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Health

Cats

Gene ExpressionBy Razib KhanFebruary 10, 2007 10:54 AM
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The "domestic" cat, Felis silvestris catus, has been with us for nearly 10,000 years. Recently, a 9,500 year old burial of a human and their companion cat was discovered on Cyprus. Cats are not indigenous to the island, so it seems that the presence of this cat must be owed to human intervention in some manner. Though we are used to thinking about how humans shaped cats through selective breeding the recent

data on Toxoplasma gondii

suggests that cats might have an impact on human behavior that could explain cultural differences! Some intellectuals have posited that the selection of companion animals (i.e., cat cultures vs. dog cultures) is reflective of their values, but the irony might be that the animals (cats) might shape those values. A friend once quipped that we did not domesticate the cat, the cat domesticated us. Though functional explanations that cats served to rid agricultural communities of vermin are plausible (the extermination of cats in Medieval Europe might have encouraged plague becaues of the reproduction of black rats), it may also be that the domestic cat's "niche" is the human propensity toward small friendly furry creatures. In any case, it seems that the dominant ancestral contribution to the domestic cats the world over is from the North African Wildcat, Felis silvestris libyca, though contributions and admixture with other subpecies of Felis silvestris, such as the European Wildcat, seem likely (certain European breeds exhibit tabby markings remiscient of local wild populations, and hybridization is common through the range of the two species where they overlap so admixture and introgression has occurred).

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There are differences between the domestic cat and its wild cousins and ancestors. Like most domesticated subspecies its brain is smaller, and it is also relatively social and naturally tame. We are used to considering cats solitary creatures, and in the wild the various silvestris subspecies tend to be loners unless mating or raising kittens, but the existence of feral cat colonies attests to the fact that there are important behavorial differences between domesticated cats their wild cousins. Unlike wildcats domesticated cats can live together (as attested by tens of millions of cat owners), even if they are not pack animals such as dogs or herd animals like many herbivores. Some evidence exists that hybridization with European Wildcats results in offspring who tend to be less naturally tame or sociable than the domesticated cat, implying some level of recessiveness to these characteristics (perhaps because of loss of function mutations which can be complemented by one "wild type" allele at various loci in the heterozygote hybrid ). Another interesting hypothesis regarding the nature of domestic cat evolution is that they have been bred for neotenous characteristics. The same evaluation has been made of dogs, and yes, even humans. Not only do many cats exhibit the morphological characteristics of kittens, some of their behavorial biases might be due to a retention of the necessity of toleration of others in the litter.

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Of course cats come in many varieties, and there are official "breeds." I'm to understand that though there are some ancient physical types (e.g., the Persians are an old breed), the vast majority of certified breeds emerged within the last 100 years, though some were conscious attempts to resurrect the physique of older lines and there is often ambiguity in regards to the question whether these breeds are partially descended from their model antecdents. Because of the interest in breeding (for cat shows, etc.) we know a few rules of thumb in regards to cat genetics. First, the "longhair" type seems to be recessive to the "shorthair." That is, shorthair cats may produce longhair offspring in the litter, but longhair cats bred together should only produce longhairs because their genotype is homozygous. Wildcats are invariably shorthaired, and it seems that the reason here is clear, as longhairs tend to require a great deal more daily grooming, and owners of these breeds are often advised to aid in cat hygiene through regular brushing. The fitness implications of being longhaired seem pretty unambiguous, though there is some speculation that a few breeds, such as the Norwegian Forest Cat, might have developed longhair because of its superior insulation properties. Another interest of cat breeders is color and pattern of coat, and here there is one locus which exhibits a dominance-recessive inheritance pattern, the Agouti gene. In short, the "tabby" pattern which is common in many cats and is also the norm in the wild is dominant to "solid" color patterns. Agouti is a peptide which is an antagonist to the MC1R locus (which has a central role in the regulation of various pigment production genes), and the banding patterns exhibit a similar genetic architecture throughout mammalian taxa. The camouflage value of a striping pattern is well known, so it is reasonable that in the wild this would be the norm. There are also many color genes which have been localized for cats. The polygenic nature of this characteristic shouldn't be particularly surprising, humans are less varied in skin pigmentation (i.e., we modulate one pigment, the two forms of melanin, to generate a black to white range in a scalar manner) and that trait is polygenic. One can see a truncated chart of the genetic architecture for cat coloration here (the reality is more complex, but you can conceptualize the nature of polygenic multiallelic inheritance). An interesting point to note is that white blued-eyed cats are more likely to be deaf, and this tendency toward greater likelihood for deafness among depigmented individuals is found across mammalian taxa, evidence of deep seated pleiotropy in the genetic pathways. Note: I have used the terms dominant and recessive promiscuously in this post. I do so with some trepidation, but the terminology is so ingrained in cat breeding that I was hesitant to introduce what I feel would be a more precise lexicon.

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