«Investigating Bear and Panda Ancestry Adapted from Maier, C.A. (2001) “Building Phylogenetic Trees from DNA Sequence Data: Investigating Polar Bear ...»
Investigating Bear and Panda Ancestry
Adapted from Maier, C.A. (2001) “Building Phylogenetic Trees from DNA Sequence Data: Investigating
Polar Bear & Giant Panda Ancestry.” The American Biology Teacher. 63:9, Pages 642-646.
We have already used some molecular databases to examine evolutionary relationships among reptiles,
bats, and birds, as well as conducting our own studies of evolutionary relationships. We have used
amino acid sequences for proteins to examine these questions. We will now go “one level deeper” as we explore DNA sequences and how these can also be used to create evolutionary relationships.
You might wonder, “how do I decide if to use protein or DNA sequence data, and for which sequences?” The answer is to use as many resources as you have. You may not obtain the SAME result when using different resources. That is, a phylogenetic tree based on hemoglobin amino acids may not look the same as a tree based on DNA encoding rRNA. Likewise, trees based on morphology may not agree with molecular based trees. But scientists look at all these data, and draw conclusions based on overlaps and consistent patterns between the various data sources.
For this exercise you will investigate these questions: 1) “What is the relationship between the polar bear and the American Brown bear?” The geographic ranges of these two species are in close proximity. Did the polar bear evolve from a population of American brown bear that were isolated far north (allopatric speciation)? 2) How about the giant panda? This species superficially resembles the bear family and yet its diet, behavior, and genital structure are quite different (Morris and Morris, 1981). 3) Finally, red pandas may not be named appropriately. Although facial features look “panda-ish,” the life history is not that much like a panda and the tail certainly isn’t bear-ish. Is the giant panda a bear or did it branch from some other lineage? We will address these questions by building genetic distance matrices and phylogenetic trees based on molecular sequence data.
It is possible to create a molecular phylogenetic tree of the panda and bear families because the 12s ribosomal RNA gene sequences from several bear species and the giant panda are available from GenBank (a molecular database). Since the 12s ribosome gene is carried on the mitochondrial chromosome, it does not undergo recombination and is inherited directly from mother to offspring through the egg. These characteristics make mitochondrial genes excellent tools for studying the family trees of closely related species (Cann et al, 1987).
You will be performing two sets of analyses. First, you will look at several bear species and examine their DNA (and therefore evolutionary relationships). Next, you will include the giant panda and determine its relationship to other bear species. From your findings, we will discuss whether the data support the giant panda and red panda being classified among the bears, or whether they belong in separate groups.
These are the steps to access bear and giant panda 12s rRNA sequence from GenBank, import them into Biology workbench, then use Biology Workbench nucleic acid tools to align chosen sequences, create genetic distance matrices, and develop phylogenetic trees. Because you have experience searching for sequences by name, we have provided you with accession numbers to make things go faster (see table on the top of page 2).
1. Access the 12s rRNA sequences and import them into sequence analysis software. Go to www.ncbi.nlm.nih.gov. Select “Nucleotide”, enter the accession number for the following 12s rRNA gene sequences one at a time, and select “GO.”
2. Click on the accession number.
3. Next to the “Display” window, select “FASTA” to bring up the gene sequence in FASTA format.
4. Highlight and copy the entire FASTA sequence, up to and including the initial “” symbol.
Paste into a WORD document.
5. Repeat steps 2-4, finding, displaying, highlighting, copying and pasting until all 12s rRNA sequences have been found.
6. SAVE your WORD document!!!
7. From the WORD menu, under FILE, select SAVE AS. In the resulting window, look for the format option, then select TEXT ONLY (the default is a formatted WORD document with hidden codes), then click on SAVE. This step saves your sequences as a simple text file, which is preferred by most bioinformatics application. The hidden formatting commands in WORD often cause errors in sequence analysis. You should now have two copies if your 12s rRNA sequences; one as a word document (file name ends in.doc, such as bears.doc) and one as a text document (file name ends in.txt, such as bears.txt).
7. Go to Biology Workbench. http://workbench.sdsc.edu (Notice: NO www)
8. Select “Session” then “Start New Session,”
9. Name the session then select Start
10. Select “Nucleic Tools”
11. Select “Add New Nucleic Sequence” then click on “RUN”
12. In the Label box, name the session then click on RUN
13. Now you could toggle back and forth from NCBI to WORKBENCH and copy and paste FASTA sequences but you saved them all in a WORD document, so let’s explore another way to add sequences to WORKBENCH. Click on “Browse” and find your TEXT document (bears.txt), select it, then select “Upload File.” Just like magic, your sequences will all end up in separate windows. IF they don’t, you have probably selected the bears.doc instead of bears.txt.
14. Edit the FASTA label in the “Sequence” box (the box containing all the A, T, C, and G). Leave the but delete all but the name of the critter – in other words the names you want to appear on the tree.
15. Select “SAVE.” Now you have a session with a bunch of sequences, all individually listed. You can select two or more as you need them.
Align the Bear 12s rRNA Gene Sequences.
1. For now, select all the bears but omit the raccoon, dog, and skink.
2. Choose CLUSTALW then “RUN” then “SUBMIT”.
3. Copy and Paste the Phylogram into a WORD Document. This is data you will be turning in.
4. Click on “IMPORT ALIGNMENT” to enter the Alignment Tools section of the Biology Workbench.
2Determine the Genetic Distance between sequence pairs.
1. Click on the box to the left of the imported and aligned sequences, then choose “CLUSTALDIST,” “Run”, and “SUBMIT.” Print the distance matrix so that you can examine it closely later.
2. Copy and paste the “Clustal Distance Matrix” table into a WORD document. This is also data!! You will examine it and turn it in.
3. The last piece of data that could be helpful in the future (so you should include) is a BOXSHADE.
You can also copy and paste this into a WORD document.
Repeat the alignment, phylogram, and matrix after including the dog and raccoon, then again after adding the cow and the skink.
Analysis Questions. Answer these questions after analyzing the data you obtained from alignments, matrices, and phylograms..
1. What are we trying to show by constructing phylograms, matrices and sequence alignments?
2. As you examine the phylogenetic tree of the bear species’ 12s rRNA gene sequences, which two are most closely alike? Which “pair of bears” seems to be the most distantly related?
3. Which are closer relatives; “American Brown and American Black Bears” or “American Black and Asiatic Black Bears”? Explain your answer referring to your data.
4. Is the giant panda more of an “American” relative or “Asiatic” relative as compared to the bear group? Explain.
5. Further research the spectacled bear. Where is it found today? Explain its possible evolutionary origin.
6. Does the Giant Panda “belong” in the bear family at all? Explain your answer.
7. Where does the Red Panda fit into the phylogenetic tree? Who are its closest relatives?
8. Why did we include a cow and a crocodile skink in the final phylogram? What other organisms could have served the same purpose?
9. Finally, if you were a taxonomist, how would you answer this question: What is a bear? Is a bear a dog or is a dog a bear?
Include with your answers, your data, phylograms and any outside research that you used to answer these questions.
TEACHERS GUIDEExamining Phylogeny of Bears using Biology Workbench.
Sequences Used to prepare this key.
American Black-gi|1871572|emb|Y08520.1|MIUA12SR U.americanus mitochondrial 12S rRNA gene