Spiny-tailed Iguana (Ctenosaura sp. (conspicuosa and macrolopha))
Spiny-tailed Iguana (male), Arizona-Sonora Desert Museum, AZ. Photo by Jim Rorabaugh
Spiny-tailed Iguana eating cactus fruit, Arizona-Sonora Desert Museum. Photo by Jim Rorabaugh
Mating Spiny-tailed Iguanas. Photo by G. Miller
Spiny-tailed Iguana with cactus fruit stains, Arizona-Sonora Desert Museum, AZ. Photo by Jim Rorabaugh
Juvenile Spiny-tailed Iguana, Arizona-Sonora Desert Museum. ©2012 Dancing Snake Nature Photography
Editor’s Note: Spiny-tailed Iguanas from the mainland of Sonora are now recognized as Ctenosaura macrolopha.
Most visitors to the Arizona-Sonora Desert Museum (ASDM) are fascinated by the large spiny-tailed iguanas that inhabit the museum grounds. Basking on the artificial rock outcrops, they are as charismatic as the bighorn sheep and their head-bobbing displays impress even the most herpetophobic people. Spiny-tailed iguanas are large, mnivorous lizards in the genus Ctenosaura, which contains 14 species that collectively range from Mexico to Panama, including Baja California and various offshore islands in the eastern Pacific, western Caribbean and Gulf of California (Grismer 2002). Their origin as a “wild” population on the Desert Museum grounds is quite intriguing. In the mid-1970’s, ASDM staff released at least one pair of individuals on the grounds. It is unknown if other individuals were released or escaped since then. Without management by ASDM staff, these individuals have propagated freely and maintain a current population of 25-40 animals. The current staff at the Desert Museum, aware of the potential dangers of invasive species, has been researching the introduced population for the last 8 years. Although the lizards maintain a healthy population on the ASDM grounds, there is no evidence that they have spread into the surrounding Tucson Mountains. The Museum grounds may provide habitat and resources (abundant water and food) not available in the surrounding desert to this mostly warm-climate genus of lizards. Perhaps most critical to the lizard’s survival on the ASDM grounds are the deep cavities within the artificial rockwork and buildings that the lizards may use to overwinter; these cavities maintain much warmer temperatures throughout the winter than would be available naturally. The spread of introduced Ctenosaura in Florida (Krysko et al. 2003) is probably a result of the more favorable climate there.
For this study, we used molecular methods (DNA typing) to investigate further the origins of the ASDM spiny-tailed iguanas. In particular, we see this as a case study for how invasive species might establish and maintain a population from only a few original individuals. Invasive species are of growing interest to conservation biologists because of their potential to extirpate native species. Although many cases have been documented, biological attributes that give a species the potential to become invasive are poorly understood (Marchetti et al. 2004). It is proposed that invasives are commonly generalists that can easily adapt to new environments (Hänfling and Kollmann 2002). From a molecular standpoint, it might be expected that a species with the potential to become invasive would come from a population with very high genetic diversity, thus increasing the chance for an individual to have the genetic make-up to survive in the new area (Hänfling and Kollmann 2002; Lee 2002). However, there are typically only a few founders of a new population. Once established, the genetic diversity in the introduced population would remain extremely low as a result of rapid population growth and inbreeding (Patti and Gambi 2001). How invasives survive over time with such a small pool of genetic diversity is intriguing and warrants study.
We obtained blood samples from specimens captured on the desert museum grounds. Captured individuals were uniquely marked (PIT tag, tattoo, paint, or colored beads) to prevent multiple sampling and to aid in behavioral studies. Samples were stored in standard veterinary tubes with EDTA buffer and brought to the Genomic Analysis and Technology Core (GATC) at the University of Arizona. The process of extracting DNA from tissue involves popping open the cells (“lysis”), breaking up the extraneous cellular material, and then isolating the DNA from the remaining proteins, lipids, and other cellular debris. We isolated total DNA by overnight lysis with proteinase K at 55°C, followed by extraction using phenol/chloroform and isopropanol/sodium acetate precipitation (Goldberg et al., 2002). We resuspended the DNA in a storage buffer (low TE; 10 mM Tris pH 8.0, 0.01 mM EDTA) and quantified it using a FLx 800 Microplate Fluorescence Reader (Bio-Tek Instruments, Inc.). We diluted working stock solutions to 5 ng/µl.
