Inbred Prairie Voles (Microtus Ochrogaster) Under Captive and Semi-Natural Conditions

SURVIVAL AND REPRODUCTIVE SUCCESS OF INBRED AND NON- INBRED PRAIRIE VOLES (MICROTUS OCHROGASTER) UNDER CAPTIVE AND SEMI-NATURAL CONDITIONS A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science Department of Zoology by Kathryn Lynn Williams Miami University Oxford, OH 2008 Advisor________________________ (Dr. Brian Keane) Reader ________________________ (Dr. Nancy G. Solomon) Reader_________________________ (Dr. Thomas O. Crist) ABSTRACT SURVIVAL AND REPRODUCTIVE SUCCESS OF INBRED AND NON- INBRED PRAIRIE VOLES (MICROTUS OCHROGASTER) UNDER CAPTIVE AND SEMI-NATURAL CONDITONS by Kathryn Lynn Williams Inbreeding’s harmful consequences have been well documented under artificial conditions; however, studies under natural conditions are limited. I examined the effects of inbreeding on the fitness of prairie voles (Microtus ochrogaster) in captivity and the field. In captivity, sibling and non-sibling pairs did not differ with regard to time to the first litter, litter size, or offspring weight. Another laboratory experiment examined these same variables in the following un-related pairs: non-inbred female/ male, inbred female/ male, non-inbred female/ inbred male, and inbred female/ non-inbred male. The only significant result was that the weaning weight of offspring born to non-inbred pairs was greater than offspring born to non-inbred female/ inbred male pairs. There also was no significant difference in the survival and reproduction of unrelated inbred and non-inbred voles released into semi-natural enclosures. This study did not find any evidence that inbred adults have lower fitness than non-inbred adults. Table of Contents List of Tables iii List of Figures iv Dedication v Acknowledgements vi Chapter One: Introduction 1 Methods 4 Results 6 Discussion 8 Chapter Two: Introduction 12 Methods 14 Results 19 Discussion 20 Tables and Figures 25 Literature Cited 34 ii List of Tables Table Page Chapter One Table 1.1 25 Table 1.2 26 Chapter Two Table 2.1 27 Table 2.2 28 Table 2.3 29 iii List of Figures Figure Page Chapter Two Figure 2.1 30 Figure 2.2 31 Figure 2.3 32 Figure 2.4 33 iv Dedication For my Dad William Randolph Williams v Acknowledgements I would like to thank my advisor, Dr. Brian Keane, and the members of my committee, Dr. Nancy Solomon, and Dr. Thomas Crist. I would also like to thank the Solomon Lab and the Meikle Lab. In addition, I would like to thank Dr. Douglas Meikle, Loren Hayes, Lana Knoch, Mark Spritzer, Charity Crowe, Chrissy Anderson, Chris Wood, and Bob Davis. I could not have completed this thesis without the friendship and support of Sheri and Chris Barton, Aaron Roberts, Jennifer Hoffman, Carrie Smith, and Sarah Harvey. Thanks to my family ~ My brother, Andrew. My Mom deserves special thanks for all of her love, support, friendship, and guidance throughout my life. And finally, to my husband and muse, Jeremy Shuck. “A riddle, wrapped in a mystery, inside an enigma” —Winston Churchill Thank you for the motivation to finish this degree and to finally go to law school. Most of all, thank you for putting up with me throughout this tenuous process. vi Chapter One INTRODUCTION “Nature abhors perpetual self-fertilization” −Charles Darwin The consequences of inbreeding have been thoroughly documented since the writings of Charles Darwin (1868). Inbreeding depression is the reduction in reproductive success and survival due to mating between relatives. Studies have documented inbreeding depression in a wide variety of taxa including the following: plants (Darwin 1868; Charlesworth and Charlesworth 1987), insects (Saccheri et al. 1998; Armbruster et al. 2000), fish (Waldman and McKinnon 1984), reptiles (Waldman and McKinnon 1984), birds (Keller et al. 1994), and mammals (Jimenez et al. 1994; Meagher et al. 2000; Slate et al. 2000). Fitness components such as inter-birth-interval, offspring survival, parental behavior, mating ability, sperm quality, plant height, and offspring weight may be negatively affected by inbreeding (Crnokrak and Roff 1999; Frankham et al. 2002). Inbreeding increases the chance an individual will inherit the same alleles at a particular locus, thus being homozygous at that locus (Frankham et al. 2002). Increased homozygosity may decrease survival or reproductive success (Frankham et al. 2002). The importance of two non-mutually exclusive hypotheses, overdominance and partial dominance, are debated as genetic mechanisms of inbreeding depression (Roff 2002). The overdominance hypothesis suggests that heterozygotes will have greater fitness compared to homozygotes (Mitton 1993). Because inbreeding may lead to an increase in the homozygous state, fitness may be reduced if heterozygous individuals have an advantage in the