Large Cat Genetic Variation:
Genetic Divergence of Two Large Cats and their Hybrids
By:
Ashley Wetzel
Throughout the world, there are 36 species of wild cat ranging from North America to Africa. These 36 species all fall under the biological family, Felidae. These various wild cats typically fall under three different subfamilies including Felis, Patherinae, and Acinonychinae. The two large cats being focused on in this paper include two of the four great cats, the lion and tiger. Both belong to the subfamily Pantherinae.
There has been a considerable amount of research done on these cats and a large percent of that research has been done over the genetic diversity available to these cats and their hybrids. Large cats have been limited to a small gene pool due to poaching and habitat destruction and this small gene pool may eventually be the cause of future large cat extinction. Throughout this paper, I will survey the research that has been done on the genetic diversity available to large cats. Based on this research, a determination of genetic diversity and gene pool variation will be made.
(Jae-Heup, K., Eizirik, E., O'Brien, S. J., & Johnson, W 2001) The very basis of the research being done on large cats involves a great deal of genetics. Population genetics research studies are primarily used to study the distribution of genetic differences within populations and how these differences have changed over time. These studies have been performed on large cats by various researchers throughout the years. The IUCN (International Union of Conservation of Nature and Natural Resources) has been using these scientific studies for years to determine the population number, population trends, and hardiness of various species. A large amount of genetic research involving genetic variation includes Mitochondrial DNA. Mitochondrial DNA, unlike nuclear DNA, is stored in the mitochondrion and it copies itself within the same mitochondrion. Due to this type of recombination, mitochondrial DNA does not differ much between parent and offspring. The use of mitochondrial DNA is considered a better tool in genetic research due to the larger number of mitochondrial DNA per cell in comparison to nuclear DNA per cell. The higher number increases the chance of obtaining a useful sample. Mitochondrial DNA is almost exclusively inherited from the mother due to the mitochondrial destruction performed by the egg after fertilization. Genetic mutation is another staple for research on large cats. Throughout many large cats, there are coat variations that are resultant of genetic mutation. DNA tests have been done to determine how harmful or beneficial these genetic mutations are to the large cats.
(World Wild Cats; Garman, Andrew 1997) Obviously, each wild cat is extremely unique and extremely different from humans. While humans have 23 pairs of chromosomes, cats only have 19 pairs. It is their 38 chromosomes that give each species of cat their distinctive features. Each chromosome carries the genes that determine the physical makeup of each species. These genes determine jaw structure, coat color, coat markings, eye colors, and various other physical attributes unique to each cat. It is at these genes that many mutations can occur. During this paper, I will also discuss a few examples of these mutations.
(2009 IUCN Red List) The lion, Panthera leo, is considered one of the four great cats. It is the second largest of all big cats with adults ranging from four-hundred pounds to five-hundred-and-fifty pounds in weight. Today, wild lions are found in parts of sub-Saharan Africa and in India’s Gir Forest. This is quite a different range considering wild lions once roamed most of Africa and parts of Asia and Europe. There are eight subspecies recognized today (only two recognized by the IUCN), however mitochondrial research has lead animal researchers to believe that there are really only two subspecies that should be recognized. The eight subspecies recognized include Panthera leo persica (the Asiatic Lion), Panthera leo leo (the Barbary Lion), Panthera leo senegalensis (the West African Lion), Panthera leo azandica (the Northeast Congo Lion), Panthera leo nubica (the East African or Massai Lion), Panthera leo bleyenberghi (the Southwest African or Katanga Lion), Panthera leo krugeri (the Southeast African Lion), and Panthera leo melanochaita (the Cape Lion). Out of these eight subspecies, two are extinct in the wild. The Barbary lion became extinct in the wild in 1922 due to excessive hunting. It is believed that captive individuals may still exist, but no genetic research has been done to clarify this belief. The Cape lion became extinct in the wild around 1860 due to excessive hunting as well. However, based on similar mitochondrial DNA sequences, researchers believe that the Northeast Congo, the East African, the West African, the Southwest African, the Cape, and the Southeast African lion are all of one subspecies found throughout Africa. The Cape lion is also believed based on mitochondrial DNA research, to just be a population of the Southeast African lion. The two major subspecies (the only ones recognized by the IUCN) being focused on in this paper are the African (all five subspecies) and the Asiatic lion.
