16 September 2007 Coalescent Consequences for Consensus Cladograms J. H. Degnan 1, M. Degiorgio 2, D. Bryant 3, and N. A. Rosenberg 1,2 1 Dept. of Human.

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Presentation transcript:

16 September 2007 Coalescent Consequences for Consensus Cladograms J. H. Degnan 1, M. Degiorgio 2, D. Bryant 3, and N. A. Rosenberg 1,2 1 Dept. of Human Genetics, U. of Michigan 2 Bioinformatics Program, U. of Michigan 3 Dept. of Mathematics, U. of Auckland

Outline  Species trees vs. gene trees  Consensus tree background  Asymptotic consensus trees  Finite sample consensus trees  Consistency results  Conclusions

Gene trees vary across the genome

Why? Incomplete lineage sorting, horizontal gene transfer, sampling, etc.

Gene tree discordance  From one true species tree, we expect there to be different gene trees at different loci as a result of lineage sorting, independently of problems due to estimation or sampling error.  Gene tree discordance depends especially on branch lengths in the species tree, measured by the number of generations scaled by effective population size, t / (2N).

Consensus (majority-rule)

Asymptotic consensus trees  Consensus trees are usually statistics, functions of data like x-bar.  We consider replacing observed (estimated) gene trees with their theoretical probabilities under coalescence and determining the resulting consensus tree.  Motivation: if there are a large number of independent loci, observed clade proportions should approximate their theoretical probabilities.

Types of consensus trees  Strict—only clades that are included in observed trees are in the consensus tree. In the coalescent model, all clades have probability > 0.  Democratic vote—use the gene tree that occurs most frequently.  Majority rule—consensus tree has all clades that were observed in > 50% of trees.  Greedy—sort clades by their proportions. Accept the most frequently observed clades one at a time that are compatible with already accepted clades. Do this until you have a fully resolved tree.  R*—for each set of 3 taxa, find the most commonly occurring triple e.g., (AB)C, (AC)B or (BC)A. Build the tree from the most commonly occurring triples.

Unresolved zone for majority-rule and too-greedy zone

What about finite samples?  If you sample 10 loci, you could have:  All 10 match the species tree  9 match the species tree, 1 disagrees  8 match the species tree, 2 disagree, etc.  You can consider gene trees as categories and use multinomial probabilities for the probability of your sample  By enumerating all multinomial samples, you can compute the probabilities of every possible consensus tree.

Are consensus trees inconsistent estimators of species trees?  Theorem 1. Majority-rule asymptotic consensus trees (MACTs) do not have any clades not on the species tree.  Theorem 2. Greedy asymptotic consensus trees (GACTs) can be misleading estimators of species for the 4-taxon asymmetric tree and for any species tree with n > 4 species.  Theorem 3. R* asymptotic consensus trees (RACTs) always match the species tree.

Conclusions  Coalescent gene tree probabilities are useful for understanding asymptotic behavior of consensus trees constructed from independent gene trees.  R* consensus trees are consistent and more resolved than majority-rule consensus trees.  Greedy consensus trees can be misleading, but are quicker to approach the species tree than majority-rule or R* when outside of the greedy zone.