population (Charlesworth and Charlesworth 1987). The partial dominance hypothesis states that inbreeding could increase the chance that harmful recessive alleles may be expressed in offspring (Mitton 1993). Because inbreeding increases the chance of the homozygous condition, deleterious recessive alleles may be exposed resulting in deleterious consequences for an individual (Frankham et al. 2002). In a unique study designed to separate the genetic mechanisms at work, Roff (2002) crossed several different lines of 14 generations of brother-sister matings in sand crickets (Gryllus firmus). Following out-crossing, if the partial dominance hypothesis was operating, trait means were expected to exceed original non-inbred levels (hybrid vigor), while if the overdominance hypothesis was operating, 1 trait means would return to the original non-inbred levels (Roff 2002). The partial dominance hypothesis was supported by the results because trait means exceeded original levels suggesting deleterious recessive alleles were causing a decline in fitness (Roff 2002). The fitness decline was alleviated after crossing with a population that did not have the same deleterious recessive alleles (Roff 2002). Additional indirect support for the deleterious effects of inbreeding comes from the behaviors that may have evolved in many species as a means to reduce the opportunity for mating between close relatives. In mammals, behavioral mechanisms such as sex-biased dispersal and kin recognition may have evolved to limit inbreeding (Charlesworth and Charlesworth 1987; Pusey and Wolf 1996; Berger et al. 1997). Although inbreeding avoidance may not be the only reason for sex-biased dispersal, if either female or male offspring disperse, the chance of breeding with relatives is reduced via geographic separation. A review by Pusey and Wolf (1996) outlined two vole species (Townsend vole- Microtus townsendii and meadow vole- Microtus pennsylvanicus) that were more likely to disperse in the presence of relatives. The authors also looked at one species of mouse (white-footed mice- Peromyscus leucopus noveboracensis) that was less likely to disperse when the opposite-sex parent was not present (Pusey and Wolf 1996). If the sexes do not differentially disperse, individuals may recognize kin via association or phenotype matching to actively avoid inbreeding (Fadao et al. 2000). An experiment on the root vole (Microtus mandarinus) showed a breeding preference for unfamiliar individuals, regardless of actual relatedness (Fadao et al. 2000). This demonstrates that individuals avoid breeding with the animals with which they were reared, such as littermates (Fadao et al. 2000). A cross-fostering study on the montane vole (Microtus montanus) found that males caused sexual maturation in unfamiliar females and unfamiliar biological daughters, but not in daughters or foster daughters when housed together (Berger et al. 1997). Most evidence for inbreeding depression in mammals comes from data in laboratory and zoological populations (Lacy et al. 1993). Captive inbreeding studies have mainly focused on only juvenile mortality as one fitness component (Shields 1993). A review, of the costs of inbreeding among captive populations of 38 species of mammals, estimated juvenile mortality was an average of 33% higher among inbred offspring compared to non-inbred offspring (Ralls et al. 1988). However, a focus on only juvenile mortality may not accurately reflect the effects of inbreeding on reproductive success because inbred offspring could be more competitive, parental, or fertile (Shields 1993). Thus, a trade- off of “quantity for quality” would go undetected if inbred offspring that survived were more successful 2 at reproduction despite having a higher chance of mortality as juveniles (Shields 1993). Alternatively, looking solely at juvenile mortality may underestimate the deleterious effects of inbreeding because inbred individuals may also have lower reproductive success. The objective of my study was to determine if inbreeding reduced total reproductive success by affecting several components of fitness. A socially monogamous rodent, the prairie vole (Microtus ochrogaster), was used as a model organism to examine the effects of inbreeding. The prairie vole was used in this study because they are abundant and socially monogamous with bi-parental care (Carter and Getz 1993; Getz and Carter 1996). Additionally, inbreeding avoidance can be overcome in prairie voles by separating animals for short periods of time (Gavish et al. 1984). Prairie voles also have relatively short gestation and weaning periods, making them an ideal model organism (Nadeau 1985; Carter and Getz 1993; Getz and Carter 1996). First, I measured the reproduction of sibling

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