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Adaptive mutations recorded in wild carnivores
Genes controlling spotting and striping patterns in Carnivora are unique in that they appear to have evolved separately from the basic mammalian genotype for coat colour. The variation of alleles at a putative Tabby locus, acting on an agouti or yellow background to give blotched, striped or lined patterns would have an evolutionary advantage for carnivores. Such markings within wild and domestic cats have an evolutionary relevance (Ragni & Possenti 1996). Eizirik et al. (2003) reported that populations of jaguarundi (Herpailurus yaguarondi) were exhibiting high frequencies of a mutated gene variant of the MC1-R gene. The mutated gene is responsible for a gradation in darkening of the pelage. The possibilities of adaptive advantages of such melanistic mutants under certain ecological circumstances in the dense South American jungles are raised by the authors. In case of the Tabby pattern, non-Tabby felids might actually represent a phenotypic variation and a rarity in the wild state (Kitchener 1991). An earlier study on Iberian lynx (Lynx pardinus) reported evidence that Tabby pelages in this species might have been actively selected for and become more frequent in feral populations, possibly as an adaptation towards crypsis for hunting. The authors describe three specific pelage types that vary in size, pattern and distribution of spots in the coat (Beltran & Delibes 1993). Kitchener (1991) notes that in the serval (Felis serval), there is a servaline phenotype that is recognised as the result of a mutation at the Tabby locus (Kitchener 1991). Melanism is the most common morph among wild mammals and black melanistic variants of the common wild tabby phenotype have been reported in at least 20 other felid species (Robinson 1978). Eizirik et al. (2003) state that felid melanistic variation has reached high population frequencies in few among the 37 felid species investigated. It has however failed to reach complete fixation in the populations investigated. This might be representative of how adaptive evolution occurs within the felid species.
The King phenotype and evaluation of evidence for it being an adaptive mutation
The change in pattern between the cheetah and King may be due to a simple shift in a pattern formation parameter, caused by the genetic change that underlies the phenotype. It is debatable whether the area of occurrence of the pelage variant ‘King’ phenotype is an adaptation for the particular forest or woodland biome. Most historical sightings of the King phenotype have been in the woodland biome area around eastern and south-eastern Zimbabwe, the north-eastern region of former Transvaal and few in eastern Botswana (Rogers 1997). A parallel may be drawn with the finding of high population level frequencies of the mutated jaguarundi (Herpailurus yaguarondi) gene variants of the MC1-R gene (Eizirik et al. 2003). The juvenile cheetah cub is different from other felid juveniles in having on its back, a ‘mantle’ of long bluish-grey or smoky coloured hair, 70–80 mm in length. The pattern is said to mimic the pelage of the honey badger (Mellivora capensis), a particularly aggressive mammal with coloration that is among the best known examples of warning coloration (Skinner & Chimimba 2005). This contrasts with the assumption that the juvenile coat pattern represents the ‘primitive’ condition of the species (Werdelin & Olsson 1997). The adult felid coat colour pattern, on the other hand, is influenced by adaptation to hunting behaviour and other characteristics of the species (Ortolani 1999). It is unclear if the blotched pattern of the King phenotype confers any particular advantage in the reported habitats. To get a clearer picture on whether the King phenotype is a type of adaptive mutation, the biochemistry of the mutation will have to be compared to other reported types of adaptive mutations, as seen in the arctic fox and the jaguarondi. Whether the persistence of such a mutation within a wooded biome in the areas described confers any evolutionary advantage is a possibility that is inherently difficult to rule out. Due to the extreme rarity of the King pattern, it might be inferred that the advantage, if any, is minor. Indeed it can be conversely argued that the King’s rarity makes it likely that it is detrimental for evolutionary success.
Genetics of the Tabby locus in Felidae
The difficulty in studying the Tabby type of allelomorphism that is common in felids is mainly due to its absence in rodents and other commonly researched laboratory animals. However, the study of melanism in humans and laboratory animals indicate that the different phenotypes of melanocytes are mostly due to differential transcription of melanocyte-specific genes and melanocyte-specific transcription factors. The cause for a pigmentation phenotype is therefore likely due to the mutation in the genes encoding specific transcription factors. The Tabby series of alleles is not common in rodents, but in cats, it leads to the formation of darker transverse stripes or blotches on the coat.
The standard of comparison is the phenotype seen in the natural, wild state of the species and is the short coated, mackerel striped Tabby as found in the European wild cat (Felis silvestris). In the wild cat of Europe, the standard pattern is similar to the striped Tabby, except that the tail is ringed a deeper black distally. The mackerel Tabby has characteristic vertical, gently curving stripes on the sided of the body (Figure 2.4). This may be continuous or broken into bars and spots, especially on the flanks or stomach. The pattern differences in the African bush cat (Felis lybica) from the European wild cat include less conspicuous striping and a slender and tapering tail.
Chapter 1: Introduction
1.1 The captive African cheetah
1.2 Hypothesis
1.3 Objectives
Chapter 2: Literature review
2.1 Distribution of wild cheetahs in South Africa and Namibia
2.2 The natural history of cheetahs
2.3 Territorial and dispersal behaviour in wild cheetahs and implications thereof
2.4 Phylogeny of the African cheetah
2.5 Loss of genetic diversity and inbreeding in populations
2.6 Molecular genetics of the King cheetah (Acinonyx jubatus) phenotype
Chapter 3: Materials and Methods
3.1 Research animals
3.2 Sample collection and storage
3.3 Quantifying diversity and inbreeding: methods
3.4 Experimental design and analytical/statistical procedures: population genetics and heritability estimates
3.5 Experimental design and analytical /statistical procedures: genetic linkage analysis
3.6 Data analysis
Chapter 4: Spatial Bayesian clustering clarifies admixture and founder origins in a captive cheetah (Acinonyx jubatus) population
Abstract
4.1 Introduction
4.2 Methods
4.3 Results
4.4 Discussion
Chapter 5: Heritability estimates and genetic correlations to inbreeding in a captive cheetah (Acinonyx jubatus) population
Abstract
5.1 Introduction
5.2 Materials and Methods
5.3 Results
5.4 Discussion
Chapter 6: Linkage analysis of the King phenotype in the African cheetah (Acinonyx jubatus) to Tabby-linked markers
Abstract
6.1 Introduction
6.2 Materials and Methods
6.3 Results
6.4 Discussion
Chapter 7: Conclusions and Discussion
7.1 Hypothesis 1: The captive cheetah population has retained genetic variation and population differentiation compared to wild conspecifics
7.2 Hypothesis 2: The captive cheetah population has Namibian ancestry and levels of ancestry can be quantified
7.3 Hypothesis 3: Unknown ancestry of a cheetah can be described using trained spatial Bayesian clustering
7.4 Hypothesis 4: There is a correlation between inbreeding and development of pathology and susceptibility to infection
7.5 Hypothesis 5: Heritability of complex conditions like gastritis in the cheetah is low
7.6 Hypothesis 6: Selection can be improved by dam selection for litter size in cheetahs
7.7 Hypothesis 7: In cheetahs, there is a detectable genetic component that contributes to maladaptation or stress in captivity has a genetic component
7.8 Hypothesis 8: Genetic linkage analysis – the described linkage between Tabby locus in domestic cats and microsatellite markers that flank chromosome B1 is replicable for detecting linkage between King locus and markers in Acinonyx jubatus
7.9 Discussion
7.10 Recommendations for future genetic management
Chapter 8: References
