Taxonomic Classification of Cnemidophorus velox Springer Based on Origins and Relative Age

Jesse Everett

Of the approximately fifty recognized species of lizards of the Genus Cnemidophorus, nearly one third are parthenogenic, or made up entirely of females. Cnemidophorus velox Springer is a member of the sexlineatus subgroup of these parthenogenic species, as it also has a triploid genotype in addition to reproducing via parthenogenesis. However, this classification is problematic, primarily because it is based on the ‘Biological Species’ concept, which defines species as “groups of interbreeding natural populations that are reproductively isolated from other such groups” (Frost, 1990). In the case of organisms which reproduce by parthenogenesis, there is no interbreeding. All members of the species are female, and reproduce by creating clones of themselves. In a loose sense, each individual could be called a species of its own.

Because this is obviously unrealistic and impractical, numerous attempts have been made to come up with species concepts which can reasonably apply to both bisexual and unisexual species. To this day, there is still no species concept which has been agreed upon to classify unisexual species along with bisexual ones. In the field of Herpetology, however, a temporary species concept for unisexual lizards has been proposed. Each parthenogenic “population” which is considered to have a single hybrid-origin is considered to be a distinct species, and is given an official binomial classification (Frost, 1990). While this definition is straightforward enough, when applied to the unisexual populations of Genus Cnemidophorus, more than one species breakdown has been suggested, each backed by a considerable amount of research.

The historical classification of Cnemidophorus velox Springer was given to the lizard population upon its discovery in 1928, and was based upon geographic distribution and physical distinctiveness from other lizard populations in the area. The population covered an area of hundreds of square miles in the four corners region of the United States, and its distinctive bold striped coloration made it easily identifiable as a unique species. Unknown at the time of discovery was the fact that the species was parthenogenic. This was not determined until 1962, when it was discovered that five Cnemidophorus species (Including C. velox) were actually entirely female (Walker, 1986).

Shortly after this discovery, an allozymic analysis of a number of Cnemidophorus species (including C. velox) was completed (Neaves, 1969), roughly comparing the complete genotypes of the various species. In the case of C. velox, the researchers were only able to conclusively match two of the three haploid genomes. Interestingly, both of the genomes identified came from the diploid Cnemidophorus inornatus, suggesting it as the probable source of the hybridization which created C. velox. Another triploid species, Cnemidophorus exsanguis, was found to have a single genome from C. inornatus, placing it as a possible sister species to C. velox. The third genome was much later identified as being similar to that of the diploid Cnemidophorus burti. While the study was unable to determine exact lineage for any of the triploid species, it did verify that C. velox is genetically distinct from both its possible parents and its sister species, confirming the classification of velox as a distinct species.

One of the first studies on the widespread populations of C. velox was conducted by Orlando Cuellar in 1977, using a technique of skin grafting to attempt to determine the level of genetic similarity throughout the population. The theory behind skin grafts is that the subject bodies’ acceptance or rejection of a skin graft from an individual of the same species would be a determinant of histocompatability. An individual which rejected a graft would be less genetically similar to the donor individual than an individual which accepted the same graft. C. velox were collected from five locations throughout their range, some populations as far as 375 miles from each other. Of the fifty skin grafts performed on various individuals, twelve were found to be rejected (Cuellar, 1977). It was determined that individuals from the same or nearby populations tended to accept skin grafts from each other, while individuals from distant populations mutually rejected them. This is unusual for a species which is assumed to have very low genetic diversity due to reproduction by parthenogenesis. In a study of Cnemidophorus neomexicanus, another parthenogenic lizard with a similar range to C. velox, it was found that individuals throughout the range of the population mutually accepted grafts. In fact, 99% of the grafts performed on C. neomexicanus were accepted, regardless of distance between the populations the individuals were captured from. For C. velox, this indicates the possibility of multiple hybrid-origins. The velox populations of Colorado, Utah, and New Mexico each appear to be distinct, with histocomplexes entirely different from that of the other groups.

Making a clear case for three distinct groups of C. velox from the above study is difficult, however. While most of the rejected grafts between individuals of two distant groups, one unusual specimen rejected a graft from another individual of its own local population. The reason for this is unclear, and not explained in the study. Also, the populations sampled were collected from only five sites: one in Colorado, and two each from Utah and New Mexico. This by no means accounts for the complete range of velox populations, and can be misleading as it does not discuss that velox occur in a continuous range between these isolated sites. Further, as histocompatibility is only a rough measure of genetic similarity, the study was insufficient in determining whether or not the three groups of velox are distinct due to mutations accumulated after a single hybrid-origin, or the result of three seperate hybrid-origin events.

