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UniFrac: Comparing Microbial
Communities
Presented by:
Donovan Parks
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Overview
Motivating example
Quick look at Bray-Curtis Index (BCI)
UniFrac: distance measure, significance test, community tree, sequence jackknifing
Contrast BCI and UniFrac
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Why compare microbial communities?
Soil suppressing Tree Pathogens
Soil not suppressing Tree Pathogens
Is this guy protecting our trees?
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Bray-Curtis Index (BCI): Distance Measure
Measures the difference in OTU diversity between microbial communities.
Community 1
Community 2
Cluster
ClusterDifferences
Clusters defined by taxonomy or sequence similarity
BCI = 0.4 / 2 = 0.2
Cluster
Normalize
Normalize
0.3
0.3
0.4
0.4
0.4
0.2
0.1
0.1
0.2
Normalize so measure is between [0, 1]
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BCI: Pros and Cons
Pros: Simple (both conceptually and experimentally)
Considers both richness and evenness
# OTU in a Community
Relative Abundance
Cons: All clusters considered equally different
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UniFrac: Distance Measure
Measures the evolutionary distance between microbial communities
Similar Communities Maximally Different Communities
UniFrac Distance Measure = (------) / (------ + ------)
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UniFrac: Significance Test
...#
Tre
es
UniFrac Distance
Magic level of significance!
Randomize Community Labels
Observed Tree
Do two communities differ significantly?
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UniFrac: Community Distance Matrix
Pairwise Comparison
0
0
0
0.6 0.5
1.00.6
1.00.5
UPGMA
Distance Matrix Community Tree
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UniFrac: Sequence Jackknifing
How does the number and evenness of sequences from each community affect the results?
Sequence Jackknifing
UniFracAnalysis
UniFracAnalysis
Replicate 100 to 1000 times
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Weighted UniFrac
What about the relative abundance of sequences (i.e., evenness)?
Since there are twice as many -- sequences, they get half the weight.
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UniFrac: A Practical Look
Multiple Sequence Alignment
(e.g., MUSCLE)
16S Gene Sequence
Library
Tree Construction(e.g., RAxML)
UniFrac Analysis16S Gene Sequence
Library
16S Gene Sequence
Library
Filter Sequences
Filter Sequences
Filter Sequences
Tree must be rooted
Tree must have meaningful branch lengths
Remove duplicate sequences if not doing weighted UniFrac.
Community Tree
0.7
0.8
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Final Remarks: BCI vs. UniFrac
They are different, equally valid measures!
BCI: taxon-based method which considers richness and evenness
UniFrac: phylogenetic-based method which considers evolutionary divergence
Which to prefer depends on the questions you are trying to answer
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Final Remarks: Many Ways to Compare
Many richness and evenness measures (e.g., Jaccard, Sörensen)
Other phylogenetic distance measures (e.g., TreeClimber)
Many interesting and meaningful ways to compare microbial communities:
Richness and evenness Evolutionary divergence Functional diversity
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References
UniFrac: Lozupone C, Knight R, UniFrac: a New Phylogenetic Method for Comparing Microbial Communities, Appl. Environ. Microbiol., Vol. 71, December 2005.
Lozupone C, Hamady M, Knight R, UniFrac - An Online Tool for Comparing Microbial Community Diversity in a Phylogenetic Context, BMC Bioinformatics, Vol. 7, August 2006.
Online Tool: http://bmf2.colorado.edu/unifrac/index.psp (excellent help page)
BCI: Bray R, Curtis T, An Ordination of the Upland Forest Communities of Southern Wisconsin, Ecological Monographs, Vol. 27, 1957.
TreeClimber (source of soil sample example): Schloss PD, Handelsman J, Introducing TreeClimber, a test to compare microbial community structures, Applied and environmental microbiology, Vol. 72, No. 4., April 2006.