Quick Lesson on dN/dS Neutral Selection Codon Degeneracy Synonymous vs. Non-synonymous dN/dS ratios Why Selection? The ProblemWhat does selection “look” like? Yokoyama S et al. PNAS 2008;105:13480-13485 When moving into new dim-light environments, vertebrate ancestors adjusted their dim-light vision by modifying their rhodopsins •Functional changes have occurred •Biologically significant shifts have occurred multiple times •How do we know whether these shifts are adaptive or random?Neutral Selection Mutations will occur evenly throughout the genome. Pseudogenes? Introns? Promoters? Coding Regions?Codon DegeneracyCodon Degeneracy AA #3 AA #2 AA #1 Wobble effect – an AA coded for by more than one codon 1st position = strongly conserved 2nd position = conserved 3rd position = “wobbly” Pos #3 Pos #2 Pos #1Synonymous vs Non-synonymous Synonymous: no AA change Non-synonymous: AA changeSynonymous vs Non-synonymousdN/dS ratios N = Non-synonymous change S = Synonymous change dN = rate of Non-synonymous changes dS = rate of Synonymous changes dN / dS = the rate of Non-synonymous changes over the rate of Synonymous changesSelection and dN/dS dN / dS == 1 => neutral selection dN / dS <= 1 => negative selection dN / dS >= 1 => positive selection No selective pressure Selective pressure to stay the same Selective pressure to changeWhy Selection? Identify important gene regions Find drug resistance Locate thrift genes or mutationsdN/dS Problem Analyzes whole gene or large segments But, selection occurs at amino acid level This method lacks statistical power Thus the purpose of this paperSLAC single likelihood ancestor counting The basic idea: Count the number of synonymous and nonsynonymous changes at each codon over the evolutionary history of the sample NN [Ds | T, A] NS [Ds | T, A]SLAC E40K L10ISLAC Strengths: Computationally inexpensive More powerful than other counting methods in simulation studies Weaknesses: We are assuming that the reconstructed states are correct Adding the number of substitutions over all the branches may hide significant events Simulation studies shows that SLAC underestimates substitution rate Runtime estimates Less than a minute for 200-300 sequence datasetsFEL fixed effects likelihood The basic idea: Use the principles of maximum likelihood to estimate the ratio of nonsynonymous to synonymous rates at each siteFEL Likelihood Ratio Test Ho: α = β Ha: α ≠ β fixedFEL Strengths: In simulation studies, substitution rates estimated by FEL closely approximate the actual values Models variation in both the synonymous and nonsynonymous substitution rates Easily parallelized, computational cost grows linearly Weaknesses: To avoid estimating too many parameters, we fix the tree topology, branch lengths and rate parameters Runtime Estimates: A few hours on a small cluster for several hundred sequencesREL random effects likelihood The basic idea: Estimate the full likelihood nucleotide substitution model and the synonymous and nonsynonymous rates simultaneously. Compromise: Use discrete categories for the rate distributionsREL 1. Posterior Probability 2. Ratio of the posterior and prior odds having ω > 1REL Strengths: Estimates synonymous, nonsynonymous and nucleotide rates simultaneously Most powerful of the three methods for large numbers sequences Weaknesses: Performs poorly with small numbers of sequences Computationally demanding Runtime Estimates: Not mentionedSimulation Performance 64 sequences 8 sequencesSelection and dN/dS dN / dS == 1 => neutral selection dN / dS <= 1 => negative selection dN / dS >= 1 => positive selection No selective pressure Selective pressure to stay the same Selective pressure to
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