New Published Research: Population Genetics of the Spotted Turtle

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Spotted Turtle whose genetics were assessed as part of this study.
One of the Spotted Turtles whose genetics were assessed as part of this study. – Ben Stegenga

One component of our long-term work with Spotted Turtles in the southeastern U.S. has been to collect tissue samples from captured Spotted Turtles so that we can examine the genetics of focal populations. The results from this research were recently published in the journal Chelonian Conservation and Biology (Brown et al. 2023). To complete this project, we worked closely with collaborators from the University of Southern Mississippi and Florida Fish and Wildlife Conservation Commission. This work was the first assessment of genetics in Spotted Turtle populations from the southeast and adds to our understanding of this iconic species.

Spotted Turtle habitat from one of the sites in this study.
Characteristic turtle habitat from one of the sites included in this study. – Alex Greenwood

While conducting standardized turtle surveys, we collected tissue samples from Spotted Turtles at two sites in Georgia, two sites in Florida, and one site in South Carolina. In total, we collected tissue samples from 204 individuals. We analyzed all collected tissue samples using standard population genetic techniques and performed analyses across 200 individuals at 10 microsatellite loci, following previous research on Spotted Turtle genetics (King and Julian 2004; Davy and Murphy 2014). The metrics reported in this paper provide important conservation insights that can only be assessed through analysis of genetic data. Understanding the population genetics is a critical aspect of conserving rare species.

We found that metrics of genetic diversity were similar across all five study sites, and there were no significant differences in important metrics describing genetic diversity across populations. This was true even when comparing southern populations to Canadian populations that had been previously assessed (Davy and Murphy 2014). Our analyses did suggest that there was genetic structure among sites. One of the Florida sites separated as a distinct population and additional analyses indicated that other populations could also be identified as being genetically distinct (see below STRUCTURE plot). Finally, we found no evidence that populations had gone through a historic bottleneck event (i.e., reduced population size followed by population growth from just a few individuals), when the assumptions of this analysis were met. In one of the Florida populations, there was evidence of a bottleneck, but this may have been caused by low sample size.

STRUCTURE bar plots of individual assignment probabilities for each inferred genetic cluster. Above shows high level of population structure at a K of 2, and below shows a K of 4 for remaining populations when FL-2 was excluded from analysis. Each bar represents a different individual turtle and colors represent different genetic clusters. – Grover Brown

Overall, our results suggest that all five populations included in this study are genetically distinct and show few signs of experiencing negative genetic effects. Interestingly, the largest divergence among populations was between the two Florida sites, even though these populations are only 50 km apart. This suggests that there may be historic barriers to movement and gene flow other than simple geographic distance. Estimates of effective population size ranged from 31.8 to 122.6, although there was high uncertainty associated with these values. In general, population size estimates from this study were equivalent or larger than estimates from other Spotted Turtle studies (although estimation techniques are variable; e.g., Enneson and Litzgus 2009; Buchanan et al. 2019b; Howell and Seigel 2019). It will ultimately be interesting to compare future estimates of population size generated from the mark-recapture data that we have collected at these same sites.

This research provides important insights into the population genetics of southern Spotted Turtles but is just a first step to better understand how genetic metrics relate to the conservation of this species. Additional range-wide analyses are needed, and there are new genetic techniques that could be applied to provide additional insights or answer other questions. Future work may ultimately provide a mechanistic understanding of the processes that drive genetic variation in Spotted Turtles, generating a clearer picture of the genetic health of this species in the southeastern United States.

The full publication can be viewed on the Publication page of our website.

 

Literature Cited

Brown, G., J.D. Mays, H.C. Chandler, B.S. Stegenga, B. Kreiser, and D.J. Stevenson. 2023. Spotted Turtle (Clemmys guttata) Population Genetics in the Southeastern United States. Chelonian Conservation and Biology 22:127–136.

Buchanan, S.W., J.J. Kolbe, J.E. Wegener, J.R. Atutubo, and N.E. Karraker. 2019. A comparison of the population genetic structure and diversity between a common (Chrysemys p. picta) and an endangered (Clemmys guttata) freshwater turtle. Diversity 11:99.

Davy, C.M., and R.W. Murphy. 2014. Conservation genetics of the endangered Spotted Turtle (Clemmys guttata) illustrate the risks of “bottleneck tests.” Canadian Journal of Zoology 92:149–162.

Enneson, J.J., and J.D. Litzgus. 2009. Stochastic and spatially explicit population viability analyses for an endangered fresh-water turtle, Clemmys guttata. Canadian Journal of Zoology 87:1241–1254.

Howell, H.J, and R.A. Seigel. 2019. The effects of road mortality on small, isolated turtle populations. Journal of Herpetology 53:39–46.

King, T.L. and S.E. Julian. 2004. Conservation of microsatellite DNA flanking sequence across 13 emydid genera assayed with novel bog turtle (Glyptemys muhlenbergii) loci. Conservation Genetics 5:719–725.