(2009 IUCN Red List) The Asiatic lion (Panthera leo persica) once had a wide spread distribution across southwest Asia, but is now limited to one population in India’s Gir Forest. This subspecies of lion lives in a single isolated population consisting of approximately 175 mature lions. Based on this population number, the Asiatic lion has been considered endangered since 1986 and was considered critically endangered from 2000 to 2007. In 2008, the Asiatic lion was changed from critically endangered to simply endangered. Although, poaching laws and conservation acts are in action, 34 Asiatic lions were killed in 2007. The threats affecting this subspecies include poaching, habitat destruction, and habitat invasion. However, the largest threat to this subspecies is that it exists as a single population and is considered extremely vulnerable to extinction caused by natural disaster and/or epidemic. In order to increase this cat’s population, conservation actions have been taken. These actions have included an attempt at establishing other wild populations to increase genetic diversity and to re-establish the Asiatic lion as a component in one of their previous ranges.
(2009 IUCN Red List) The African lion (Panthera leo) is found in sub-Saharan Africa. The African lion once ranged from northern Africa through southwest Asia, western Europe, eastern India, and even parts of Egypt. It is believed that, throughout the last three decades, the African lion has been secluded to only 22% of its historical range. Throughout the last two decades, there have been many attempts at determining the population number of this subspecies. However, none of them have been inclusive with a series of very different ranges (30,000 – 100,000). Despite the uncertainty, it is believed that since 1950, the African lion population has been cut in half. Based on this uncertainty and possible population number, the African lion has been considered vulnerable since 1996. The threats affecting this subspecies include indiscriminate killing (as a result of defensive killing to protect livestock and humans), prey depletion, habitat loss, habitat invasion, disease, and poaching. Perhaps the largest threat to these lions is the forced coexistence with humans. Between 1997 and 2007, there have been 400 lion-related human fatalities (mainly in Tanzania). As a result of these lion attacks, retaliatory killings of lions have become common. The West and Central African Lion Conservation Strategy was put in place in 2006. This strategy focuses on three objectives to decrease threats and increase genetic diversity. These three objectives include an attempt to reduce lion-human conflict, to conserve and increase lion habitat, and to increase wild prey availability.
Based on the threats effecting Asiatic and African lions, the gene pools available to both subspecies have been drastically impacted. Through habitat destruction and invasion, lion prides have been pushed closer and closer together leading to inbreeding and increased territorial disputes between lion prides resulting in lion death that would not have occurred if lion ranges had not been limited. Inbreeding results in poor morphology, poor fertility, and immunological problems. For example, maneless male lions with little body hair have been seen in Kenya. It is believed that the maneless feature and poor coat quality is a result of inbreeding. If Asiatic and African lion ranges had not been limited throughout the years, inbreeding would not have become such a large problem and more fertile lions would be available to the populations which would also increase the gene pool and genetic diversity of these cats.
(Shankaranarayanan & Singh, 1997) Many studies have been done on Asiatic and African lions to determine the genetic diversity of each subspecies and the species Panthera leo as a whole. In 1997, a study was done in Hyderabad, India, on the genetic variation in Asiatic lions and Indian tigers. The study was done based on the belief that Asiatic lions are highly inbred, exhibiting very low levels of genetic diversity. This study was used to determine the degrees of polymorphism in Asiatic lions. Polymorphism is described as having multiple alleles of a gene within a population with each allele expressing a different phenotype. In the study, 38 Asiatic lions from the Gir Forest Sanctuary in India were analyzed through Random Amplified Polymorphic DNA analysis. The results showed an average heterozygosity of 25.82% with four primers. After these results were determined, other tests were administered. These tests included a sperm motality study, of which the results correlated with the RAPD results. Along with the sperm motality study, microsatellite analysis was performed on 50- to 125-year-old skin samples from Asiatic lion museum samples. Based on the microsatellite analysis, the researchers determined that the Asiatic lions in the Gir Forest Sanctuary are hybrids of the Asiatic subspecies and the African subspecies. Basically, Asiatic lions, before their population bottleneck, had contact with African lions and bred to form an AsiaticXAfrican hybrid cross. Throughout the years later, these AsiaticXAfrican crosses were bred with pure Asiatic lions, resulting in lower and lower levels of the Africanized lion genes. Thus, very few of the Asiatic lions in the Gir Forest Sanctuary are pure Asiatic lions. Based on the study, it was determined that Asiatic lions typically did not have very high levels of genetic variability. This is believed to occur naturally and is not a cause of inbreeding. To determine the individual lions with the highest genetic variability, DNA fingerprinting studies have been done to choose individual lions for conservation breeding programs.
