Genetic uniformity In Cheetah

Importance of Genetic diversity represented by polymorphism (P), the proportion of loci known to vary in the population, and heterozygosity (H), the proportion of loci which vary in the average individual.

• Positive relationship between H and fitness, as longetivity, growth rates, fecundity, metabolic efficiency and developmental stability (MItton & Grant, 1984).
• Genetic diversity is thus viewed as contributing to the fitness of individuals as well as the evolutionary potential of a species.
• Lerner (1954) suggested hetrozygote superiority as a buffering capacity to overcome environmental challenges and develop toward a phenotypic optimum for the species, termed developmental homeostasis.
• This is reflected in the symmetry of bilaterally paired traits.
• However the level of P and H corresponding to homeostasis varies between species (Kat, 1982).
• Average levels of P and H vary dramatically between species (Baccus et al., 1983).

Cheetahs Low genetic variability (LGV)

Cheetahs exhibit high uniformity at the major histo-compatibility complex (MHC), normally the most polymorphic cluster of genes in the mammalian genome (O'Brien, 1990).

Perception of LGV in the cheetah as extreme is influenced by comparisons of cheetah with other animal groups, but these vary widely, and appear to conform to taxonomic grouping to some degree (Nevo, 1978). It is more appropriate to compare within carnivora. Large mammals tend to have LGV than other mammals.

• Terrestrial carnivores significantly LGV than other mammals. 8 of carnivores examined show no polymorphism.
• Cheetah Polymorphism = 0.02 - 0.04, heterozygosity = 0.0004 - 0.0014
• LGV is equivalent to low heterozygosity
• generally believed to result from population bottleneck followed by high level of inbreeding (O'Brien et al., 1983).
• Sighted as posing a critical threat the future of the species. Led to emphasis upon captive breeding programmes.
• Evidence: present level of genetic variability and symptoms of inbreeding depression in captive populations.
• Fluctuating asymmetry and variation in other carnivores challenges the usual assumption.
• Several carnivores exhibit lower levels than cheetah.
• Phenotypic effects attributed to inbreeding depression.
• Infertility, reduced litter size, susceptibility to disease are limited to captive individuals.
• Comparison of presumptive fitness based on H are valid only within species.
• Within the Felidae there is a wide range of H and P.
• In the Leopard Panthera parldus (H = 0.029), H is less than 1/2 of the Ocelot Leopardus parldalus (H = 0.072). Yet this is not considered as evidence for genetic impoverishment.
• DNA fingerprinting and mitochondrial DNA sequencing calculations supports the hypothesis of an ancient bottleneck about 10,000 years ago O'Brien et al., (1987), proposed a recent bottleneck due to overhunting at the turn of the century to be responsible for the relatively lower GV of the South African Cheetah A.J. jubatus than the East African Cheetah A.J. rayneyi .
• There is no evidence for high GV prior to the hypothetical bottleneck.
• The case of the Eastern Barred Bandicoot demonstrates the need for caution with assumptions.
• An extreme bottleneck reduced to just 2 individuals can result in 75% conservation of original GV (Frankel & Soule, 1981).
• It is possible for variation to increase following such and event (Carson, 1990).
• A formerly polymorphic population is unlikely to be reduced to one of near-monomorphism following a bottleneck (Pimm et al., 1989).
• Gradual reduction in invariability eliminates deleterious recessives by selection; though homozygous, without inbreeding effects (Lande, 1988).
• Persistence of LGV species assisted by metapopulation dynamic (Gilpin, 1991).
• GV as soluble protein not necessarily equivalent to LGV in inheritance characters, nor necessarily idicative of inbreeding.
• Fluctuating asymmetry (FA), i.e. deviations from perfect symmetry in bilaterally paired traits (Van Valen, 1962), and morphological variants.
• FA shown to reliably indicate environmental and genetic stresses across various taxa (Parsons, 1990). LGV is reflected in a decrease in homeostasis, which is expressed as increase in FA and phenotypic variants. Also, both increase with inbreeding.
• 16 characters of skull and dentition were compared with large cats (Wayne et al., 1986). Correct analysis showed no difference. Repeated studies.
• Suggests cheetah have not suffered depletion of GV, but exhibit developmental stability at present level of variability.
• Also contradicts notion that cheetahs are seriously inbred.