We choose to examine the mitochondrial DNA (mtDNA) in our samples because it has been used in previous studies of Ctenosaura, allowing us to compare our results to a database of individuals throughout their native range. MtDNA is commonly used in population genetic studies because it has a faster mutation rate than other parts of the genome and is fairly easy to isolate. MtDNA is passed on to offspring only by the mother and it does not recombine in each successive generation. Therefore, the mtDNA’s signature is the same when traced back to the mother, grandmother, great-grandmother, etc. If a change or mutation were to occur, it would remain with that individual’s maternal lineage as a “genetic footprint.” This way, we can trace back to the original population from which a sample’s lineage originated.
Once we isolated the DNA from our samples, we used polymerase chain reaction (PCR) to make copies of the specific region (“locus”) of mitochondrial DNA that we were interested in examining. PCR uses “primers” that act like keys to unlock a specific section of DNA and then through a process of heating and cooling makes millions of copies of that section so that it can be distinguished from the rest of the genetic material. We used standard methods of PCR to amplify the COX3 region of the mitochondrial genome using primers 8618L and 9323H described by Cryder (1999). We sequenced PCR “amplicons” (copies of the locus) at the GATC DNA Sequencing Laboratory using an ABI Prism® 3730 DNA Analyzer (PE Biosystems). This gives us the actual DNA code of the sample as a sequence of the four possible nucleic acids (or “base pairs”), e.g. GATACCAGA… We examined sequences using Sequence Navigator version 1.0.1 (Applied Biosystems, Inc.) and verified the mtDNA code for each of our samples. We then compared the sequences to see how their codes differed from each other and solicited the assistance of Dr. Robert Murphy and Amy Lathrop at the Royal Ontario Museum/University of Toronto for comparison of our samples to their database of Ctenosaura COX3 sequences from samples collected throughout their native range.
In total, we examined ~730 continuous base pairs of sequence for each of 14 individuals. We found only two unique sequences in our sample and those two sequences differed by 25 nucleotides out of the 730 (e.g., a DNA sequence of GATACA in one individual might read GATCCA in another). A unique sequence of non-recombining DNA like mtDNA is called a “haplotype”, thus we found two different haplotypes in our sample set. Twelve of the individuals were an identical match to 5 samples in Dr. Murphy’s database from mainland Mexico in Sonora: one from Alamos; three from the road to Hermosillo, ~10 km north of Guaymas; and one labeled only “Sonora”. The other two individuals from the Desert Museum were identical to two samples in the database collected from Isla San Esteban. To roughly estimate the time of divergence between the two haplotypes (how long the two populations have been isolated from each other to allow these changes to naturally develop), we used a mitochondrial mutation rate (µ) of 2% change in nucleotide sequence per 1 million years (suggested to be appropriate for lizards; Arévalo et al. 1994). We then divided our observed sequence divergence (number of variable sites/total length of sequence) by 2 µ, for an estimated 890,000 years of divergence between the two maternal lineages.
Based on our molecular evidence, the introduced population of spiny-tailed iguanas at ASDM came from at least two, very different, maternal lineages. (i.e., at least two females from different locations have been released on the grounds and have successfully contributed their genes to the population). The majority of individuals came from a maternal line (i.e., their mother, or mother’s mother, etc.) of Ctenosaura hemilopha macrolopha, native to Sonora, Mexico, while two individuals carried a mitochondrial genome from Ctenosaura conspicuosa that originated from San Esteban Island, in the Sea of Cortéz. (Grismer  recognizes these as C. macrolopha, Mainland Spiny-tailed Iguana, and C. conspicuosa, Isla San Esteban Spiny-tailed Iguana). That the lineages remain distinct does not imply that individuals do not interbreed or that there are currently two distinct populations on the museum grounds. Since the mtDNA does not recombine in each successive generation, it maintains its original sequence. The fact that there are more of one type than the other could imply that: 1) more individuals of one type were originally released than the other; 2) one type is more successful in breeding than the other (“selection”); 3) our sampling methods favored one type more than another; or 4) it could be a result of random sorting in the population (“genetic drift”).
Ctenosaura conspicuosa, known only from Islas San Esteban and Cholludo, Sonora (Grismer 2002), is more closely related to another insular Ctenosaura species, C. nolascensis (Isla San Pedro Nolasco Spiny-tailed Iguana), than it is to mainland Sonora Ctenosaura (Cryder 1999). The divergence we calculated between these two maternal lineages (Ctenosaura hemilopha macrolopha and C. conspicuosa) is deep, roughly 890 thousand years, coinciding roughly with the formation of Isla San Esteban approximately 1 million years ago as it broke off from mainland Sonora (Murphy 1983).
