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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Texas A&M University THE ARCHITECTURE OF PHENOTYPES IN A NATURALLY HYBRIDIZING COMPLEX OF XIPHOPHORUS FISHES A Dissertation by JAMES BRADLEY JOHNSON Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, Gil G. Rosenthal Committee Members, Adam G. Jones Kirk O. Winemiller Lee A. Fitzgerald Head of Department, Thomas McKnight May 2013 Major Subject: Zoology Copyright 2013 James B. Johnson ABSTRACT The origin and maintenance of phenotypic variation has generated considerable interest among students of functional morphology, sexual selection and behavioral ecology. In particular, hybridization has been suggested as a phenomenon which may generate novel phenotypic variation. In this dissertation I focus on the Xiphophorus birchmanni - X. malinche hybrid system to assess the role of hybridization in altering behavioral, morphological, sexual and non-sexual traits. I determine the relationship between the sword sexual ornament and body condition to support previous work which suggests that the sword is an inexpensive means to increase apparent size. My findings support the prediction that, while body size is condition-dependent, the sword is not. I show a trend toward hybrid populations displaying increased phenotypic variance and reduced phenotypic integration in sexual ornaments and body size. These findings provide evidence for a potential answer to a central question in the study of sexual selection, that of reduced genetic and phenotypic variance in sexual ornaments as the result of persistent direction selection generated by female choice. I take advantage of reduced phenotypic integration in hybrids allowing the evaluation of locomotor performance across a broad range of multivariate trait values. Sexual ornaments did not impair swimming performance per se. Rather, the sword negatively affected performance only when paired with a sub-optimal body shape. I evaluated how natural hybridization changes the relationship between boldness and anti-predator response. In poeciliid fishes, bold individuals have increased survival in the presence of predators. ii This non-intuitive observation may result from bold individuals being more likely to engage in anti-predator behaviors. Counter to my prediction, bold individuals were less likely to perform a fast-start response to a predator threat. This correlation was consistent among populations and species but was only significant in hybrids. My findings suggest that hybridization could influence correlations between behavioral traits in a manner similar to that documented for morphological traits. iii ACKNOWLEDGEMENTS I would like to thank my chair, Dr. Gil Rosenthal and committee members Drs. Adam Jones, Kirk Winemiller and Lee Fitzgerald. I would also like to thank the instruction I have received at Texas A&M, in particular from faculty connected with the Ecology and Evolutionary Biology (EEB) program. Regardless of the fact that my family has no concern for science they never dismissed my interest in the topic and have enthusiastically encouraged my educational purists. In particular I would like to thank my mother Betty Sue Johnson (maiden name Carroll; 1936-2008) and my father James Edward Johnson for loving me without question providing me with unbelievable amounts of financial support in the pursuit of my education. I would also like to thank my brother Billy Johnson and his wife Becky, my sister Shelia Miller and her husband Marlon, my sister Virginia and my sister Delores Montgomery and her husband Pat. I would also like to thank my nephews and nieces, Brent Johnson, Britney Johnson, Cameron Noble, Duane Lister, Brianna Sterns and Demetrick Montgomery. I would like to thank my beautiful girlfriend Laura who endured what the process of obtaining a doctorate has done to me. I would also like to thank her parents Peter and Judy who have treated me as their own son and given me considerable professional advice. College Station is a difficult place to like even for someone from small town Texas. The following people (in alphabetical order) have made my life in this town iv intellectually stimulating and a lot of fun: Allison Pease, Amanda Subalusky, Andrew Jackson, Anthony Papadopoulos, Arron Gray, Ashley Ratley, Bibiana Correa, Brandon Bowers, Brian Langerhans, Caitlin Nessner, Carrie Deans, Charles Carlson, Charlyn Partridge, Chris Lee, Chris Schalk, Christian Kaufman, Clay Small, Collin McMichael, Cory Adams, Courtney Franssen Tobler, Courtney Passow, Dan Powell, Dan Saenz, Danielle Macedo, David Baumgardner, David Laurencio, Drew Dittmer, Emily Rose, Gage Dayton, Gaston Jofre Rodriguez, Holly Kindsvater, Jack, Jerry Huntley, J.V. Montoya, Katie Roach, Kevin Deitz, Kim Paczolt, Liz Marchio, Machteld Verzijden, Marion LeGall, Mattie Squire, Michi Tobler, Mike Treglia, Nick Ratterman, Nicole Smolensky, Olivia Ochoa, Pablo Delclos, Pam Willis, Paul Lenhart, Rhonda Struminger, Rich Serva, Robert Powell, Rongfeng Cui, Ryan Easterling, Sarah Flanagan, Scott Wahlberg, Seth Coleman, Sunny Scobell, Suzanne Simpson, Toby Hibbitts, Trisha Butkowski and Zack Culumber. v TABLE OF CONTENTS Page ABSTRACT ....................................................................................................................... ii ACKNOWLEDGEMENTS .............................................................................................. iv TABLE OF CONTENTS .................................................................................................. vi LIST OF FIGURES ........................................................................................................ viii LIST OF TABLES ............................................................................................................ ix CHAPTER I INTRODUCTION ....................................................................................... 1 CHAPTER II NEGATIVE CONDITION-DEPENDENCE OF A SEXUAL ORNAMENT IN XIPHOPHORUS MALINCHE .............................................................. 7 Introduction .................................................................................................... 7 Materials and Methods ................................................................................... 9 Results .......................................................................................................... 13 Discussion .................................................................................................... 13 CHAPTER III HYBRIDIZATION AND THE STRUCTURE OF PHENOTYPIC VARIATION IN SWORDTAILS (TELEOSTEI: XIPHOPHORUS) ............................. 19 Introduction .................................................................................................. 19 Materials and Methods ................................................................................. 22 Results .......................................................................................................... 27 Discussion .................................................................................................... 29 CHAPTER IV SEXUAL ORNAMENTS, BODY MORPHOLOGY, AND SWIMMING PERFORMANCE IN NATURALLY HYBRIDIZING SWORDTAILS (TELEOSTEI: XIPHOPHORUS) .................................................................................... 35 Introduction .................................................................................................. 35 Materials and Methods ................................................................................. 37 Results .......................................................................................................... 47 Discussion .................................................................................................... 55 CHAPTER V BOLDNESS AND PREDATOR EVASION IN NATURALLY HYBRIDIZING SWORDTAILS (TELEOSTEI: XIPHOPHORUS) .............................. 60 Introduction .................................................................................................. 60 Materials and Methods ................................................................................. 62 vi Results .......................................................................................................... 64 Discussion .................................................................................................... 66 CHAPTER VI CONCLUSIONS ..................................................................................... 69 REFERENCES ................................................................................................................. 71 vii LIST OF FIGURES Page Figure 1 The morphology, performance and fitness paradigm .......................................... 1 Figure 2 An illustration of a X. malinche male with digitized landmarks used to calculate centroid size. ...................................................................................... 10 Figure 3 Surface plot describing the relationship between log sword length, log centroid body size and condition.. .................................................................... 12 Figure 4 The summed relative weights (importance) for the five best-supported model effects. ............................................................................................................... 14 Figure 5 A, hybridization between two species for two traits (Z1 and Z2). ....................

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