Individual Differences As Artefact of Captivity

• Reproductive success varies dramatically between captive facilities. Striking difference in management skills. 9-12% of mature females produced live cubs in North American zoos, compared to 60-80% at a South African research centre (Brand, 1980).
• North American record improved with changes in husbandry.
• In S.Africa cheetahs fed whole animal carcasses, in N.America cat food.
• Cat food is high in phyto-oestrogens and has been linked with liver disease, the leading cause of death for adult cheetahs in N.A. zoos (Munson, 1993). Several products contain toxic levels of vitamin A. Change in diet led to resumption of normal oestrous cycle and a marked reduction in liver pathologies (Setchell et al., 1987).

Behavioural Aspects of Fertility

• In the wild males and females associate only briefly, at the time when the female comes into oestrous in a period of courtship.
• Keeping males and females together leads to suppression of oestrous in the female. The cheetah is an induced ovulator, ovulating only under the correct conditions.
• Confines of captivity prevent performance of ritual courtship chases (Eton, 1973), contributing to reluctance to breed.
• Absence of male courtship groups and lack of male to male competition may contribute to low levels of testosterone (Wildt et al., 1993).
• Small litter sizes (mean 1.5) compare to 3,4 and even 6 young in the wild.
• Reproductive capacity is equal or greater than other large felids. High fecundity, rapid rates of litter production in wild cheetahs (Caro & Lorensen, 1994). Fewer than 0.5% of 48 cub deaths could be attributed to genetic defect.
• High level of sperm abnormalities (70.9%, O'Brien et al.) doesn't appear to impair fertility in the cheetah, though such levels are associated with extremely inbred livestock and mice. 83.3% of male cheetahs produce pregnancies; 89.5% of these achieved during a single oestrous (Lindberg et al., 1993).
• 5 species of great cat show high sperm abnormality levels (Rasch, 1990).
• Increased susceptibility to disease confounded by information from captive populations.
• The coroner virus, feline infectious peritonitis in an Oregon wild animal park; mortality rate 50-60% of captive cheetah compared to 1-10% in domestic cats.
• These captive cheetah were 24 individuals in 3.5 acre area.
• In the wild bands of males roam 12-36 square km; solitary females range 60-800 square km (Schaller, 1972). Wild cheetah scrupulously avoid contact with con specifics.
• Cheetah in their behaviour would not need to develop an immune system for the context of high population density and con specific contact.
• Cheetah are slow to reject skin grafts from con specifics, indicating high compatibility at the MHC complex.
• Variability at the MHC is a critical defence against pathogens.

• Disease transmission appears to be exacerbated under captive conditions.

  • No evidence of elevated susceptibility to disease in wild populations.
  • In the presence of the fatal virus, feline infectious enteritis, cheetah in the wild did not suffer devastating mortality seen in the wild animal park
  • Half the cheetah did survive the virus; they are not incapable of immune defence.
  • Wild cheetah tested sero-positive to a variety of pathogens and parasite.

• Cheetahs great similarity at the MHC may be a liability as the species is increasingly relegated to populations of artificially high density in game reserves and captive breeding programmes.

Conclusion

• Decline by 50% in the wild between 1960-75 (Myers, 1975) is likely due to loss of native habitat and to its genetic composition. Restricted to high density populations 1 per 6 km square, instead of the usual 1 per 100 km square.
• Increased disease transmission.
• Increased predation by lions, leopards and hyena is primary cause of death (Eton, 1974).
• 73% of cheetah cub deaths observed, due to predation.
• Range lands for native ungulates depleted by growth of cattle ranching.
• Cheetahs hunted by farmers, and for international markets.
• No factor has genetic basis for interaction.
• Genetic invariability of the cheetah raises questions: traditionally considered perilous. Reproductive viability suggests homozygosity may not be universally deleterious.
• Population fluctuation asymmetry indicates no genetic stress.
• Other terrestrial carnivores also have genetic uniformity, exists in viable populations.
• Reproduction and survival of wild cheetah is unimpaired, and suggests problems attributed to inbreeding in captive cheetah are behavioural and physiological consequences of the captive environment.
• Cheetah survived LGV for 1,000's of years only last century, a marked decline.
• Short term effect of LGV on survival unwarranted.
• Genetic concerns distract from the real issue of natural habitat, and relegate the species to parks and institutions.
• Where emphasis is on captive propagation experience shows genetic composition and behaviour of cheetah may be detrimental under artificial conditions.
• Long term effects of cheetahs genetic uniformity on its evolutionary future as a natural population most likely will remain unanswered.
• Unless priorities shift to the protection of habitat, it is certainly human impact, not homozygosity that will lead the cheetah to extinction.