Tortuga Gazette 25(8): 10-11, August 1989
Molecular Biology and the Turtle:
The desert tortoise and its relatives
by Michael J. Connor
As many long time turtle hobbyists can tell you, our native California desert tortoise has long been known by the scientific name Gopherus agassizii. Gopherus is a uniquely North American genus consisting of tortoises that are structurally adapted to a burrowing and digging lifestyle. Three other species of gopher tortoise are alive today - the Texas tortoise (berlandieri), the Mexican Bolson tortoise (flavomarginatus), and the Florida gopher tortoise (polyphemus). Over the years, experts in the field came to realize that the genus Gopherus really included two groups of tortoises, separable on the basis of differences in their bone and shell structure. In recognition of these differences a new genus (Scaptochelys) was proposed in 1982 that would include the desert and Texas tortoises. This new genus name was short lived, however, because it was soon pointed out that the genus name Xerobates had been proposed for the desert tortoise back in 1857, and the rules of scientific nomenclature strictly assign priority in naming to the earliest date. So, in 1984 the desert tortoise became Xerobates agassizii and the closely related Texas tortoise became X. berlandieri.
Unfortunately, name changes like these happen quite often. Usually this is because a new species or sub-species is discovered, or a new relationship is identified between already known species. On occasion, a tortoise species has apparently been named twice, as happened with Geochelone forsteni which used to be known as G. travancorica or G. forsteni depending upon where it came from. Now, thanks to the application of recently developed molecular biological techniques, the study of the classification of species and their inter-relationships is undergoing a revolution that will eventually reduce the need for many of these periodic name changes.
Instead of relying on a physical comparison of species, scientists can now compare fragments of DNA to construct "gene trees". DNA, found in the cells of all animals and plants, is the genetic blueprint that carries the genes coding for what an organism is and what it will look like. Molecular biologists have developed ways to examine DNA by cutting it into smaller fragments that can be separated by their size into characteristic patterns. Since offspring inherit their DNA from their parents the fragment patterns obtained from offspring DNA are similar to those of the parents. On the other hand, the DNA fragment patterns from unrelated individuals tend to differ. Therefore, a comparison of DNA fragment patterns can reveal how closely related two individuals or species are. Also, since species evolve over time due to the accumulation of favorable mutations in their DNA, analysis of DNA fragment patterns can give valuable information on the evolutionary relationships between species.
The first major application of these techniques to the study of chelonian biology was published recently [1]. Professor Avise of the University of Georgia, and Drs. Lamb and Gibbons of the Savannah River Ecology Laboratory used analysis of DNA fragment patterns to study the evolution and classification of Gopherus tortoises, placing particular emphasis on the desert tortoise.
The scientists studied a form of DNA (mitochondrial DNA) that is inherited only from the mother. They found that the desert tortoise population consists of 3 major distinct assemblages of genotypes. The first or "western" assemblage is the most wide-spread, occurring north and west of the Colorado River, and includes at least 3 sub-groups that differ only by 1 or 2 fragments. The commonest sub-group occurs throughout the Colorado and Mojave deserts in California extending into southern Nevada along the Piute Valley. Interestingly, the two other sub-groups both lie in the most northern part of the desert tortoise range. One of these sub-groups extends along the Ivanpah valley in California eastward through Nevada to extreme northwest Arizona and southern Utah. The last and smallest sub-group is restricted to the area where the borders of Arizona, Utah and Nevada meet.
The second assemblage includes most of the Arizona desert tortoises and extends south into Sonora (Mexico), a linear distance of nearly 500 miles. This "eastern" genotype is distinguished from the western assemblage by at least 17 fragmentation site changes! This difference between the western and eastern groups is quite remarkably large for animals that form a single species, and is one of the largest differences reported for any animal. Such a sizeable difference suggests that the western and eastern tortoises have been separated geographically for a long period of time, probably as long as 2-3 million years. This corresponds to a geological period during which great rivers and shallow seas (such as the Bouse sea) periodically covered much of the lower Colorado desert. Since these were eventually replaced by the modern Colorado River, tortoises originally located on the west and east sides of the Bouse sea have remained reproductively isolated from each other up to the present day. The scientists also discovered a third "southern" assemblage, almost as different from the western tortoises as the western tortoises differ from the eastern, in a small area of southern Sonora. Although it is not yet clear where the boundary is between the eastern and southern assemblages, southern Sonora is an area of deciduous forest, ecologically very different from the desert habitat of the other two major genotypes.
Although these observations may at first glance seem rather academic they establish a couple of very important points: (a) the desert tortoise is scientifically extremely interesting, and (b) the desert tortoise we know to-day includes a variety of populations with differing genotypes. Direct evidence that the desert tortoise exists in several genetically different forms advances the cause of those of us interested in preserving and protecting these remarkable animals, since moves to place the more critically endangered populations under federal protection are now clearly justified on scientific grounds.
When the scientists compared DNA from all four species of gopher tortoise they confirmed the close link between X. agassizii and X. berlandieri, and between G. flavomarginatus and G. polyphemus. Their results suggested that the Xerobates and Gopherus forms last shared a common female ancestor some 5-6 million years ago. The Texas tortoise, X. berlandieri, proved to be very closely related to the "eastern" agassizii genotype. It is highly likely that berlandieri evolved from this eastern agassizii assemblage, probably from Xerobates stock inhabiting the north central region of Sonora. Perhaps the closeness of the relationship between the eastern agassizii assemblage and berlandieri explains the occasional occurrence of hybrids from matings between captive Texas and desert tortoises.
The development and refinement of genotyping techniques such as these will continue, and, hopefully, will be extended to other chelonian families. Eventually it may become feasible to identify the origin of an individual turtle or tortoise on the basis of a simple blood test. This is of great practical significance, since it could be used to identify undocumented long term captives at the sub-species level. Knowing the correct identity of the hundreds of giant tortoises currently exhibited in zoos, for example, could conceivably result in an expansion of the available breeding pools for the more critically endangered sub-species.
References
[1] T. Lamb, J. C. Avise, J. W. Gibbons: Phylogeographic patterns in mitochondrial DNA of the desert tortoise (Xerobates agassizi) and evolutionary relationships among the north American gopher tortoises. Evolution, 43: 76-87, 1989.