Genetic Diversity of White Tigers and Genetic Factors Related to Coat Color
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GENETIC DIVERSITY OF WHITE TIGERS AND GENETIC FACTORS RELATED TO COAT COLOR An Undergraduate Research Scholars Thesis by SARA ELIZABETH CARNEY Submitted to Honors and Undergraduate Research Texas A&M University in partial fulfillment of the requirements for the designation as UNDERGRADUATE RESEARCH SCHOLAR Approved by: Research Advisor: Dr. Jan Janecka May 2013 Major: Biomedical Sciences Wildlife and Fisheries Sciences TABLE OF CONTENTS Page TABLE OF CONTENTS .....................................................................................................1 ABSTRACT .........................................................................................................................2 ACKNOWLEDGEMENTS .................................................................................................4 CHAPTER I INTRODUCTION .......................................................................................5 Candidate genes for the white coat phenotype ................................8 Use of microsatellites to determine genetic diversity ......................9 II METHODS ................................................................................................11 Samples ..........................................................................................11 PCR methodology ..........................................................................11 Microsatellite analysis ...................................................................12 MC1R analysis ...............................................................................13 III RESULTS ..................................................................................................14 IV DISCUSSION ............................................................................................19 V CONCLUSION ..........................................................................................22 REFERENCES ..................................................................................................................24 1 ABSTRACT Genetic Diversity of White Tigers and Genetic Factors Related to Coat Color. (May 2013) Sara Elizabeth Carney Department of Veterinary Medicine and Biomedical Sciences Texas A&M University Research Advisor: Dr. Jan Janecka Department of Veterinary Medicine and Biomedical Sciences White tigers are greatly cherished by the public, making them valuable to zoos and breeders. Unfortunately, a number of health issues have occasionally surfaced within some of the white tiger population such as neurological and facial defects. There is interest amongst private tiger breeders to determine if these maladies are associated with the coat color or breeding practices, and to find ways to prevent these health issues. The genes involved in producing the white phenotype and the disease phenotype are currently unknown. Furthermore, the relationship between the genes associated with coat color and levels of inbreeding also remain unknown. Microsatellites are a tool frequently used within by geneticists and ecologists alike. These segments of highly repeatable DNA mutate frequently and are variable in length. Thus microsatellites can be used to determine heterozygosity within a population by detecting the alleles present at the loci of interest. The amount of heterozygosity within a population can be indicative of the amount of inbreeding present and overall levels of genetic diversity. A panel of twelve microsatellites was used to analyze heterozygosity, thus inferring the levels of genetic 2 diversity present. Among the tigers sampled, estimated heterozygosity was determined to be 0.761 in white tigers and 0.772 in orange tigers. The genes Melanocortin-1-Receptor (MC1R) and Agouti Signaling Protein (ASIP) have been found to affect coat color phenotypes in other species similar to that of the white tiger, making them ideal candidates for this project. These genes work antagonistically to each other in production of melanin. MC1R is responsible for the production of α-melanocyte stimulating hormone (α-MSH) while ASIP silences this activity. Thus, a loss-of-function associated with MC1R or a gain-of-function associated with ASIP could lead to reduced pigment production. This study continues the initial investigation by focusing on sequencing MC1R. Differences in the nucleotides and amino acids of the sequences were compared though alignment in Sequencher. At this time a causal mutation has not been found within exons 1 and 2 of ASIP or MC1R. 3 Acknowledgements First, I would like to thank everyone in the Texas A&M Molecular Cytogenetics and Genomics Lab for providing a helpful and friendly environment for me to begin exploring the scientific process. I would also