Lo tensional como expresión de potencia: las músicas comunitarias en Bogotá como Prácticas instituyentes

In contrast to mouse models where one century of standardised breeding experience already exists and numerous suitable strains have been established and characterised,

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systematic breeding of experimental herds for large animal models is only in the early stages of development. A large proportion of current knowledge about pig breeding derives from reproduction of conventionally utilised pig breeds in agriculture. The term breed refers to a group of animals of a defined species that has been selected by people for their heritable similarity in terms of appearance or productive and reproductive characteristics (Porter 1993).

In Europe, directed breeding of conventional pig lines for pork or lard production developed in England during the 18th century, when the increasing industrialisation led to urbanisation and an enhanced demand of food supplies that could not be derived from self-subsistence. Pigs from a number of different countries were imported to England and a crossbreed between Asian and Italian pigs with the existing landraces turned out the first modern pig breed, the Leicester pig. Small, Middle and Large (=Yorkshire) White followed during the first half of the 19th century. These breeds were spread throughout Europe and were used to improve the prevalent landraces (Nickels 1997). In Germany, a wide variety of new breeds developed until the mid- 20th century. The late 1950s brought a change in the demographics of pig breeds. Pigs that had been bred for lard production were successively eliminated and breeding began to centralise on meat production to satisfy the increasing demand during the economic miracle (v. Lengerken and Wicke in v. Lengerken et al. 2006). German Landrace became the predominant breed by far. Other breeds such as Hampshire, Duroc, Angler Saddle Pig or Swabian-Hall were at some point during the 1970s on the verge of extinction. However, over the course of the 1980s, a few of these breeds were rediscovered (Nickels 1997). Today, the most common commercial pig breeds in Germany are German Landrace, German Large White and Pietrain or, comprising the majority of animals in conventional pig production, hybrids of these breeds (Horst and Gregor in Kräußlich and Brem 1997). Some of the rediscovered breeds such as Swabian-Hall, Duroc or Hampshire pigs constitute niche populations in pork production on the one hand, and an important component in the conservation of old farm animal breeds on the other hand.

In contrast to pig livestock, miniature pigs have been bred specifically for experimental use and, later and to a lesser extent, as pets. Miniature pig is the generic

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term for growth-restricted pig breeds or lines with varying heights. Generally, pigs with an adult weight of less than 100 kg are called miniature pigs, even though the majority are much smaller, most weighing around 15-45 kg when they reach sexual maturity (Fisher 1993). The goal in miniature pig breeding was to create a conveniently sized animal for research that would be able to compete with other experimental animal species and make use of the anatomical and physiological similarities between pigs and humans but without the disadvantages of commercial pig breeds. Miniature pigs require less housing space, are easier to handle and are more cost-effective in terms of feed and experiments due to lower material requirements, compared to larger pigs (Swindle 2007). In 1949 the Hormel Institute of the University of Minnesota initiated a project to develop a breed of miniature swine specifically for use in biomedical research (Bustad 1966). One of the first miniature pigs, the Minnesota miniature pig, arose from this effort. Today, there are over 50 other breeds of miniature pigs worldwide, but only a few of them are regularly used for research. The most commonly utilised breeds include the Göttingen, Sinclair, Yucatan, and Hanford miniature pigs (Swindle in Conn 2008).

The Hanford miniature pig, for example, has served as a model for a number of neonatal and paediatric diseases (Glauser 1966; Cohen et al. 1990; 1991). Yucatan and Sinclair pigs have been extensively used in diabetes research (Phillips et al 1982; Dixon et al. 1999; 2002) and as models of cardiovascular disease (Wissler and Vesselinovitch 1968; Gal et al. 1990). Sachs et al. (1976) reported on the establishment of a partially inbred herd of miniature swine homozygous for a specific swine leukocyte antigen allele. This model has been put to use in various transplantation settings, such as skin grafting (Leight et al. 1978), bone marrow transmissions (Pennington et al. 1988) and liver transplantations (Flye et al. 1999). By sequential brother-sister matings, the co-ancestry, also known as inbreeding, of this herd has reached levels greater than 90% (Mezrich et al. 2003; Cho et al. 2007). This so-called inbreeding is defined by the probability that the two alleles at an autosomal locus of an individual are identical by descent, meaning that the parents of an individual must have one or more common ancestor for that individual to be termed inbred. Inbreeding is measured by the inbreeding coefficient (Wright 1922).

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Dickerson et al. (1954) showed that in pigs inbreeding in the mother leads to a reduction in litter size of approximately 0.2 piglets per 10% rise in the inbreeding coefficient. As Dettmers and Rempel reported in 1968, this correlation became apparent during the recurrent mass selection for small size in the establishment of the Minnesota miniature pig breed. Although litter size underlies great variation depending on the exact breed but also on environmental factors, large, commercial pig breeds generally farrow more than 10 live piglets per litter (v. Lengerken and Wicke in v. Lengerken et al. 2006). Contrary to this, miniature pigs produce average litters of no more than 5-6 piglets (Swindle 2007). Due to the limited population size of miniature pig breeds, inbreeding becomes inevitable if specific characteristics of the breed are to be conserved. Other reproductive parameters such as the number of stillborn piglets per litter, the piglets raised in a litter or the average weight gain until weaning have been equally shown to negatively correlate with an increase in the inbreeding coefficient (Bradford et al. 1958). These unwanted effects, in addition to others, are summarised under the term inbreeding depression.