Redesigning Animal Agriculture

112 D.N. Wells and G. Laible
Adams et al., 2002). For loss- of-function
transgenes, both alleles in progeny need
to be inactivated. This will require an
extra generation of breeding, with preferably
another non-related transgenic sire
harbouring the same genetic modification.
The resulting homozygous knock-out
individuals are then available for further
selection and progress towards stably fixing
the new genetic change into the herd or
wider population. Animal industries may
choose to regularly introduce the same
transgene, ideally at the same locus, on a
new genetic background using cell lines
derived from the most recently selected
progeny-tested sires, so as to capture the
annual incremental genetic gains from conventional
animal breeding and maintain
genetic diversity. The economic benefits of
a genetic modification affecting a production
trait must be sufficiently large to compensate
for the lag in genetic gain during
the time taken to introduce the transgene
and test its performance before wider dissemination.
It has been estimated in some
introgression schemes that the value of the
transgene must be greater than 10% of the
‘economic value’ of the trait in question,
to at least match what could have been
achieved through conventional selection
and breeding (Gama et al., 1992).
Perspective
Since the domestication of livestock species,
humans have been inadvertently redesigning
the genetic make-up of their animals by
selective breeding based on desirable phenotypic
characteristics. This practice alters the
frequency of many genes in an often unregulated
manner. The new technologies of cloning
from cultured cells and trans genesis with
site-specific integration have the potential
to allow a more targeted approach towards
animal breeding. A tremendous opportunity
exists with transgenesis to take production
and health traits beyond the normal biological
boundaries. Animals can be engineered
for specific purposes in ways that are not
possible with either conventional breeding
or genomic selection, which exploits the
natural variation amongst the existing
population. However, major advances are
still required to realize this vision. This
includes improved reprogramming of the
donor genome following NT, an increased
frequency of gene targeting in somatic cells
and an intimate understanding of the biological
systems involved. The identification
of genes and regulatory elements that
influence livestock production traits will
enable the effective utilization of cloning to
duplicate entire genotypes and for transgenesis
to introduce precise genetic enhancements
to progress animal breeding in the
21st century.
Despite the advances being made in
science, there is continued debate as to the
value of transgenic animals in agriculture. It
is important to note, however, that the majority
of the public do appear to be conditionally
supportive of genetic modification and
they want to see good practical examples
demonstrating the benefits of the technology
with minimal risk. Benefits improving
the health of farmed livestock and/or of the
consumer, along with solutions to environmental
impacts of agriculture will undoubtedly
be met with the greatest acceptance by
the public. But for any of these outcomes
to be realized the producer needs to benefit
also, at least financially. A difficulty is that
the technology is complex and each application
often has a unique set of pros and
cons. Research data are required to evaluate
each potential transgenic application
before industry, regulatory, and consumer
acceptance are sought. These results need to
be communicated effectively to the public,
producers and regulatory agencies by members
of the scientific community. It has been
more than 20 years since the first transgenic
farm animals were created and 10 years since
the birth of Dolly. And it will be some time
yet before specific genetic modifications in
livestock are possibly adopted on farms and
accepted in segments of the marketplace.
This might not happen in isolation and
alternative genetic technologies in plants
may emerge with the potential to complement
modified livestock and significantly
enhance some common agri- biotechnology