(Shankaranarayanan & Singh, 1998) Shankaranarayanan and Singh did another study in 1998 on AsiaticXAfrican lion crosses, Asiatic lions, and African lions. In this study, mitochondrial DNA sequence variation was used to distinguish between various big cat species and their hybrids. This study only confirmed their 1997 study’s results. Mitochondrial D-loop sequencing revealed only one haplotype among Asiatic lions and multiple haplotypes in AsiaticXAfrican crosses. Based on the basics of mitochondrial DNA and its maternal inheritance, this concluded that African female lions were responsible for the multiple haplotypes involved in AsiaticXAfrican crosses. Based on this information, it’s been determined that the two subspecies once shared the same ranges and only separated 80,000-100,000 years ago.
(Shankaranarayanan & Singh, 1997 and 1998) Based on these two studies, Asiatic lions exhibit very low genetic variation levels due to the small population in the Gir Forest Sanctuary. It is also believed that there are not many (if any) pure Asiatic lions in existence, with many exhibiting low levels of AsiaticXAfrican hybridization due to historic subspecies breeding. Also based on these two studies, it is believed that the African lion exhibits much higher levels of genetic variation than Asiatic lions.
(Robinson & De Vos, 1982) Another factor affecting genetic variation is the incessant breeding of White lions. Throughout the world, there are approximately 400-500 white lions in various zoos all around the world. The white-colored coat variation is a genetic occurrence that happens only once (if any) every 50-60 years. In reality, it does not happen at all in the wild and, if it does, the individual animal does not survive long enough to have any type of conservational effect on the lion population. The white color is a genetic mutation that occurs when both lion parents have a copy of the genetic mutation. This genetic mutation is caused by the Chinchilla mutant that causes a white coat change similar to albinism. Some consider the mutation a recessive trait similar to albinism, however, through genetic research, the Chinchilla genetic mutant has been deemed responsible for the coat change. A study done in 1982 among prides of lions inhabiting the Kruger National Park and the Timbavati Game Reserve in South Africa helped to determine the culprit for this coat variation (whether it was albinism or the genetic mutant). The Kruger National Park had various reports of white variants among certain prides of lions. Park caretakers termed these white lions albinos. This was deemed incorrect based on the eye color of the white lions. Albino individuals would exhibit a pink-eyed feature, but these cats displayed a normal yellow eye color. Based on the small territory and the free-roaming of the lion prides between the Kruger National Park and the Timbavati Game Reserve, it was determined that the lion population inhabiting those two areas had been subjected to a low rate of inbreeding. All of the white cubs produced in the Kruger National Park were produced by parents with normal coats. Based on this information and the litters produced, inbreeding is a direct cause of the high population of white lions. In 1959, a pride with two cubs was spotted, but it wasn’t for another sixteen years, that the white coat variation appeared in a litter in 1975. The first pride with white cubs possibly lead to the second appearance of the white coat variation, by one of the white adults (ww) breeding with a normal adult (WW), resulting in multiple normal cubs (Ww) with copies of the genetic mutant. If these lions (Ww) bred with another set of lions (Ww), they would produce 25% normal cubs (WW), 50% normal cubs (Ww) and 50% mutated cubs (ww). As well as this, if a normal lion (Ww) with copies of the genetic mutant bred with a white lions (ww), they would produce 50% normal (Ww) and 50% mutated (ww). There are many explanations for the random appearances of the white mutant, but the most accurate one is inbreeding. The only way these lions could survive is in a controlled environment such as a reserve or a zoo. White lions do not survive in the wild. If by mere luck one survived, every white lion produced by that lion and after that lion would be related to that same original white lion and would thus be very, very inbred. These animals, every once in a while, may look pretty, but unfortunately are not healthy and often exhibit facial deformation, immunological problems, and ocular problems. As well as the production of faulty and unhealthy creatures, these animals serve no conservation value. White lions cannot be (at least not ethically) returned to the wild. If they were returned to the wild, survival rate would be less than 10%. They lack the natural camouflage necessary for being a lion and sending them back into the wild would result in an unfortunate slaughter via lion attacks, hyena attacks, and starvation due to inability to hide when hunting prey.