like to thank Dr. Jan Janecka, Dr. Bhanu Chowdhary and Dr. Samantha Steelman for allowing me the opportunity to embark on such an exciting project and for providing ample guidance and encouragement. I am also grateful to my friend Emilee Larkin, who sparked my interest in the subject and was always available to answer questions and provide support. I would like to thank my friends and family for their enthusiasm and support. I would also like to express my gratitude to those who provided samples for this project including the San Francisco Zoo, In-sync Exotics (Vicky Keahey), Big Cat C.A.R.E, (Heidi Riggs Berry), Sierra Endangered Cat Haven (Dale Anderson), Big Cat Rescue, Tiger Creek, the Exotic Feline Rescue Center, Ferdinand and Antonin Fercos, T.I.G.E.R.S. (Doc Antle) and REXANO (Zuzana Kukol). Your generosity has not only helped us begin to understand the genetics of white tigers, but it has also provided me with the opportunity explore the world of research and for that, I thank you. 4 CHAPTER I INTRODUCTION To many the white tiger, Panthera tigris, has been a source of awe, combining the power and grace exhibited by the standard orange tiger with the rare beauty from its unusual coat color. Though many find the white tiger to be inspiring, this is not a universally held opinion. Critics contend that the white tiger is a detriment to tiger conservation, claiming that the tigers must be inbred in order for the white coat to be present. Furthermore, they attribute the ailments faced by some white tigers (eg. crossed-eyes and cleft palates) (Roychoudhury and Sankhala 1978) to the white coat trait, believing it to be inseparable from inbreeding. In light of this controversy, it is important to determine the white tiger’s role in conservation of the species. Though some do not place priority on the preservation of the white tiger, it is evident that the species as a whole is facing the threat of extinction. Three of the original eight tiger subspecies, Bali (Panthera tigris balica), Caspian, (Panthera tigris virgata), and Javan (Panthera tigris sondaica), have recently become extinct (Luo et al. 2004). The tiger population has faced recent rapid decline. Within the last 100 years the wild tiger’s habitat has been reduced to only 7% of the land in once roamed (Dinerstein et al. 2007). Poaching as well as habitat loss and fragmentation poses the greatest threat to the wild tiger population. Deforestation has significantly impacted the wildlife present in these areas particularly the tiger and its prey (Kinnaird et al. 2003). The tiger faces additional risks associated with its dwindling population, primarily decreased genetic diversity. Frequently, populations facing significant decline may resort to inbreeding, potentially leading to inbreeding depression (Hedrick and Kalinowski 2000). Consequently, deleterious homozygotic traits that were once masked in a healthy 5 population of heterozygotes may become rampant in a genetically isolated population. Thus, this genetically compromised population becomes increasingly vulnerable to disease (Lynch 1977). While the wild tiger population faces steady decline, the captive population has successfully propagated. Tigers have relatively few complications associated with reproduction, which often plagues captive breeding programs. Additionally, captive-bred populations are protected from many of the threats that face their wild counterparts, such as habitat degradation, disease and poaching. Though the captive tiger has escaped many of these issues, loss of genetic diversity is still a present concern within segments of the population (Lacy 1987). The white tiger is particularly vulnerable to increased homozygosity due to selection for this phenotype. In many ways the captive environment has allowed rare coat color polymorphisms such as that of the white tiger to persist. Though there are early reports of white tigers in India, the first lineage of captive white tigers originated in what was known at the time as Rewa, (which is now Madhya Pradesh), from a single male known as Mohan who was captured in 1951 (Thorton et al. 1966). The first breeding of Mohan to Belgum, a wild orange female, was unsuccessful in producing a white offspring. Mohan was subsequently bred to his daughter, Radha, produced from the previous cross. This resulted in four litters, all producing white offspring (Thorton et al. 1966). It can be inferred that Rewa, an F1, was heterozygous for the white coat allele. Thus the Rewa-Mohan cross gave offspring of the union a 50% chance of being homozygous for and therefore expressing the white coat allele. The white coat polymorphism is an autosomal characteristic inherited in a Mendelian-recessive fashion (Thorton et al. 1966). Although inbreeding was prevalent in early 6 breeding of white tigers, it is not essential to