Bruce W. Christensen and Dr. Jack W. Sites, Zoology
The family Iguanidae contains eight living genera of large, herbivorous lizards that by virtue of their size occupy important ecological niches wherever they are found. They are important elements of the food web, both as seed dispersers and prey for other animals, including humans (1). Of ecological and evolutionary significance, the family also contains several island endemics, including iguanas in the Carribean and on the Tongan, Fijian, and Galapagos archipelagoes.
A phylogenetic study is one that seeks to unravel the evolutionary relationships between the study organisms. Previous to 1996, researchers based phylogenetic studies of the family Iguanidae primarily on morphological (physical or structural) data. Two prominent studies proposed conflicting results. The most significant disagreement was the placement order of three genera: Iguana, Sauromalus, and Cyclura. One study (2) placed the genus Iguana as a group “older” than the other two. Another (3) hypothesized that Cyclura and Iguana were the “newest” groups with Sauromalus belonging to an older line, more closely related to the two Galapagos genera.
In 1996, some workers (4) published the results of the first detailed molecular study of the phylogenetic relationships of all genera in the Iguanidae. Depending on which characters were included, they recovered two hypotheses. Their most preferred hypothesis grouped Iguana and Sauromalus as most closely related to each other and had Cyclura occupying an “older” position. The other possibility shown from their study grouped Iguana most closely with Cyclura and left their relationship with Sauromalus unresolved.
Two later molecular studies, using different markers, showed support for both the Sites et al. (1996) preferred tree (5) and the alternative tree (Rassmann, 1997).
My study added a protein data set to the available knowledge. We used a technique called protein gel electrophoresis to gather information about the relatedness of the genera. Taking protein samples from the genera in question, along with a few other genera for comparison, we ground it up in an aqueous solution and soaked it into paper wicks. We placed these wicks in a horizontal starch gel matrix connected to an electrical power source. The power source created a positive and negative pole on either side of the matrix. The migration distance and direction of the molecules of each genus’ proteins depended on the charge of the molecule, which in turn depended on it’s conformation and physical entities. After allowing sufficient time for the proteins to move and separate, chemical stains were added to the gel which caused the protein of choice to become visible. Groupings could then be made based on which genera migrated the same distances and which ones differed. I attained 32 such protein allozyme relationships.
The data from these experiments were entered into a computer program (7) and combined with molecular and morphological data sets from previous studies (4 and 3, respectively). A most parsimonious relationship was found that resembled the preferred hypothesis of the 1996. The resulting hypothesis showed stronger support for the relationships between the three genera in question than the original 1996 hypothesis due to the additional data set from my study.
Acknowledgements (8)
Works Cited
- Burghardt, G. M. and A. S. Rand (eds.). 1982. Iguanas of the World. Noyles Publications. New Jersey.
- Avery, D. F. and W. W. Tanner. 1971. Evolution of iguanine lizards (Suaria: Iguanidae) as determined by osteological and myological characters. Brigham Young Univ. Sci. Bull. Ser. 12: 1-79.
- Norell, M.A. and K. deQueiroz. 1991. The earliest iguanine lizard (Reptilia; Squamata) and its bearing on iguanine phylogeny. Am. Mus. Novit. 299:1-16.
- Sites, J. W., Jr., S. K. Davis, T. Geurra, J. B. Iverson, and H. L. Snell. 1996. Character congruence and phylogenetic signal in molecular and morphological data sets: a case study in the living iguanas (Squamata, Iguanidae). Mol. Biol. Evol. 13: 1-87-1105.
- Petren, K. and T. J. Case. 1997. A phylogenetic analysis of body size evolution and biogeography in chuckwallas (Sauromalus) and other iguanines. Evolution 51:206-219.
- Rassmann, K. 1997. Evolutionary age of the Galapagos iguanas predates the age of present Galapagos islands. Molecular Phylogenetics and Evolution. 7:158-172.
- Swofford, D. L. 1993. PAUP: phylogenetic analysis using parsimony. Version 3.1. Illinois Natural History Survey, Champaign, Ill.
- I would like to thank the Office of Research and Creative Activities, the Department of Zoology, and the Department of Honors at BYU for financial support of this project. I thank J. W. Sites, Jr. for advising, revising, and use of his laboratory.