1. Phylogenetic Trees Tutorial 5

2. Agenda How to construct a tree using Neighbor Joining algorithm Phylogeny.fr tool Cool story of the day: Horizontal gene transfer

3. 3 Neighbor Joining Algorithm Constructs an unrooted guide tree from a distance matrix We do not assume constant rate of evolution

4. 1.Calculate all pairwise distances. 2.Find 2 nodes i and j, such that the relative distance between i and j is minimal. 3.Remove the rows and columns of i and j 4.Add a new row and column k (the parent of i and j), and compute the distance from k to any other remaining node. 5.Continue until two nodes remain – connect them with an edge. Neighbor Joining Algorithm 2 matrices

5. EDCBA 4139 220A 4341 022B 201804139C 100184139D 010204341E Step 1. Calculate all pairwise distances A, B, …, E are tree nodes. Each character represents a sequence. How can we measure distance between sequences?

6. Euclidean Distance: Given a multiple sequence alignment, calculate the square root of the sum of the score at every position between two sequences The score increases as the dissimilarity between residues increases. Step 1. Calculate all pairwise distances distance between sequences

7. If we assume constant evolution rate we may construct a wrong tree. Closest leaves aren’t necessarily neighbors: i and j are neighbors, but (d ij = 13) > (d jk = 12) Step 2. Two nodes with minimal relative distance

8. Find a pair of leaves that are close to each other, but far from other leaves. This is called “relative distance”.

9. Relative distance between i and j Distance between i and j (from the distance table) Distance between i and all other nodes Number of leaves (=sequences) left in the tree Step 2. Two nodes with minimal relative distance

10. EDCBA 4139 220A 4341 022B 201804139C 100184139D 010204341E Distances matrix: A B C D E

11. EDCBA -44 -47.3-74-A -44 -47.3--B -57.3 ---C -64----D -----E A,B is the pair with the minimal M i,j distance. The Mij Table is used only to choose the closest pairs (lowest value) and not for calculating the distances Step 2. Two nodes with minimal relative distance The relative distance table:

12. Steps 3+4. Remove i, j and add k to the matrix D km = (D im + D jm – D ij )/2 Compress i and j into k, iterate algorithm for rest of tree the distance from k to any other leaf m can be computed as:

13. EDCk 3129 0k 2018029C 1001829D 0102031E Now we’ll calculate the distance from X to all other nodes: Steps 3+4. Remove i, j and add k to the matrix A B C D E K

14. The final tree: Steps 5. Continue till 2 nodes remain A B C D E K Y 12 10 20 5 6 4 9 What is missing? Z

15. Phylogeny.fr

16. One click mode

18. Multiple sequence alignment (MSA) is a basic step in phylogenetic tree construction and is the base for the distance matrix

19. “A la carte” mode

20. “On-click” default settings

21. Cool Story of the day Horizontal gene transfer

22. Is horizontal gene transfer possible?

23. Viruses

24. Horizontal gene transfer is the primary reason for bacterial antibiotic resistance and plays an important role in the evolution of bacteria. Horizontal gene transfer in Bacteria http://en.wikipedia.org/wiki/Horizontal_gene_transfer Horizontal gene transfer is very abundant in bacteria, it is hard to talk about a bacteria’s genome, but more of the genome of a “society of bacteria”.

25. Sea slug The sea slug Elysia chlorotica incorporates chloroplasts from the algae that it eats into its body. Photosynthesis continues for up to 12 months using genes within the chloroplast, which are directed by algal nuclear genes that were transferred to the nuclei of the slug.Elysia chloroticachloroplastsalgaePhotosynthesis http://en.wikipedia.org/wiki/Horizontal_gene_transfer

26. Bioinformatics/ David W.Mount p. 244 Until the full speciation…