A later study of C. velox was conducted, this one an analysis of Mitochondrial-DNA. For the experiment, 20 specimens from 13 locations were selected (Moritz et al., 1989). MtDNA from the specimens was cut using a number of restriction enzymes, and run on electrophoresis gels, using several known cut genomes as a guide. It was determined that the mean pairwise comparison of C. velox mtDNA with that of C. exsanguis mtDNA diverged by only 0.22%, a difference which is not statistically significant (Moritz et al., 1989). This once again confirms the genetic similarity between the two species, and suggests that both species are derived from the same maternal lineage. Also, it was found that, even when compared to other triploid species, the overall variation found in C. velox is very low, which suggests a quite recent hybrid-origin.

The data was used to construct a cleavage site map for C. velox and C. exanguis, and this map was compared to the cleavage site maps of seven diploid Cnemidophorus species (Moritz et al., 1989). The comparison found that the closest diploid mtDNAs to C. velox and exsanguis are from the currently named Cnemidophorus burti stictogrammus and Cnemidophorus costatus barrancorum. The mtDNA of C. inornatus, previously considered to be the closest probable ancestor of C. velox, was determined to be far removed, sharing only three of the thirty cleavage sites examined.

Combining mtDNA data with the previously determined allozyme data, Moritz et al. concluded that the most probable course of evolution for C. velox involves first the founding of a hybrid dipliod lineage by hybridization between a female of either Cnemidophorus costatus barrancorum or Cnemidophorus burti stictogrammus and a male C. inornatus. The suggestion is that this diploid hybrid was for some reason either parthenogenic, or rapidly became so after hybridization. The creation of C. velox is thought to have occurred when this diploid hybrid backcrossed with a C. inornatus male to produce the first triploid. While this is a strange set of circumstances, it is considered plausible because it also poses the evolutionary origin for the sister species, C. exsanguis: Instead of the diploid hybrid backcrossing, it mated with a male Cnemidophorus septemvittatus, also producing a triploid.

While the study does pose a possible, although unusual, route by which a parthenogenic triploid species like C. velox could be produced from a number of diploid species, a large amount of guesswork and extrapolation is involved. A hybrid formed from two closely related diploid species living in the same general area is quite plausable, but the method by which this hybrid diploid became parthenogenic (or even if it did) is not described at all. Also, in taking mtDNA from a broad group of individuals, the study assumes that all individuals are evolved from a single hybrid, contrary to the theory suggested by Cuellar in his study. Given the suggestion posed by Moritz et al., that both C. velox and C. exsanguis were created by the hybridization of the unusual diploid parthenogenic lizard with one of two males of other diploid species, it stands to reason that if such an event necessarily happened twice (to produce both species) it could have happened any number of times, allowing for the possibility of multiple hybrid-origins.

Many current works on the Genus Cnemidophorus currently classify C. velox as the C. velox “complex”, which contains a variable number of groupings, although most often three (Frost, 1988). The difficulties involved with the classification of such an unusual group of animals are many, but mostly this is due to the weight of the history of the discovery of these lizards. For being named over forty years before it was even known that C. velox was parthenogenic, it’s something of a miracle that the classification is still even remotely fitting. But the genetic evidence does indicate that C. velox is at the least a genetically distinct group of animals, having a unique mix of genomes from various other Cnemidophorus species. Further, the allozymic similarity between all individuals of the species, despite the absence of stabilizing gene flow between individuals, reaffirms the appropriateness of a single “species” classification. The possibility of multiple-origins remains a possibility, never the less. Without a classification system which fits unisexual populations as well as bisexual ones, however, the single species classification of Cnemidophorus velox Springer fits this group of genomically unique, genetically similar animals.

References:

Cuellar, O. 1977. Genetic Homogeneity and Speciation in the Parthenogenic Lizards Cnemidophorus velox and C. neomexicanus: Evidence from Intraspecific Histocompatibility. Evolution 31:24-31.

Frost, D. and D. Hillis. 1990. Species in Concept and Practice: Herpetological Applications. Herpetologica 46:87-104.

Frost, D. and J. Wright. 1988. The Taxonomy of Uniparental Species, with Special Reference to Parthenogenic Cnemidophorus (Squamata: Teiidae). Syst. Zool. 37:200-209.

Moritz, C., J. Wright, and W. Brown. 1989. Mitochondrial-DNA Analyses and the Origin and Relative Age of Parthenogenic Lizards (Genus Cnemidophorus). III. C. velox and C. exsanguis. Evolution 43:958-968.

Neaves, W.B. 1969. Adenosine deaminase phenotypes among sexual and parthenogenic lizards in the genus Cnemidophorus (Teiidae) J. Exp. Zool. 171:175-184.

Walker, J. 1986. The Taxonomy of Parthenogenic Species of Hybrid Origin: Cloned Hybrid Populations of Cnemidophorus (Sauria:Teiidae). Syst. Zool. 35:427-440.