Agriculture Reference
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into the genome of an animal such that the
rDNA modification is stably transmitted to their
offspring in a Mendelian fashion. Traditional
animal breeding methods are typically used for
the propagation of the transgene once the
founder animal has been produced. The use of
GE is most appealing when the allele substitu-
tion effect is very large resulting in a profound
change in phenotype that would be difficult, if
not impossible, to achieve using traditional
breeding approaches (e.g. expressing a protein
not found in that species). This approach can
include gene addition (i.e. transgenesis), or tar-
geted gene editing of the endogenous genome.
The latter enables precise changes to be made at
a specific location in the genome (e.g. creating a
gene knock-out) without any other changes to
the genome of an animal (i.e. without selection
markers, or even the genome-wide changes
caused by crossbreeding). This approach, which
is being enabled by several new molecular meth-
ods e.g. zinc finger nucleases, and transcription
activator-like effector nucleases (TALENs), can
be used to supplement or replace a target allele
present in one population with a preferred allele
with known effect from another population
(Fahrenkrug et al ., 2010).
Whether GE livestock fit in with sustainabil-
ity goals will be greatly dependent upon the BO
and production system. However, some GE live-
stock applications (e.g. disease resistance) would
seem to align with many sustainability goals,
such as improving animal well-being. Infectious
diseases have major negative effects on poultry
and livestock production, both in terms of eco-
nomics and animal welfare. The costs of disease
are estimated to be 35-50% of turnover in
developing countries and 17% in the developed
world. Improving animal health using GE has
the added benefit of reducing the need for veteri-
nary interventions and the use of antibiotics
and other medicinal treatments. Efforts are
underway to generate trypanosome resistance
in cattle, which is a major problem for beef and
dairy population in East Africa (Willyard, 2011).
GE could also provide a humane method for sex
selection in dairy and egg industries, where
females provide the animal product (i.e. milk
and eggs). Gene supplementation that feminizes
male embryos (Smith et al ., 2009) or eliminates
the production of male sperm in sires (Herrmann
et al ., 1999) is technically feasible; the latter
approach has the desirable outcome that the
animals that are produced are not themselves
GE (Fahrenkrug et al ., 2010). This change to sex-
biased or sex-specific production of offspring
would have the additional advantage of increas-
ing overall efficiency of the production system
(Hume et al ., 2011).
Similarly, the use of more productive GE
animals should be given due consideration in
the context of sustainability. Consider the con-
troversial example of the AquAdvantage (AA)
salmon, the first GE animal destined for the
human food chain to attempt US regulatory
approval (Van Eenennaam and Muir, 2011). The
AA salmon is an Atlantic salmon carrying a
Chinook salmon growth hormone gene con-
trolled by an antifreeze protein promoter from a
third species, the ocean pout. The mature weight
of these fish remains the same as other farmed
salmon, but their growth rate is increased by
400-600%, with a concomitant 25% decrease
in feed input, decreased waste per unit of product
and a shortened time to reach market weight
(Du et al ., 1992). The AA salmon application
that was reviewed by the US Food and Drug
Administration (FDA, 2010) included manage-
ment measures to abate the potential escape
and interbreeding of GE fish by limiting the pro-
duction to land-based tank culture facilities. The
proposed locations were FDA-inspected and
featured simultaneous, multiple and redundant
physical and geographical containment meas-
ures. And as an extra precaution, additional
levels of biological containment were proposed,
including the production of 100% female fish
(unable to interbreed) and triploidy induction
(which results in sterility), with an average suc-
cess rate of 99.8% (range 98.9-100%). The FDA
review concluded that the likelihood that AA
salmon could escape from confinement was con-
sidered very low. Despite these proposed contain-
ment measures, some groups maintain that any
genetic risks associated with AA salmon are
unacceptable. Absent from the debate over the
AA salmon has been any discussion of the
genetic implications of the escape of growth-
selected lines of Atlantic salmon from conven-
tional net pen aquaculture.
Atlantic salmon remains the most impor-
tant farmed food fish in global trade. Since the
mid-1980s, the yield of food fish from capture
fisheries has been static at about 60 mMT year −1 .
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