Inbreeding Depression


There is currently 1.5 to 1.8 million species in the world that have been successfully named and classified. Of this number a vast amount already has, or is in the process of extinction. Loss of habitat, over-exploitation of wildlife for commercial purposes, the introduction of harmful exotic species, environmental pollution, and the spread of diseases pose serious threats to the world’s biological heritage. This is of key concern as the removal of a single species can set off a chain reaction in the ecosystem affecting many others. This is especially true for keystone species, whose loss can transform or undermine the ecological processes or fundamentally change the species composition of the wildlife community.

(U.S Fish And WildLife Service, 2005)Is there a method in which these species can be saved from extinction and reintroduced into the wildlife? A zoological garden shortened to zoo, is an institution in which animals are exhibited in captivity. In addition to providing visitors with an entertaining spectacle, modern zoos are also involved with conservation biology (PA, 2005). Conservation biology is the scientific study of the nature and status of the earth’s biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction (Soule, 1986). Zoos meet there conservation role through captive breeding, the process of raising plants or animals in controlled conditions to produce stock for release into the wild, education, research, animal welfare, environmental enrichment, reintroduction and support for in situ conservation of species (F, 2008). Despite many efforts being made into conserving species, there are however some major constraints faced by zoos in meeting their conservation role while at the same time providing opportunities for the public to learn more about and enjoy nature in artificial forms. One major constraint faced by zoos, is the cost and time to conserve some species. Large species breed at a much slower rates than other species and undergo seasonal breeding and require extensive fostering. An average cost of 1 million dollars in the USA is needed per year/per zoo. Therefore zoos require continued funds by tourism, and other methods in order to keep these species conserved. Other consequences arise within species themselves. Species that are kept conserved in zoos for a long period of time, result in a loss of behaviour in the wildlife, and essentially the species become adapted to zoo life. Species that have been kept in zoos for a long period of time, are in the habit of extensive care and nourishment, and therefore become adapted to living in a predator free environment. However, when let out into the wild, these species have no experience in searching for food and avoiding predators therefore natural selection will select for these traits that are advantageous in captivity, but deleterious in the wild population.

A major approach that is taken to conserve an endangered species is inbreeding, which is breeding between closely related species, most often a result of population bottleneck. This is a major comeback for conservation as inbreeding often results to -inbreeding depression which is a reduction in the species fitness, and eventually leads to reductions in genetic variability. In a population where inbreeding occurs most offspring's will have recessive deleterious traits and these traits will be masked by heterozygosity, and so natural selection will select against heterozygote’s. (Lynch, 2005) Inbreeding depression is a major consequence of captive breeding, as seen in species such as Lion population, South China Tiger, and the California Condor. These are just a few out of the numerous numbers of endangered species, which have undergone captive breeding and have been made an attempt at to keep conserved in the wildlife population.

In order to illustrate the detrimental effects of Inbreeding depression, Packer et al, 1990 examined the declining lion population in Ngorongoro Crater which is an extinct volcanic caldera located at the western edge of the Gregory Rift. The lion population estimated to be at about 60 to 75 individuals over the period 1957-1961. In 1962 however, the lion population suffered an extraordinary outbreak of Stomoxys calcitrans biting flies that reduced the population to about nine females and one male. (C.Packer, 1991) Because of the drastic reduction in population size, there was extensive inbreeding amongst the survivor species. As examined by Packer et al, 1990 Ngorongoro Crater was largely repopulated by descendents of a group of four females. Each successive cohort of cubs belonging to these four females subsequently became established breeders and mated with only seven breeding males that immigrated to the Crater all during the period shortly after the outbreak. However once the surviving females had bred successfully, the large coalitions of males prevented any further immigration which further increased inbreeding and therefore levels of heterozygosity subsequently declined by 10 percent. With a decline in levels of heterozygosity there is evidence that shows impaired reproduction in the Crater lions and higher proportion of abnormal sperm, which reduces sperm quality such that it penetrates female eggs at a very low rate. As a result productivity of the Crate population has declined since the 1970’s and as predicted the average levels of heterozygosity in this population will continue to decline in the future in the absence of any male immigration.

A similar study of a lion species named Panthera Leo, abundant in Africa suffered population bottle neck due to loss of habitat. (Bjorklund, 2003) And this resulted in reduced genetic diversity which is correlated with reduction in survival. This results in a higher population decline and can increase the rate of extinction.

Therefore, as demonstrated by Packer et al, and Bjorklund et al, reduction in population size of a species promotes inbreeding which causes a decline in genetic diversity, and ultimately has detrimental effects on the species.

Inbreeding depression has also been observed in the South China tiger, by study done by (Y.C. et al, 2007). The South China Tiger (Panthera Tigris Amoyensis) is a subspecies of tiger native to forests of southern China. Of the currently present 5 tiger species, the South China Tiger is the rarest and the most critically endangered due to factors including hunting, poaching and habitat loss. However one of the most serious threats to tiger’s survival is the use of tiger products in traditional Chinese medicines. The tiger bone is considered to be one of the most valued as a treatment to various conditions. (Tilson, 1997) Due to these factors the individuals declined from a reputed number of 4000 species in 1950’s and continued to decline until the mid 1990’s when the number of free ranging species was estimated to be fewer than 20 individuals(Gilson et al.2004). In the 1950’s however four females and two males were captured from the wild and were used to found a captive population. (Y.C.Xu, 2007). Because of the vast decrease in population number, the 6 founder population were enforced to inbreed in the 1970’s and continues to inbreed because of comeback such as their mating behaviour. Female tigers can only mate when they reach maturity at 3 years of age and only in temperate climates seasonally. Because of this only a small number of tigers are able to breed in each generation which lead to inbreeding depression in 1972. As observed by (Y.C.XU et al, 2007) inbreeding depression in the population shows low level of juvenile survivorship due to the loss of genetic variation and because of this infants were not able to adjust to the changing environmental conditions. Also because of low fertility and fecundity, reproductive difficulties were observed amongst the species. As a result the small number of founders and inbreeding depression contribute to a major challenge of conserving the South China Tiger.

