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Sexual Selection on Females: Comparing Two Estimates of Mating Success in a Sex-Role Reversed Insect

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Sexual Selection on Females: Comparing Two Estimates of Mating Success in a Sex-Role Reversed Insect SEXUAL SELECTION ON FEMALES: COMPARING TWO ESTIMATES OF MATING SUCCESS IN A SEX-ROLE REVERSED INSECT by Laura Jane Robson A thesis submitted in conformity with the requirements for the degree of Master of Science Graduate Department of Ecology and Evolutionary Biology University of Toronto © Copyright by Laura Jane Robson 2009 Abstract please enjoy this thesis and con ranting e Sexual selection on females: comparing two estimates of mating success in a sex-role reversed insect Laura Jane Robson Master of Science Department of Ecology and Evolutionary Biology University of Toronto While there has long been interest in the form of sexual selection in males, studies characterizing this selection in females remain sparse. Sexual selection on females is predicted for sex-role reversed Mormon crickets, where males are choosy of mates and nutrient-deprived females compete for matings to gain nutritious nuptial gifts. I used selection analyses to describe the strength and form of sexual selection on female morphology. There was no positive sexual selection on the female body size traits predicted to be associated with male preferences and female competition. Instead, I detected selection for decreasing head width and mandible length. Additionally, I tested the validity of a commonly-used instantaneous measure of mating success (mated vs. unmated) by comparing selection results with those determined using a more detailed fitness measure (cumulative mating rate). The two fitness measures yielded similar patterns of selection, supporting the common sampling method comparing mated and unmated fractions. ii Acknowledgements I hope at you ca n find l I must give my sincerest thanks to my lab-mates, family and friends whose help and encouragement made this degree so rewarding. Above all, I am grateful to Darryl Gwynne for giving me the opportunity to work in his lab. While I can’t thank Darryl enough for letting me chase Mormon crickets and for vastly improving my thesis, I am equally grateful for the exceptional lab that he has created. Kevin Judge is endlessly generous with his time, and I benefitted from his guidance constantly. Kyla Ercit was the best field companion imaginable, and without her help I would have collected half the crickets while being twice as panicked. Edyta Piascik helped measure eggs and was always available for advice and editing. Jill Wheeler and Andrew MacDonald were incredibly patient in coaching me through R, Murray McConnell livened up lab meetings and David Punzalan provided eleventh-hour statistics advice. As always, I must thank my parents, Debbie and Gord Robson, for their support and encouragement throughout this degree and all of my endeavours. I am grateful for too many reasons to list here, but please know that I’m aware of how lucky I am to have you. Thank you. I am also thankful to Emily Robson and Brock Hart for feeding and entertaining me, and to Ian Young for always letting me “talk it out”. Many thanks to Emma Stewart, Stephanie Scott, Jacqueline Saint-Onge, Jamie Munro, Emma Rogers, Emily Macleod and Brechann McGooey for enjoyably distracting me from my thesis. I owe you. Finally, I am grateful to the Mormon crickets. While some of their habits may not be appealing, they are fascinating animals and I hope to see them in the field again someday. This research was supported by a Natural Sciences and Engineering Research Council (NSERC) Canadian Graduate Scholarship to Laura Jane Robson and a NSERC Discovery Grant to Darryl Gwynne. iii Table of Contents I hope that you can find eve r ABSTRACT ii ACKNOWLEDGEMENTS iii TABLE OF CONTENTS iv LIST OF TABLES vi LIST OF FIGURES vii LIST OF APPENDICES viii CHAPTER 1: Sexual selection on females: comparing two estimates of mating success in a sex-role reversed insect 1.1 Introduction 1 1.2 Materials and methods 5 Life history of Mormon crickets 5 Sample collection 7 Calculation of cumulative mating rate 8 Morphological traits measured 8 Measurement of selection 9 Comparison of fitness measures 11 1.3 Results 11 Comparison of collection sites 11 Selection on original trait axes 12 Canonical analyses of selection 12 Correlations of morphological and fecundity traits 13 Comparison of fitness measures 13 iv 1.4 Discussion 14 REFERENCES 21 TABLES 29 FIGURES 33 APPENDIX A 41 v List of Tables T 1.1 Selection coefficients estimating sexual selection on nine female morphological traits using cumulative mating rate as the measure of fitness 29 1.2 Eigenvectors from the canonical rotation of the γ matrix presented in Table 1.1 30 1.3 Spearman rank correlations among the female morphological traits and fecundity measures 31 1.4 Selection coefficients estimating sexual selection on nine female morphological traits using instantaneous mating success (mated vs. unmated) as the measure