78 Utilisation and conservation of farm animal genetic resources
Miguel Toro and Asko Mäki-Tanila
For example, the European horse breeders have tried over several centuries to domesticate zebras ater settling in South Africa in the 17
th
century. Finally, they abandoned the idea, because zebras have an incurably vicious habit of biting the handler and not letting
go they injure more zoo-keepers that tigers, and they have a better peripheral vision than horses making the use of lassoes impossible.
1.1.2. Are there genes for domestication?
Darwin 1868 was the irst to note that most domesticated species have undergone similar changes during domestication: appearance of dwarf and giant varieties, piebald
coat colour, depigmentation, curly hair, rolled tail, loppy ears and early sexual maturity. Many, but not all, characteristics associated with domestication seem to be linked to
pedomorphosis: the retention of juvenile characteristics in the adult body. his suggests that domestication may be the result of changes in a relatively small number of regulatory
genes afecting development.
here has been searches for genes speciically responsible for these traits, e.g. for genes underlying diferences in coat colour pattern in pigs and horses, plumage in chickens
or muscle mass in pigs Andersson and Georges, 2004. In the 1950s, Belyaev see Trut, 1999 started a selection trial for tameness in the Silver Fox. He hypothesised
that morphological, physiological and behavioural traits were simultaneously modiied by domestication and that selection for an important behavioural trait would modify
the others. Belyaev measured tameness by the ability of young sexually mature foxes to behave in a friendly manner towards their handlers, by wagging their tails and whining.
Ater more than forty years of selection, 70-80 of the test population accepted human contact and oten licked the persons looking ater them, like dogs do. As predicted
by Belyaev, additional changes appear such as piebald coat colour, drooping ears and shorter tails and snouts. Physiological changes also occurred: in domesticated animals
the corticosteroid levels rise signiicantly later and they have much lower adrenal responses to stress and more serotonin in their blood.
1.1.3. Multiple versus single domestication
It has been a long debate, whether domestic animals are the result of a single domestication event in a restricted geographical area or of multiple, independent
domestication events in diferent geographical regions. he ample distribution of domesticated Eurasian mammals from Portugal to China supports the idea of diferent
independent domestications. his has been conirmed by molecular genetic data see reviews: Bruford
et al., 2003, Mignon-Grasteau et al., 2005, Dobney and Larson, 2006, Zeder
et al., 2006, but the answer depends on species. In some cases pigs and horses
Utilisation and conservation of farm animal genetic resources 79
Chapter 4. Genomics reveals domestication history and facilitates breed development
the hypothesis of more than two domestication events is well supported by the scientiic data, whereas in the case of cattle Lotus
et al., 1994 and donkeys the evidence points to two Table 4.1.
Tracing mitochondrial DNA mtDNA, Box 4.1 is the standard method for tracking the history of domestication. It is well known that cattle can be classiied into two
groups: zebu Bos indicus from East Eurasia and East Africa, and taurine Bos taurus
from Europe and Mid, North and West Africa. Looking at mtDNA analyses, the two breed groups are very distant suggesting two separate domestications from diferentiated
subspecies of the auroch Bos primigenius cattle. he African zebu cattle only have
taurine mtDNA markers but nuclear and Y chromosomes markers are similar to those of the Asian zebu. In this case mtDNA studies were not able to detect zebu genes that
were passed by zebu bulls to a few taurine female founders Bruford et al., 2003, Zeder
et al., 2006. he mtDNA studies have to be supported by analysis of nuclear DNA data to get clear results.
In sheep and goats there is one major geographical mtDNA lineage that probably represents initial domestication in the Fertile Crescent, with two more restricted
lineages representing later independent domestication events. In goats, the irst line
Box 4.1. Molecular markers for studying domestication. Mitochondrial DNA polymorphism
Variation in mtDNA is extremely useful in studying genetic diversity. here are a number of reasons for this:
mtDNA is maternally inherited with no recombination and, all markers in this genome are efectively linked as a single haplotype. Hence the number of nucleotide diferences between
mitochondrial genomes is a direct relection of the genetic distance that separates them; each cell has thousands of copies of mtDNA; and
regions of mtDNA mutate 5-10 times more than nuclear DNA making it ideal for studying the divergence between wild and domestic populations under the relative short timescale of
domestication 10,000 years.
he usual way to analyse mtDNA is the sequencing of the cytochrome b gene and of the control region that shows greater variation than the other parts of the mtDNA molecule.
Y chromosome In the same way as mtDNA could be used to identify maternal lineages in the populations, Y
chromosomes sequences provide similar information on paternal lineages. here, recombination is restricted to a small area of the chromosome.
• •
•
80 Utilisation and conservation of farm animal genetic resources
Miguel Toro and Asko Mäki-Tanila
expands from the Fertile Crescent all over the world. Another, limited to West Pakistan, is related to cashmere lines, whilst the last is of uncertain origin Bruford
et al., 2003, for sheep; see also Tapio
et al., 2006. In pigs the irst mtDNA studies suggest two domestication events Asia and Europe, but the most recent mtDNA study suggests up
to seven events across Eurasia Larson et al., 2005. Dog history appears to resemble that
of pigs rather than of cattle, with multiple events in several locations occurring through domestication of wolves. In contrast, the domestication of horses was neither limited in
time nor space. here is extensive matrilinear diversity seen in mtDNA variation that does not match with a patrilinear diversity of the male speciic Y chromosome. his is
consistent with a strong sex-bias in the domestication process with only a few stallions contributing genetically to the domestic horses Lindgren
et al., 2004. Donkey seems to be the only ungulate domesticated solely in Africa and probably as the result of two
domestication events Beja-Pereira et al., 2004.
he South American camelids present a complex situation. he modern-day alpaca and llama could descend either from the wild guanaco or from the wild vicuña. Both llama
and alpaca have mtDNA haplotypes from both these wild species whilst microsatellites indicate a close relationship between alpaca and vicuña and between llama and guanaco,
respectively. his implies diferences in how mtDNA and microsatellite variation are revealing the history, and in the future data on the Y chromosome will help to clarify
the picture Zeder
et al., 2006.
1.2. Breeds and modern genetic improvement
Developing farm animal populations into breeds or pure lines is a very recent event from a domestication history perspective. here were numerous local populations that
provided the basis for the breeds in the 19
th
century. he sequence of events started with a relatively high market value of a speciic group of animals and continued towards
the demand for a purebred pedigree. his demand represents an initial step towards scientiic breeding as a means of reducing risks in breeding either for the market
or as a hobby. Domestic trade and export of animals have stimulated the eforts to set the formal deinition of a breed, although this remains very diicult chapter 3,
Box 3.2. Livestock shows have had an efect on emphasising the type and ideas about the correct conformation and colour. herefore breeds have been selected for exterior
traits conirmation, colour, horns with reasons which may have been quite weak and occasionally with quite serious consequences for welfare e.g. in companion animals.
he selection has been accompanied with assortative mating and intentional inbreeding, which can also bring unforeseen and unwelcome consequences. However these risks
have been oten successfully avoided, for example the famous Shorthorn breed was founded by extensive use of only a very few bulls. here do exist breeds that do not
