Taylor Maxwell and Dr. Duke Rogers, Zoology
Mexico is classified as a “megadiversity” country, which means that it has more species of plants and animals than the vast majority of other countries in the world. For example, there are ca. 4,750 species of mammals known to science, and more than one of every seven can be found in Mexico. The reasons that Mexico has so much biodiversity are complex, but in part, it is due to the fact that the region is climatically and topographically diverse. Habitats range from lowland tropical rain forest in the south to dry deserts in the north with a multitude of intervening mountain rages throughout. In Mexico, systematic biologists estimate that the number of species of well-known groups of animals (such as mammals and birds) may double once we have more complete biological information.
This study focuses on northwestern Mexico, particularly central Sonora. The deserts there are isolated from similar habitats elsewhere by mountain ranges to the east and north, the Sea of Cortez to the west and by subtropical habitats to the south. Several species of small mammals are found in this area and nowhere else. In other cases, species of mammals have larger geographic distributions, but have populations that are isolated in central Sonora by one or more mountain ranges. The purpose of this study was to examine the levels of genic differentiation within the silky pocket mouse (Parognathus flavus). This species is distributed from Texas westward into northern Sonora. Morphologically, this population has a smaller body size relative to other populations throughout the range of this species. Our purpose was to determine whether or not this isolated population is a distinct evolutionary entity. To address this question DNA sequence information was used.
We were able to obtain 20 of the expected 25 tissue samples for the analysis. They covered the major parts of the range from Texas, New Mexico, and down to southern Mexico. Most of the tissues were liver samples, which are great for extracting DNA. All of the extractions were successful.
My goal was to sequence the cytochrome b gene in the mitochondrial genome. I obtained primers, which had been previously developed for another group of distantly related mice (Peromyscus). I was able to obtain the full sequence (1129 base pairs) for samples that covered the full species range including a sequence from Parognathus parvus, its closest relative, which was used as the outgroup for the analysis. I used the computer program Sequencher to edit and align the sequences in preparation for the analysis. The aligned sequences were exported to the program PAUP, developed by David Swofford. PAUP is a program used to reconstruct phylogenetic trees from genetic data. It has many options depending on what criterion one adheres to (there are many competing methods with different assumptions) I chose the parsimony criterion. The philosophy of parsimony uses shared derived characters as information to reconstruct evolutionary relationships. The tree with the “fewest steps” is taken as the best tree.
The analysis was run multiple times varying different parameters. Because of the small size of the dataset I was able to do an exhaustive search which explores every possible tree. Cytochrome b is a protein encoding gene, therefore its sequence has codon positions one, two, and three. Each position may changes at different rates. Different weights were assigned to each position in various combinations to find out if the outcome was sensitive to different weights. The same tree was obtained regardless of the weighting scheme.
The Sonoran sequence was not significantly different from the other samples contrary to our expectations. Another significant finding concerns P. merriami gilvus, a subspecies proposed to be related to P. flavus. Mark Engstrom (1), from the Royal Toronto Museum, did a chromosomal and allozyme study which grouped it solely with P. merriami. While visiting with him about my project he was very interested to find out if his results would be supported by my project. He was unsure if his data was enough to answer the question. The P. merriami sample was basal to everything supporting his conclusion that it is not within the P. flavus group.
References
- Lee, T. E., Jr., and Mark D. Engstrom. 1991. Genetic variation in the silky pocket mouse (Parognathus flavus) in Texas and New Mexico. Journal of Mammalogy 72: 273-285.