The dN/dS ratio quantifies the effectiveness of selection by comparing the rate
ID: 212305 • Letter: T
Question
The dN/dS ratio quantifies the effectiveness of selection by comparing the rate of substitution of mutations at silent sites (dS), which do not affect the amino acid sequence and so are presumed neutral, to the rate of substitution of mutations at non-silent sites (dN), which result in changes in amino acid sequence and therefore may experience selection. Note that most changes in amino acid sequence are deleterious. A dN/dS ratio close to 0 indicates that natural selection has removed nearly all mutations that cause protein changes. A higher dN/dS ratio indicates that natural selection is less effective at removing deleterious mutations.
In a study of stick insects, researchers sequenced three coding genes and estimated the number of silent and non-silent mutations that have fixed since divergence in six pairs of species. Each pair contained one sexual species and a related species that has evolved to be asexual. In asexual species, females produce clonal offspring in “virgin births”. Inspect the figure below and provide a plausible explanation for the differences observed between species. dN/dS ratios are averaged across six sexual and six asexual species.
1.01 Asexuals 0.8 Sexuals 0.6 0.4 0.2 0.0 Actin Hsp70 COI FIG. 1. The per codon dN/dS rates (±standard deviation) for sexual and asexual Timema lineages for the three analyzed genes.Explanation / Answer
The asexual individuals of the Timema lineages show greater dN/dS ratio in the three genes than the sexual individuals. That means mutations are mostly at non-silent sites,and natural selection is less effective at removing deleterious mutations in the asexual individuals,as because in asexual individuals due to lack of sexual mating and recombination events,the chance of removing deleterious mutations is less.
And in the sexual individuals,the mutations are mostly at silent sites,so the mutations are not deleterious. And natural selection can remove these mutations.
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