(2009 IUCN Red List) The tiger, Panthera tigris, like the lion is also considered one of the great cats. It is the largest of all wild cats, with adult males weighing up to three hundred kilograms and ranging in lengths of nine to ten feet. Today, wild tigers are found in twelve Asian range states including Bangladesh, Bhutan, Cambodia, China, India, Indonesia, Lao PDR, Malaysia, Myanmar, Nepal, Russia, Thailand, and Viet Nam. It is believed that they may still exist in North Korea, but there haven’t been any recent sightings to confirm this belief. Within the past one hundred years, the tiger has lost 93% of their historic range. Tigers once ranged across Asia, from Turkey to the eastern coast of Russia. There are nine subspecies recognized by the IUCN. The nine subspecies include Panthera tigris altaica (the Siberian or Amur tiger), Panthera tigris amoyensis (the South China tiger), Panthera tigris balica (the Bali tiger), Panthera tigris corbetti (the Northern Indochinese tiger), Panthera tigris jacksoni (the Malayan tiger), Panthera tigris sondaica (the Javan tiger), Panthera tigris sumatrae (the Sumatran tiger), Panthera tigris tigris (the Bengal tiger), and Panthera tigris virgata (the Caspian tiger). Three of the nine subspecies recognized by the IUCN are now considered extinct. These three subspecies include the Bali tiger, the Javan tiger, and the Caspian tiger. The Bali tiger is believed to have died out sometime in the 1940s, at the end of World War II, due to hunting, loss of habitat, and prey deterioration. There are no Bali tigers in captivity. The Javan tiger is believed to have died out completed in the 1970s due to the same threats that killed off the Bali tiger thirty years earlier. There are no Javan tigers in captivity. The Caspian tiger is also believed to have died out in the 1970s due to hunting of their species and their prey base, habitat loss and invasion, and increased vulnerability of small populations. There are no Caspian tigers in captivity. Each subspecies has been verified based on mitochondrial DNA sequence diversity distinct to each subspecies.
(2009 IUCN Red List)Throughout the last fifty years, tiger populations in Asia completed plummeted. Fifty years ago, approximately 100,000 tigers roamed wildly. Today, only about 5,000 tigers exist in the wild. The tiger has been considered endangered since 1986. Much like lions (only much more critical), the small population number has caused a distinct drop in genetic diversity and in the available gene pool for future generations. The three subspecies being addressed in this paper are Panthera tigris altaica (the Siberian tiger), Panthera tigris tigris (the Bengal tiger), and Panthera tigris amoyensis (the South China tiger).
(2009 IUCN Red List) The Siberian tiger is a subspecies that exhibits a low level of genetic variation, mainly due to population declines. This tiger subspecies is indigenous to many areas in Russia. In the 1930s, the population fell to 20-30 animals. From 1996 to 2000, these tigers were considered critically endangered. Fortunately, the subspecies made a comeback and there are now about 400-500 tigers inhabiting Russia. Unfortunately, the effective population is only about 40% (100-200 tigers), meaning only about 40% make it long enough to reproduce. This is in result of poaching, human-tiger contact, and prey depletion. This has, of course, slowed since the enforcement of poaching laws and other conservation acts. 90% of these tigers live in a single large subpopulation in Sikhote Alin, Russia and the another subpopulation exists in the Changbai mountains in coexistence with the China tiger populations. Thus, the China tiger population and the Russian tiger populations have created a AsiaticXSiberian hybrid. Much like the AsiaticXAfrican lion cross, this has caused a very low concentration of pure Chinese tiger and Siberian tigers. However, by creating this cross, this has increased genetic sequence divergence among both subspecies. Unfortunately, this also has limited the pure genetic pool of Siberian and Chinese tigers. Genetic variation is considered to still be on a decline considering the population trend (as of 2008) is decreasing.