Inbreeding depression has also occurred in the wild population of the California Condor that ranged from British Columbia to Baja California in the 19th century (Vickey, 2000). During that time period, the species faced dramatic population decline caused by high mortality rates due to exploitation, habitat destruction and the most prominent factor lead poisoning. It wasn’t until the 1987, that all the remaining population of 27 individuals of which 14 were males and 13 are females were captured and brought into captivity. (Phillip, 1995) Due to the small population size, there was extensive inbreeding which led to the loss of genetic variation and changes in allele frequency. The population also harvested a vast amount of deleterious recessive alleles, which are usually depleted of the population by natural selection. Before the decline in the Condor population there is much genetic variability, and therefore recessive deleterious alleles are hidden from natural selection by dominant alleles. However when the population declined, and inbreeding was enforced, close relatives that mate one another likely carry the same recessive deleterious alleles. Accordingly when relatives mate the offspring may inherit two copies of the same recessive deleterious allele expressing it. According to the study conducted by (Ralls et al. 1999), captive flocks had produced five deformed embryos that exhibited Chondrodystrophy, and died near the time of hatching. Chondrodystrophy, a recessive deleterious allele in chickens, is a condition where the abnormal development of cartilage affects bone growth before birth, and eventually leads to dwarfism. Chondrodystrophy in birds can be caused by a variety of factors such as mycoplasma infections and dietary deficiencies, particularly of manganese. (Ralls et al. 1999). Therefore inbreeding depression exposes Chondrodystrophy in offspring's, which results in a high mortality rate and ultimately reduction in population and stability of species extinction.

Other species that are affected by recessive deleterious alleles caused by inbreeding are Golden Lion Tamarin’s (Leontopithecus rosalia rosalia). According to study done by (Bush et al. 1996), Golden Lion Tamarin’s are native to the forests of Brazil and are widely endangered with a captive population originating in 1982 due to extensive deforestation. The current captive population of 494 individuals underwent inbreeding, which resulted Diaphragmatic defects, a disease caused by recessive deleterious alleles. A diaphragmatic defect is a rare syndrome characterized mainly by a defect in the diaphragm that allows some of the abdominal organs to move into the chest cavity, and involved missing or formation of abnormal bones in the arms or legs. (M., 1980) Therefore inbreeding depression was observable in the Lion population and posed major consequences.

As seen in the California Condor and Golden Lion Tamarin, low genetic diversity caused by reduction in population will result in inbreeding depression. This will unmask of recessive deleterious alleles in the offspring's of the parental generation, and will result in the depletion of these individuals due to natural selection.

Today, more species are alive on the planet than ever before. Estimates place the number of species between 10 to 100 million, with only 1.5 to 1.8 million identified with Latin names (Lacy, 1997). Of this number studies indicate that 2 to 3 species are going extinct yearly due to a wide number of reasons such as exploitation, habitat destruction, pollution, global warming, introduction of exotic species, and the spread of diseases. Why should we initially care about species going extinct? Major concern isn’t about extinction of a single species, but rather the ecosystem it belongs to, and the interaction with other species. Some species play a vital role in there ecosystem, that if lost due to extinction, it could affect a numerous number of species. Zoos across the world now hold a major role in conserving these species. Species that are headed towards extinction are captured from the wild and placed in zoos in which they are provided an artificial habitat much like their own. With the rather small population of species, professionals employ captive breeding amongst them. It is the process of capturing animals from the wild and mating them in captivity in order to increase the number of species under protected conditions. (Waugh, 1988) Despite many species that have been reintroduced into the wild as a result of captive breeding, there is a downfall. Firstly, captive breeding programs focuses only on a few charismatic endangered species, and neglects the numerous number of other species that are in the process of extinction. It is also very costly to train individuals to capture species from the wild, and provide them with their mimicry habitat and other resources they require. A major consequence faced by conservation however, is inbreeding depression seen in species that mate relatives, or siblings. Major consequences of Inbreeding depression are seen in the Lion population, South China Tiger, and California Condor. Because of their relatively small population due to factors such as habitat destruction, these species were captured from the wild, and had gone through inbreeding. A major consequence of inbreeding is the reduction in fitness and the loss of genetic diversity. As the closely related species kept on inbreeding within themselves, it was only those alleles that were being expressed. With a loss of genetic diversity, offspring's of those individuals are not able to withstand the changing environment. Other consequences of inbreeding seen in California Condor, is the expressivity of a recessive deleterious allele. The offspring's exhibited diseases caused by these recessive alleles.

Hence, when a population undergoes a bottleneck affect, most zoos try a technique commonly knows as inbreeding to try and preserve a species. Inbreeding also has its consequences as it is apparent in the three species discussed above. The most direct effect of inbreeding is inbreeding depression as it causes a reduction in population number.

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