Four to five million years ago, differential tectonic movements of the Pacific and North American plates initiated separation of Baja from the west coast of Mexico, and thereby isolated much of Baja’s flora and fauna. Similar patterns of isolation occurred on many of the islands currently found within the Gulf of California. Morphological differences distinguish the two species; C. conspicuosa has a slightly enlarged middorsal scale row, a different arrangement of caudal scales, lacks the post-thoracic banding pattern in adults, and has grayish and banded hatchlings as opposed to hatchlings with a greenish pattern in C. hemilopha (Grismer 2002). On a purely subjective level, adult males and hatchlings on the Desert Museum grounds appear more similar to C. hemilopha macrolopha than C. conspicuosa. However, no morphological data have been analyzed to test this hypothesis.
San Esteban Island (composed of volcanic rock deposited in the Miocene), as compared to other Gulf of California islands, is relatively isolated by both distance to nearest land and depth of interceding channel (Gastil et al. 1983). Ctenosaura are found on four major islands in the Gulf of California, whereas their cousins the Chuckwallas (Sauromalus) are found on roughly 25 major islands (Case 1983; Grismer 2002). Both species are known components of native human diets and their distributions throughout the Gulf of California may have been influenced by human dispersal as a reliable insular food source for semi-nomadic groups like the Seri Indians (Grismer 2002 and references therein). An interesting corollary is the relative tameness of some insular populations (Delibes and Blázquez 1998), which may influence their ability to colonize an area like ASDM where people and lizards are often in close proximity.
On San Esteban, C. conspicuosa inhabits arroyos more than hillsides, often found perched on boulders, on top of cardón cactus (Pachycereus pringlei), or otherwise associated with vegetation. Their diet is omnivorous with plants and arthropods being the most common food items. They are diurnal and active year-round with a lull in activity during mid-winter. Breeding may take place in spring and summer as hatchlings have typically been observed from August through early October. Adults are often found in small groups of one male and two females (Grismer 2002).
Less is known about the natural history of the mainland population around Alamos, but for other C. hemilopha populations, their behavior is similar to that described for C. conspicuosa. However, the diet of C. hemilopha is more herbivorous (Grismer 2002). Individuals at the Desert Museum appear to adhere to these behavioral patterns; copulation has been observed in mid-spring and hatchlings have appeared between early August through the month of September.
It is questionable how long a population can persist with low genetic diversity and consistent inbreeding, but there are natural examples of healthy populations with low genetic diversity (Wayne et al. 1991, Gray 1995). Data collected from natural populations suggest that Ctenosaura maintain a low level of mitochondrial divergence within populations (Dr. Robert Murphy, pers. comm. 2005). We are not able to determine if the two maternal lineages hybridize from these data, or if the ratio of individuals with each type is a result of sampling procedures, genetic drift, or selection. Further testing of autosomal markers or comparing morphological characteristics might resolve this question. If the two distinct maternal lineages in the introduced ASDM population hybridize, then it may be an advantage in that it increases the population’s genetic variability. However, outbreeding depression caused by mixing dissimilar populations can also result in loss of specific adaptations to a particular environment (Lynch 1991). An isolated population, such as the spiny-tailed iguanas on Isla San Esteban, can form co-adapted complexes of genes with favorable effects. Breaking up these complexes by outcrossing can have deleterious effects that build up slowly. Even if there is an advantage of outcrossing manifest in the first generation, it can be reversed in successive ones (Lynch 1991).
Further research on the ASDM population using autosomal markers may give us a better understanding of how an introduced population can persist despite originating from such a limited number of individuals. Also, since mtDNA only follows female lineages, autosomal markers would allow us to better determine how many different individuals comprise the total number of introductions over the years. Undocumented escapes or introductions at ASDM may have contributed additional variability to the ASDM Ctenosaura population, allowing it to maintain enough individuals and diversity to survive. Additional research into this interesting case study may provide insight into what characterizes a species to potentially be invasive and how alien species are able to persist.
We thank Amy Lathrop and Dr. Robert W. Murphy of Royal Ontario Museum/University of Toronto for providing primer information and allowing us to use their database for comparison. We also thank the Arizona Research Laboratories and the Genomic Analysis and Technology Core staff for their support of the project.
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Authors: Taylor Edwards, Kevine Bonine, Craig Ivanyi, and Rebecca Prescott
Originally published in the Sonoran Herpetologist 2005 18(11):121-125