1
Insulin-Like
Growth Factor 2 Gene for Carcass
Leanness
and Uniformity
Yuefu Liu, CCSI, September, 2003
IGF2
gene and its physiological functions
Insulin-like
growth factor 2 (IGF2) gene plays an important role in
mammalian growth,
influencing foetal cell division
and differentiation, and postnatal muscle growth.
It
was mapped to the distal tip of chromosome 2. IGF2 is an imprinting gene,
paternally
expressed, i.e. only the allele from father is expressed in
progeny. This imprinting
inheritance mode of the gene was reported by several
studies.
The
beneficial effects of growth hormone (GH) on swine carcass are known for a long
time. The GH does not act directly on muscle cells but is
instead an intermediate in a
series of hormonal signalling
events to increase growth (Meadus, 2000). Altering
any one
of these endocrine genes or their respective receptor genes
can modify growth. IGF2 is
one of the intermediates in the GH endocrine pathway.
IGF2
gene is a major gene for lean growth and fat composition
Based
on its physiological function, IGF2 is considered as a candidate gene for a
quantitative trait locus (QTL) in pigs affecting
muscularity. The large effects of IGF2
gene on lean meat content and backfat
thickness of swine were detected by several
studies. The evidences from these studies are summarized as
follows:
An
intercross between wild boar and Large White domestic pigs was used for QTL
mapping by Andersson-Eklund et al.
(1998). A quantitative trait locus (QTL) on the short
arm of chromosome 2 with a moderate effect on muscle mass
was detected using
conventional Mendelian inheritance
model. Jeon et al (1999) reanalyzed the data and
tested the presence of an imprinting effect. An imprinted
QTL (paternally expressed) was
detected at the distal tip of SSC2p that has very large
effects on lean meat content in ham
and explained 30.6% residual phenotypic variance of F2
population. Large effects on the
area of longissimus dorsi muscle, heart weight and backfat
thickness were also detected.
This
large effect QTL explains 15.4% of F2 phenotypic variance of Longissimus muscle
area, 14% of heart weight, 10.4% of backfat
depth. The results demonstrated that the
paternally expressed QTL locates at the same position as
IGF2. This together with the
fact that both the gene and the QTL are imprinted makes IGF2
gene a possible candidate
for the QTL effect. The Large White allele at the
IGF2-linked QTL is associated with
larger muscle mass and reduces backfat
thickness. Notably, this QTL has no effect on
abdominal fat.
Nezer et al. (1999) studied an intercross between Large
White and Pietrain pig breeds,
with 1032 F2 progeny. The QTL mapping based on the
experiment detected a highly
significant QTL at the distal end of the short arm of
chromosome 2, influencing
muscularity measurement (lean cut %, ham % and loin %) and
fat deposition (backfat
2
thickness, backfat % and fat cut %).
Sequence analysis (Nezer et
al.1999) showed that
IGF2
gene coincides with the position of the detected large-effect QTL at the distal
end
of chromosome 2p, and well within the 95% support interval. Nezer (1999) also analyzed
the skeletal muscle and liver cDNA
from 10-week-old porcine foetuses and found that
IGF2
is imprinted in these tissues of pigs as well just as Human and mouse. QTL
mapping results of Nezer et al.
(1999) based on the F2 data also confirmed that the QTL
at the end of SSC2p is imprinted and paternally expressed.
Therefore, IGF2 gene is
regarded as a positional candidate for the QTL at the distal
end of SSC2p. Its effects on
muscle mass and fat deposition is major and of the same
magnitude as those reported for
the Halothane gene (Ryanodine
receptor 1 gene). Two loci jointly explain 50% of the
Pietrain-Large
White difference for muscularity and leanness. However, they did not
find
any evidence for interaction between the QTL at IGF2 gene
and Halothane gene locus. In
sequence analysis, Nezer et al.
(1999) found a single nucleotide mutation, G to A
transition in IGF2, which increases lean yield by up to 2.7%
(Meadus 2000).
The
QTL at IGF2 and FAT1 on chromosome 4 (Andersson et
al. 1994) are two QTL with
the greatest effect on body composition and fatness,
segregating in the wild boar- Large
White cross. The QTL at IGF2 controls mainly muscle mass
whereas FAT1 has major
effects on fat deposition (Jeon et
al. 1999). The two QTL loci explain 33.1% of variance
for lean meat content in ham, 31.3% for percentage of lean
meat plus bone in back, and
26.2
% for average backfat depth.