of fitness 32 vi List of Figures F 1.1 Relationship between female spermatodose number and adult age 33 1.2 Thin-plate spline of sexual selection on female head width and mandible length 34 1.3 Contour map showing points distribution of sexual selection on female head width and mandible length 35 1.4 Thin-plate spline of sexual selection along the first and ninth major axes of nonlinear selection 36 1.5 Contour map showing points distribution of sexual selection along the first and ninth major axes of nonlinear selection 37 1.6 Thin-plate spline of sexual selection along the eighth and ninth major axes of nonlinear selection 38 1.7 Contour map showing points distribution of sexual selection along the eighth and ninth major axes of nonlinear selection 39 1.8 Frequency of mated an unmated females by spermatodose number 40 vii List of Appendices F APPENDIX A: Supplementary Figures 41 Figure A.1: Length measurement of the female’s foreleg spine 41 Figure A.2: Width measurement of the female’s last abdominal plate 42 viii Chapter 1 1 Sexual selection on females: comparing two estimates of mating success in a sex-role reversed insect Laura J. Robson 1.1 Introduction Given the ornaments, armaments and incredible displays often associated with mating (Darwin 1871), there has long been interest in understanding sexual selection in the field (Endler 1986) and this area of research has shown no decline in popularity in recent years (Owens 2006). The use of selection analyses (Lande and Arnold 1983) to measure selection on individual traits and combinations of traits (Philips and Arnold 1989, Blows and Brooks 2003) has led to a surge in the number of studies quantifying the strength and form of sexual selection in the wild (reviewed in Kingsolver et al. 2001, Hunt et al. 2009). Many of these studies rely on cross-sectional sampling of their mating population, comparing the phenotypes of mated and unmated individuals (e.g. Arnqvist 1992, Sadowski et al. 1999, LeBas et al. 2003, Bertin and Fairbairn 2005, Bussière et al. 2008). In this study, I test the validity of this commonly used fitness classification by comparing it to a more detailed measure of mating success (cumulative mating rate). The second goal of this study is to describe sexual selection on females in a system in which the sex roles are reversed: females compete for matings rather than males. Studies of sexual selection have focused mainly on males as they are nearly always the more sexually competitive sex with greater variation in mating success (Shuster and Wade 2003). However, 1 there is accumulating evidence of widespread sexual selection on females (Cunningham and Birkhead 1998; Clutton-Brock 2007, 2009). In most animal mating systems, females are more selective of mates than males, and males are the more competitive sex (Darwin 1871). The operational sex ratio is generally male-biased due to the typical differences in parental effort among the sexes: female parental effort almost always exceeds that of the male due to the large energetic investment in the nutrient-rich egg compared to that of sperm and often also due to the increased brood care provided by females (Trivers 1972). However, male investment can offset the high female parental investment via the contribution of nutrients or protection to the mating female or her offspring. In many insect taxa, these contributions take the form of male nuptial gifts, which can be prey items (e.g. dance flies, Downes 1970), seminal chemicals (e.g. pyrochoid beetle, Eisner et al. 1996), male haemolymph (e.g. nemobiine crickets, Fedorka and Mousseau 2002) and even male tissue (e.g. sagebrush cricket, Dodson et al. 1983). Some of the most nutritious mating gifts are the large glandular spermatophylaces of katydid species in which females compete for matings (Orthoptera: Tettigoniidae) (Gwynne 2001). One way to measure sexual selection on females and males is to examine the relationship between mating success (number of mates) and reproductive success (fecundity/fertility) (i.e. Bateman gradient: Bateman 1948, Arnold and Duvall 1994). For individuals of the sex with the steeper Bateman gradient, reproductive success is limited mainly by mating opportunity and they must often engage in intrasexual competition for mates (Bateman 1948). In species where males provide for their mates or offspring and show sex- role reversal, the female Bateman gradient can exceed that of males (e.g. Jones et al. 2000). Paternity data are required to calculate the male gradient (but see Lorch 2005), and regardless of the gradient slope, sexual selection will only occur when mating success depends on the 2 expression of a given trait such as an ornament or armament (Bateman 1948, Arnold 1994). Sexual selection on these traits can be measured directly by determining how the expression of the trait influences mating success using selection analysis (Lande and Arnold 1983). While selection analyses have been used to measure sexual selection on males in a diverse array of vertebrate and invertebrate taxa (Hunt et al.
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