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Clemson University TigerPrints All Theses Theses December 2019 Testing the Waters: Diversification and Selection in the Nerodia fasciata/clarkii Species Complex Mark Alexander DiMeo Clemson University, [email protected] Follow this and additional works at: https://tigerprints.clemson.edu/all_theses Recommended Citation DiMeo, Mark Alexander, "Testing the Waters: Diversification and Selection in the Nerodia fasciata/clarkii Species Complex" (2019). All Theses. 3202. https://tigerprints.clemson.edu/all_theses/3202 This Thesis is brought to you for free and open access by the Theses at TigerPrints. It has been accepted for inclusion in All Theses by an authorized administrator of TigerPrints. For more information, please contact [email protected]. TESTING THE WATERS: DIVERSIFICATION AND SELECTION IN THE NERODIA FASCIATA/CLARKII SPECIES COMPLEX A Thesis Presented to the Graduate School of Clemson University In Partial Fulfillment of the Requirements for the Degree Master of Science Biological Sciences by Mark Alexander DiMeo December 2019 Accepted by: Dr. Christopher Parkinson, Committee Chair Dr. J. Antonio Baeza Dr. Vincent Richards ABSTRACT Understanding evolution is a key component in trying to decipher the processes generating global species diversity. The strength, direction, and interaction of gene flow and selection often determine diversification patterns and the process of speciation. The Nerodia fasciata/clarkii species complex, a lineage of water snakes, is thought to have high levels of both gene flow and selective pressures due to ecological constraint. Nerodia clarkii resides in salt marsh and estuarine habitats while Nerodia fasciata is typically found in fresh water. Salinity is a strong selective pressure and is thought to play a role in the diversification process. Currently, there are five described subspecies within the complex but their validity is in question, causing concerns about the conservation status of the federally threatened Atlantic Salt Marsh Snake (N. c. taeniata). To understand the diversification of the Nerodia fasciata/clarkii complex and to resolve the noted taxonomic issues, I generated the first population genomic dataset for this group using double digest restriction site-associated DNA sequencing (ddRADseq). I first used a Discriminate Analysis of Principal Components (DAPC) to identify population structure and SVDQuartets to generate a coalescent based phylogeny. With these data, I identified 4-6 populations that approximate subspecies designations although only one assigned subspecies, N. c. clarkii, was monophyletic. Second, I estimated migration among the best supported population clustering (k=5) using Estimated Effective Migration Surfaces (EEMS). EEMS revealed a migration corridor between the mostly N. fasciata populations and a reduction in gene flow at the coasts. Third, I used two selection scan analyses and one environmental association analysis to identify genes that ii are putatively under selection with an emphasis on local adaptation to saline water. I found 10 candidate genes that may be involved in osmoregulation and multiple correlations to temperature and precipitation. My results indicate that the two species are valid, and that four subspecies are also evolutionary lineages. Although gene flow and population assignment tests provided evidence that N. c. taeniata was isolated from other populations I could not unambiguously determine its validity. Both the candidate gene frequencies and EEMS indicate that majority N. fasciata populations mostly share a selection regime and gene flow patterns separate them from N. clarkii. This may demonstrate how a reduction in gene flow and a change in selection pressures can generate species diversity. iii To my family, Even if I’m in Japan I know you’re with me iv ACKNOWLEDGEMENTS Finishing this thesis was a longer and harder process than I ever expected and I could never have done it without help. First, I want to thank my advisor and committee members. Dr. Parkinson, thank you for taking a chance on me when I had next to no research experience and sticking with me as I struggled to become a scientist. I’m sure it wasn’t easy to do so. Additionally, without your grant from USFW I’m not sure I could have funded all the work. I want to thank my committee members Dr. Baeza and Dr. Richards for their patience with me for all the delays to my thesis. I hope I was able to live up to all your expectations. Second, I want to thank all the members of the Parkinson Lab, past and present, who have provided support to me, without which I could never have finished. In the three years I was a part of the lab I have seen multiple lab mates come and go and all of them have played a part in my growth as a scientist and person. Notably Jason Strickland, Andrew Mason, and Rhett Rautsaw have been with me since I joined and without their guidance this wouldn’t be possible. In particular I want to thank Mark Margres who really took me under his wing in the last year and Rooksana Noorai who was always there to listen to me despite the fact that she is not an official member of the lab and was not obligated to do so. Then I have to thank the past Master’s students, Greg Territo and Jason Hickson, and the undergrads who laid out the foundation for this project through sample collection and data generation. I cannot forget all the contributions to the research and editing of this project that the Parkinson Lab gave me. v Third, during the course of my research I worked at both the University of Central Florida and Clemson University so I want to thank all the friends I made at both institutions. You may not have directly contributed to my thesis but you helped to keep me sane through the hard times with your friendship. Finally, I want to thank my family. I don’t know why but despite all my setbacks and delays your confidence in me never seemed to waver. This emotional support was worth so much to me, even more than the financial support! Despite the fact that I’m not near you now I know I’ll always have your support. vi TABLE OF CONTENTS Page TITLE PAGE .................................................................................................................... i ABSTRACT ..................................................................................................................... ii ACKNOWLEDGEMENTS ............................................................................................ iv LIST OF TABLES ......................................................................................................... vii LIST OF FIGURES ...................................................................................................... viii CHAPTER I. INTRODUCTION ......................................................................................... 1 Diversification, Gene Flow, and Selection .............................................. 1 Population Genomics ............................................................................... 2 Study Species ........................................................................................... 3 Objectives ................................................................................................ 5 Questions and Predictions ........................................................................ 6 II. EVOLUTIONARY HISTORY AND POPULATION GENETICS OF THE NERODIA FASCIATA/CLARKII SPECIES COMPLEX ........................................... 7 Introduction .............................................................................................. 7 Materials and Methods ............................................................................. 9 Results .................................................................................................... 17 Discussion .............................................................................................. 29 III. EVIDENCE OF SELECTION IN THE NERODIA FASCIATA/CLARKII SPECIES COMPLEX ......................................... 32 Introduction ............................................................................................ 32 Materials and Methods ........................................................................... 34 Results .................................................................................................... 38 Discussion .............................................................................................. 45 IV. DISCUSSION .............................................................................................. 48 vii Table of Contents (Continued) Page Taxonomy and Conservation ................................................................. 49 Future Directions ................................................................................... 50 APPENDICES: Supplementary Tables and Figures...................................................... 52 REFERENCES .............................................................................................................. 73 viii LIST OF TABLES Table Page 2.1 Locus name, annealing temperature (ᵒC) and alignment length used for Bayesian phylogenetic inference of the Nerodia fasciata/clarkii species complex ................................................................ 10 2.2 List of mitochondrial and nuclear genes with the model for each coding position. TATA, M, and E are introns and were not split by codon thus they only have one position……………………………………………………… 15 2.3 Fst, Gst, and Jost’s D pairwise measures of differentiation among