According
to Sheller et al. (2002), IGF2 explains 25% of the phenotypic variation of
leanness in experimental crosses. However, it does not
influence daily gain and meat PH.
Lee
et al. (2001) tested the existence of the imprinted QTL at IGF2 based a F2
population
of 512 pigs from cross between Berkshire and
confirmed that IGF2 gene region is imprinted in pigs and harbours an important QTL for
muscularity and fat deposition. The test reached the
genome-wide highly significant
threshold (p<0.01) for average backfat
thickness and loin-eye area. The favourable alleles
originated from the
backfat by 0.1 cm and increase
loin-eye area by 1.0 squared centimetre, compared to
The
IGF2 microsatellite was found to be highly
polymorphic, with three alleles among
the two wild boars founders and another two alleles among
eight Large White founders
(Jeon et al. 1999). This high
polymorphism provides a good potential for improving lean
meat content of the swine carcass by selection.
A
recent study by Laere et al. (2003) reported that a G
to A transition in IGF2 gene is the
causative quantitative trait nucleotide. This single
nucleotide regulatory mutation in IGF2
gene adds 3-4% more lean meat to hogs. The link of the
mutation with desired phenotype
is 100%, regardless the origin of the pedigree (Buys 2003).
It allows selecting carcass
leanness directly on functional nucleotide at DNA level.
Use
of IGF2 gene in swine breeding
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The
gene mapping researches consistently indicated the large effect of QTL at IGF2
gene
on muscle mass and carcass leanness. This QTL has important practical implications for the
pig industry because it is imprinted and has large
effects on lean meat content.
Use
for Uniformity: IGF2 gene is paternally expressed only. The genes from
boars
should show the full effect on progeny, regardless of the
sows’ genotypes. Besides, this
QTL
has a large effect on lean meat content of carcass. Use of homozygous terminal
sires
in producing hogs should be able to increase the lean-
meat-content uniformity of hogs
because a sire, especially AI sire, can produce a large
number of progeny and the Dams’
IGF2
gene will not cause any phenotypic variation in progeny.
Using
IGF2 gene to increase the uniformity of pork leanness is not just a theoretical
potential. It was confirmed by actual breeding trials. For
example, terminal sires that are
homozygous for the favourable
allele at the IGF2 gene were selected in Gentec
(Sheller
et
al. 2002) and a field trial with these sires was conducted to investigate whether
IGF2
gene can be used in commercial selection program in order to
increase uniformity of
commercial pigs without influencing meat quality. In this
trial the hogs from selected
boars were leaner and more uniform compared to those from
unselected boars. Backfat
thickness was reduced by 2.3 mm (0.09 inches). Average lean
meat percentage, ham
percentage and loin percentage increased by 1.98%, 0.31% and
0.43%, respectively. The
variations of these traits were reduced by 25% on average.
Compared with the average of
plant top 25%, hogs from the selected boars were leaner. The
increase was 0.64% for lean
meat percentage, 0.39% for ham percentage and 0.43% for loin
percentage, respectively.
The
meat quality traits, PH after 24 hours and meat color, were also compared. Hogs
from selected boars and those from unselected boars both had
the same PH value (5.77 to
5.78
measured after 24 hours) and Minota lightness (44.57
to 43.08) within the optimal
range. The investigation concluded that the selection of
homozygous terminal sires with
the favourable allele at IGF2 gene
increased the uniformity and carcass leanness in
market hogs without influencing meat quality.
Improving
sow longevity: Sow durability (or lifetime reproduction) is said to be
reducing as a result of the genetic selection for increasing
leanness and lowering the fat
deposition. Some studies (e.g. Brisbane and Chesnais 1996; Stalder et al.
2001) reported
the association between backfat
and sow longevity that is defined as the lifetime number
of litters produced by a sow. Besides, body fat deposition
is necessary to sustain sow
reproduction performance, for example to supply adequate
milk production and to limit
body weight loss (Ranford et al.
1994).
The
selection for leaner carcass demanded by packing industry and consumers may
conflict with the sow longevity and lead to increase the
replacement costs of sows in
swine production. IGF2 gene provides a possibility to
resolve this conflict. The
imprinting mechanism of this major gene could be used for
producing commercial hogs
with required leanness from fatter dams since only the favourable alleles from
homogenous sires at IGF2 are expressed in progeny. Dams can
be fatter.
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References
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