VIEW THE EVIDENCE
Mr. Pete Anderson attempted to sell some ancient pots to a state agent, Suzanne Smith, during a sting operation that had been organized to find and prosecute people who loot archaeological sites. The pots appeared to be from the Anasazi cultural group. Inside one of the pots were modern plant remains that looked like parts of a daisy. The state agent knew of an Anasazi site that had been looted during the summer. She went to the site and found that three types of daisies grew in that area. The three types of daisies are shown below.
Outwardly, the three types of plants look pretty much the same.X. confertifolia
The state agent investigating the possible theft of the Anasazi pots sought expert advice from the state botanist regarding the daisy-like plants she had found. The botanist told the investigating agent that the daisies might represent different species. Specifically, the agent was advised that the size of the "compositase" protein could be a clue to the identity of the plants. Proteins from all three types of daisies were examined using starch gel electrophoresis. The results of these experiments (shown below) indicate that the three kinds of plants are not identical. Two of the plants showed single, but distinct forms of the "compositase" protein, and were identified as X. tortifolia and X. confertifolia. The third plant was different from the first two in this analysis. This plant appeared to be a hybrid between the first two. Both the X. tortifolia protein and the X. confertifolia protein were present in the third type of plant. This daisy was identified as X. cronquistii. The identification of the hybrid was tremendously exciting to the state botanist! He explained that this hybrid was extremely rare, found in only a few sites in the Southwest.
Shown below is a gel comparing samples of protein derived from the three types of daisies found at the looted archaeological site, as well as samples from the daisy remains found in the pot Anderson had attempted to sell to the state agent, Suzanne Smith. (See column at right on how to read gels.)
The botanist had given Ms. Smith a general description of where different daisy species grow. Did this description conflict with the conclusions of the protein chemists regarding the identity of the daisy species? In this map of Utah, the red dot indicates the statewide distribution of the rare hybrid, X. cronquistii. As the botanist reported, this type of plant is only found in a very small region. Plus, the state agent thought excitedly, this location coincides with the location of the looted Anasazi site!
The state agent went back to her notes. Where exactly had she found each type of plant? This is a photo of the archaeological site and the spot where the daisies were found. The deep blue circle designates the archeological site, the green circles show patches of the daisy the gel study had identified as X. cronquistii, the yellow circles indicate patches of the X. confertifolia daisy, and the pink circle shows patches of the X. tortifolia daisy.
The state agent, Suzanne Smith, and the state botanist scouted the area of the archaeological site, taking notes about the locations of the different types of daisies. This is a graphical representation of the site, showing the location of the patches of plants, and the hut. Brown indicates brown clay soil; gray indicates gray shale soil.
Consulting botanical reports from the Utah State Herbarium, the state agent, Suzanne Smith, learned these facts about the daisy species that the gel electrophoresis experiments had identified:
X. tortifolia grows in gray shale soils in the washes of Southern Utah. Most plants grow on steep slopes near the bottoms of the washes.
X. confertifolia grows in brown clay soils on mesa tops, and in washes where eroded brown clay soil appears.
X. cronquistii is a rare hybrid form of the X. tortifolia and X. confertifolia species, found in only a few locations. X. cronquistii prefers gray shale soil.
Tickled pink about their find of the rare hybrid, the state botanist sent this supporting information to the state investigator. He wrote, "This is my first look at cronquistii -- I never thought I'd see one! I'd be totally happy if you had found it in happier circumstances. Go get those pot hunters!"
Scientific Name: Xylorhiza cronquistii Welsh and Atwood in Welsh
Common Name: Cronquist woodyaster
Family (Common Name): Asteraceae or Compositae (Sunflower Family)
Global Distribution: Kane (Horse Mountain) County, Utah; a Colorado Plateau endemic
Management Responsibility: BLM-Cedar City District
Habitat: Pinyon-juniper community, on the Kaiparowits Formation, at 6,200 to 6,800 feet elevation, June.
Look-alikes: Similar to X. tortifolia, but differs in the sparingly toothed leaf margins and linear-oblanceolate leaf shape.
Source: The Endangered, Threatened and Sensitive Plant Field Guide, prepared by the Utah TES Plant Interagency Committee.
Now that you have reviewed the evidence gathered by the state agent, take some time to analyze its importance. Can this evidence tie Anderson to the crime of looting the archaeological site? Can the evidence tie the Anasazi pot in question to that particular site? How convincing do you find this evidence? Examine the electrophoretic analysis of the "compositase" protein in the three collected daisies and in the daisy remains in the Anasazi pot. Review all the supporting data regarding where the three daisy varieties can be found, and how prevalent these plants are. What do you think?
How to read gels
Gel electrophoresis is a tool scientists use to separate molecules from each other. Different gel systems have been developed to separate individual DNA, RNA or protein molecules in a mixture of molecules from each other. Gels are literally just what they sound like: matrices of a gelatin-like substance (the actual substances can vary). Samples are loaded in wells formed in the gel and then are pulled through the gel, usually by electric current. How gels resolve mixtures depends on a number of factors, but most commonly, gels resolve molecules on the basis of size.
Each vertical row on a gel is referred to by scientists as a lane. When a gel is "run", samples of protein material are loaded at the top of the gel, all at the same horizontal position. Electric current is applied, causing the protein samples to be drawn into the gel matrix. As the samples are pulled toward the bottom of the gel, smaller proteins travel most quickly, larger proteins travel more slowly. As a result, a mixture of proteins can be resolved into distinct "bands" (the black horizontal blobs you can see in each lane below). After running the protein samples into the gel, the gel is stained to allow the proteins to be visualized. In this way, scientists can see how many different types of proteins are present, and what their relative sizes are. Since all the samples were initially loaded at the same horizontal level, the sizes of proteins in different lanes can be compared to each other, to see if neighboring lanes have similarly sized proteins. By running proteins of known size in adjoining lanes, the actual size of the proteins in the mixture can be estimated.
The picture above shows a stained protein gel (starch gel electrophoresis). Samples from the three types of daisies and from the daisy found in the Anasazi pot were run on this gel to resolve "compositase" proteins on the basis of size.
Lanes 1 and 2 show samples from the daisy remains found in the
Can you see that the hybrid X. cronquistii daisy displays both parental types of "compositase" proteins (both the X. confertifolia and X. tortifolia forms) ?
Understanding what you see
Each type of plant has two copies of the gene for the "compositase" protein. Each of the parental daisies has two identical copies of the gene, and therefore makes only one variety of "compositase" protein from those genes. The hybrid daisy (a cross between the two parental types) carries one copy of the confertifolia variant of the gene and one copy of the tortifolia variant of the gene. The hybrid, therefore, makes both sorts of proteins and shows two bands on the protein gel. Note: It is not uncommon for variant genes to exist, causing slightly different proteins to be made in different strains of an organism, or even in different individuals within a strain. Formally, these variant genes are mutations. However, unlike the usual way we think of mutations, these changes in the sequence of the proteins may not harm the organism, and may go entirely unrecognized.
Now go and look at the pictures of the plants themselves. Can you tell the difference between the plants just by looking at them? Not in this case. Sometimes morphology (how things look) is a rigorous measure in an effort to distinguish different species. When morphological differences are not enough, molecular differences can be very valuable tools. In this investigation, the analysis of the plant proteins allowed the agent to see unambiguous differences between the three kinds of plants at the molecular level.