Dairy Sheep Symposium - the Department of Animal Sciences ...

Different variables may affect the work effort required and the probability to produce a clone.
Although sheep was the first mammal cloned from an adult cell, no additional sheep have been
reported as a result of nuclear transfer using adult cell nuclei. The reason for this is unclear but
perhaps is due simply to the focus of most sheep work being on the production of transgenic
animals and using fetal cell lines rather than adult cell lines (Westhusin et al., 2001). The
efficiency of cloning sheep is similar to other species in terms of cloned embryo production and
live offspring produced per embryo transferred (Colman, 2000; Wilmut et al., 1997) and cloning
best animals to improve the efficiency of production in sheep will undoubtedly be explored in the
near future.
Utilising the Reproductive Potential of Females
Females in livestock species have a reproductive potential greatly in excess of that exploited
by natural mating. This is due to two particular features: (1) the presence of a relatively large
number of primordial follicles in ovaries and (2) ovarian follicular growth is initiated soon after
birth which allows the recovery of viable gametes (oocytes) from females before the puberty.
The latter has become the singular outstanding advantage of females compared with males in
genetic improvement in species with a long generation interval. Techniques as oocyte pick-up by
laparotomy with subsequent in vitro fertilization could be used to increase the production of
ewes with high genetic potential and to decrease the generation interval (Kuhholzer et al., 1997;
O’Brien et al., 1997; Stangl et al., 1999). The ability to produce embryos in vitro provides the
opportunity to apply other technologies such as embryo splitting, embryo sexing, preimplantation
genetic diagnosis etc. Furthermore, with the emergence of genetic markers that will
elucidate the molecular basis of economic traits in livestock, it will be possible, in a near future,
a genetic selection at an embryo stage and before the investment in pregnancy.
Molecular Tools in Sheep Breeding
Since domestication began, sheep farmers have been manipulating livestock genes through
selective breeding. This artificial selection of animals exhibiting desired properties has produced
an unwittingly sorting of alleles underlying phenotypes of interest. It is known that most production
traits undergoing selection are quantitative traits, that is, they exhibit a continuous distribution
and are influenced both by environmental and genetic factors. The genotypic component
reflects the joint contribution of multiple “polygenes”. Using the quantitative genetics theory,
breeders have modeled the animal phenotype as the sum of genetic and environmental components
allowing for an increase in most of animal products. This progress in animal performance
acts on polygenes without any knowledge about their identity or mode of action.
The advent of DNA molecular technologies specifically PCR and the appearance of
microsatellites as an abundant source of highly polymorphic markers has boosted the generation
of linkage maps in sheep and other livestock species (Beattie, 1994; Crawford et al., 1995;de
Gortari et al., 1998; Maddox et al., 2001). The availability of these genetic maps jointly with
other genomic resources (segregating populations, large insert libraries, radiation hybrid maps,
comparative maps, preliminary transcript maps, etc.) has opened the possibility to map by linkage
analysis and identify by positional cloning economic trait loci in livestock.
Milk production, as the majority of economically relevant production traits shows a
continuous distribution and as mentioned above, is controlled by an unknown number of genes
and influenced by environmental factors. A Quantitative Trait Locus (QTL) is defined as a region
in the genome that harbors one or more genes affecting a quantitative trait.