Phylogenetic tree construction...Rooted / Unrooted trees ! Rooted tree: directed to a unique node !...
Transcript of Phylogenetic tree construction...Rooted / Unrooted trees ! Rooted tree: directed to a unique node !...
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Phylogenetic tree construction
http://libguides.scu.edu/evolution
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Outline
q Phylogenetic tree types q Distance Matrix method
q UPGMA q Neighbor joining
q Character State method q Maximum likelihood
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Phylogenetic tree?
q A tree represents graphical relation between organisms, species, or genomic sequence
q In Bioinformatics, it’s based on genomic sequence
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What do they represent?
q Root: origin of evolution q Leaves: current organisms, species, or genomic
sequence q Branches: relationship between organisms, species,
or genomic sequence q Branch length: evolutionary time (in cladogram, it doesn't represent time)
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Rooted / Unrooted trees q Rooted tree: directed to a unique node
q (2 * number of leaves) - 1 nodes, q (2 * number of leaves) - 2 branches
q Unrooted tree: shows the relatedness of the leaves without assuming ancestry at all q (2 * number of leaves) - 2 nodes q (2 * number of leaves) - 3 branches
https://www.nescent.org/wg_EvoViz/Tree
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More tree types used in bioinformatics (from cohen article)
q Unrooted tree q Rooted tree
q Cladograms: Branch length have no meaning
q Phylograms: Branch length represent evolutionary change
q Ultrametric: Branch length represent time, and the length from the root to the leaves are the same
https://www.nescent.org/wg_EvoViz/Tree
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How to construct a phylogenetic tree?
q Step1: Make a multiple alignment from base alignment or amino acid sequence (by using MUSCLE, BLAST, or other method)
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How to construct a phylogenetic tree?
q Step 2: Check the multiple alignment if it reflects the evolutionary process.
http://genome.cshlp.org/content/17/2/127.full
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How to construct a phylogenetic tree? cont
q Step3: Choose what method we are going to use and calculate the distance or use the result depending on the method
q Step 4: Verify the result statistically.
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Distance Matrix methods
q Calculate all the distance between leaves (taxa) q Based on the distance, construct a tree q Good for continuous characters q Not very accurate q Fastest method
q UPGMA q Neighbor-joining
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UPGMA
q Abbreviation of “Unweighted Pair Group Method with Arithmetic Mean”
q Originally developed for numeric taxonomy in 1958 by Sokal and Michener
q Simplest algorithm for tree construction, so it's fast!
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How to construct a tree with UPGMA?
q Prepare a distance matrix q Repeat step 1 and step 2 until there are only two
clusters q Step 1: Cluster a pair of leaves (taxa) by shortest distance
q Step 2: Recalculate a new average distance with the new cluster and other taxa, and make a new distance matrix
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Example of UPGMA A B C D E
A 0
B 20 0
C 60 50 0
D 100 90 40 0
E 90 80 50 30 0
q New average distance between AB and C is: q C to AB = (60 + 50) / 2 = 55
q Distance between D to AB is: q D to AB = (100 + 90) / 2 = 95
q Distance between E to AB is: q E to AB = (90 + 80) / 2 = 85
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AB C D E
AB 0
C 55 0
D 95 40 0
E 85 50 30 0
Example of UPGMA cont 1
q New average distance between AB and DE is: q AB to DE = (95 + 85) / 2 = 90
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Example of UPGMA cont 2
AB C DE
AB 0
C 55 0
DE 90 45 0
q New Average distance between CDE and AB is: q CDE to AB = (90 + 55) / 2 = 72.5
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Example of UPGMA cont 3
AB CDE
AB 0
CDE 72.5 0
q There are only two clusters. so this completes the calculation!
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Downside of UPGMA
q Assume molecular clock (assuming the evolutionary rate is approximately constant)
q Clustering works only if the data is ultrametric q Doesn’t work the following case:
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Neighbor-joining method
q Developed in 1987 by Saitou and Nei q Works in a similar fashion to UPGMA q Still fast – works great for large dataset q Doesn’t require the data to be ultrametric q Great for largely varying evolutionary rates
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BM vonHoldt et al. Nature 000, 1-5 (2010) doi:10.1038/nature08837
Example Neighbor-Joining Tree for Dogs