosymbiont known to date, and has been associated with its louse hosts for only 13-25 My. Further, it is the fastest evolving p-endosymbiont with substitution rates of 19-34% per 50 My. When comparing Riesia to other insect p-endosymbionts, we find that nucleotide substitution rates decrease dramatically as the age of endosymbiosis increases.

Conclusions/significance: A decrease in nucleotide substitution rates over time suggests that selection may be limiting the effects of Muller's ratchet by removing individuals with the highest mutational loads and decreasing the rate at which new mutations become fixed. This countering effect of selection could slow the overall rate of endosymbiont extinction.

Show MeSH
Related in: MedlinePlus