Peter D. Pixton and Dr. Mark C. Belk, Microbiology
Utah chub (Gila Atraria) is a small fish native to the ancient Bonneville Basin. After a series of floods, Lake Bonneville receded approximately ten thousand years ago, isolating fish and other fauna that had been a part of the Greater Bonneville drainage. Pockets of Gila Atraria become isolated in drainages throughout Utah, Wyoming, and Idaho, and the genetic flow between these stranded populations stopped.
A recent study of eight different populations of Utah chub showed remarkable characteristics. Four of the populations coexisted 1 with Cutthroat Trout, a predator of Gila Atraria, and four of the populations were in habitats with no predators. In comparison with the non-predator populations, all of the predator populations showed a faster growth rate, a faster time to reproductive maturity, a larger reproductive effort, and a greater body mass.
Current theories suggest that predation has been a significant evolutionary pressure within the predator populations of Gila Atraria, selecting for fish that grow faster and reproduce sooner than populations that do not coexist with predators. If this hypothesis is true, perhaps some fish are better adapted genetically to survive the pressures of predation.
My project has centered upon the genetic analysis of these populations. Specifically, I have been interested in mitochondrial DNA variations between the eight different populations. Because mitochondrial DNA is maternally inherited, and because different regions show different rates of mutation, it is an excellent source of markers.
DNA was extracted from ethanol preserved tissues using a potassium acetate and phenol extraction method or by a new DNA isolation method offered by Puregene. Once DNA had been isolated from representative fish from each of the eight populations, the DNA was amplified by the Polymerase Chain Reaction (PCR) to yield a large number of copies of target DNA sequences (Fig. 1).
Using restriction endonucleases (a special set of enzymes that cut DNA at specific recognition sites), the DNA was digested into fragments and then run on an agarose gel. The fragments of DNA in the gel migrate according to their size, making a specific pattern for each fish. These patterns are then analyzed for similarities or differences. By using a variety of restriction endonucleases (each recognizes and cuts different sequences of DNA), it is possible to thoroughly examine the mitochondrial sequence of choice.
By using specific primers in the PCR step of the experiment, the ND I region of each Utah Chub was amplified and examined. Unfortunately, despite numerous digests and the usage of a variety of restriction enzymes, none of the populations showed any differences after restriction and reading on an agarose gel. Two hypotheses could explain these results: one, the fish are all very closely related maternally, or two, the ND I region of the mitochondrial DNA is highly conserved and therefore will not show variability within the short time that the populations have been separated from each other.
Since hypothesis one is highly unlikely, efforts are currently underway to find another region of the mitochondrial DNA that may be more useful as a marker. A search of the cytochrome B, cytochrome C, and ND2 regions will hopefully pinpoint differences in the mitochondrial DNA of each population. Ultimately, it is hoped that markers for each of the different populations will be found.2
References
- Johnson, Jerald B., Unpublished data, Brigham Young University, 1994. 1
- Special thanks to ORCA and Dr. R. Paul Evans for the use of his lab.