(2009 IUCN Red List) The Bengal tiger is a subspecies that also exhibits a low level of genetic diversity due to population declines as a result of a 41% decline in habitat area. These tigers are typically found in India, Nepal, Bhutan, and Bangladesh. In 2005, scientists came up with a total subspecies population of less than 2,500 tigers (1,782-2,527). Much like the Siberian tiger, only about 40% of the Bengal tiger population is active in the breeding of the entire subspecies population. These numbers have deemed this tiger subspecies endangered since 2001. Also like the Siberian tiger, the population trend (as of 2008) is decreasing and it is believed, over the next three generations, this trend will continue to be consistent unless conservation efforts become a higher priority.
(2009 IUCN Red List) The South China tiger is considered to be the most critically endangered subspecies of all tiger subspecies. In the 1950s, the South China tiger population was estimated to be about 4,000 tigers. Sometime around then, a large scale tiger eradication campaign was put into effect. With this campaign and the increased habitat loss, this subspecies fell to about 150-200 tigers. In the last couple of years, there has been no visual proof of a wild South China tiger population. In 2004, it was concluded that no viable wild populations exist. There are, however, South China tigers in captivity. It is believed that there are about 57 – 72 tigers in various zoos. These tigers are descendants of only six tigers and thus, show high levels of inbreeding, low levels of genetic diversity, and very low rates of successful breeding in captivity. As well as this, only a few of these tigers seem to be completely pure South China tigers, showing genetic evidence of cross-breeding with other subspecies (perhaps the Siberian tiger).
As a whole, the genetic diversity of all tiger subspecies are at very, very low levels making it extremely difficult to bring this animal back to the peak of its existence. Tigers used to exist in numbers in the hundreds of thousands and now they are limited to the thousands. Obviously, the gene pool has been limited to 5% of its original gene pool. Nearly every subspecies of tiger are incredibly inbred.
(Inbreeding Timeline Big Cat Rescue) Another factor in the inbreeding of tigers is the same genetic mutant that causes the white coat variation in white lions. White tigers, portrayed in various forms of entertainment, are caused by the Chinchilla mutant. Exactly the same as white lions, the only way to produce a white tiger is through severe inbreeding. This included mother to son and father to daughter inbreeding. For the white tiger population, there has been an inbreeding time line beginning in 1820, where the first white tiger was discovered. On May 27th, 1951 in Rewa, India, the first white tiger cub was captured. It was a male white cub that they named Mohan. About a year later, they captured a normal colored tigress named Begum and bred her to Mohan. They produced two litters of normal colored tiger cubs, but when Mohan was bred with his 4-year-old daughter (from the second litter), they produced an all white litter of a male and three females. Mohan was continually bred with his daughters, while the white male cub and one of the white females were also bred together. As a result of this inbreeding, the white female, who was mated with her brother, mauled her first litter, ignored and neglected her second and third litters. Despite the failure with the first three litters, they continued to breed the female until she had produced twenty white cubs. As you can see, every white tiger produced since 1951 is directly related to Mohan. This is why they are so genetically inbred. This fails to be portrayed by the media, seeing as only perfect examples of the white tiger are shown, but there are obvious reasons why the white female continually mauled and ignored her cubs. They were genetically mutated and so severely inbred and unhealthy. They exhibit a mortality rate in excess of 80% due to the fatal effects of their inbreeding which include immune deficiency, strabismus (crossed eyes), scoliosis (distorted spine), cleft palates, and mental impairments. People seem to like the idea of white tigers but cannot handle the reality and the complications that come with them, resulting in high rates of abandonment. Along with this, these white tigers have absolutely no conservational value whatsoever.
In conclusion, there are many factors that are fighting against large cat genetic variation. Due to these factors, many species and subspecies of large cat exhibit very low levels of genetic variation and overall exhibit low gene pool availability. The factors contributing to this problem include low population numbers, decreasing population trends, exclusive single population living in one area only, inbreeding, and genetic mutations. These factors, along with the genetic problems these cats face, are having an extremely negative effect on these species as a whole. These threats could easily cause the extinction of these animals which is becoming a closer and closer reality due to habitat destruction and invasion by human beings. If things continue on the way they have recently, these cats won’t exist much longer. Many subspecies are already extinct in the wild and many other subspecies face the same fate, if not worse.
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