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Unformatted text preview: Bayes factor analyses of heritability for serum and muscle lipid traits in Duroc pigs
J. Casellas, J. L. Noguera, J. Reixach, I. Díaz, M. Amills and R. Quintanilla
J ANIM SCI 2010, 88:2246-2254.
doi: 10.2527/jas.2009-2205 originally published online April 23, 2010 The online version of this article, along with updated information and services, is located on
the World Wide Web at:
http://jas.fass.org/content/88/7/2246 www.asas.org Downloaded from jas.fass.org by guest on July 16, 2011 Bayes factor analyses of heritability for serum and muscle
lipid traits in Duroc pigs1
J. Casellas,*2 J. L. Noguera,* J. Reixach,† I. Díaz,‡ M. Amills,§ and R. Quintanilla*
*Genètica i Millora Animal, Institut de Recerca i Tecnologia Agroalimentàries (IRTA)-Lleida, 25198 Lleida,
Spain; †Selección Batallé S.A., 17421 Riudarenes, Spain; ‡Tecnologia dels Aliments, IRTA-Monells,
17121 Monells, Spain; and §Departament de Ciència Animal i dels Aliments,
Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain ABSTRACT: Concern about pork quality has increased during last decades. Given the influence of fat
content and composition on sensorial, nutritional, and
technological variables of pork meat, an accurate knowledge about genetic control of pig lipid metabolism is
required. This study focused on providing a broad characterization for serum and meat lipid trait heritability
estimates in pigs. Analyses were performed on a population of 370 Duroc barrows and measured the additive
polygenic background for the serum concentrations of
cholesterol, triglyceride, and low- and high-density lipoproteins at 45 and 190 d of age (at slaughter), as well as
intramuscular fat, cholesterol content, and C:12 to C:22
fatty acid content in longissimus thoracis et lumborum
and gluteus medius muscles at slaughter. These traits
were analyzed under Bayesian univariate animal linear
models, and the statistical relevance of heritability estimates was evaluated through Bayes factor (BF); the
model with polygenic additive effects was favored when
BF >1. All serum lipid traits showed relevant genetic
determinism, but the BF reached greater values at 190 d of age. Serum lipid traits displayed moderate modal
estimates for heritability that ranged from 0.18 to 0.30.
On the other hand, the genetic determinism for meat
quality traits showed a heterogeneous behavior with
large and less-than-1 BF. In general, longissimus thoracis et lumborum and gluteus medius muscles showed a
similar pattern, with strong evidence of polygenic additive effects for intramuscular fat and palmitic, stearic,
and cis-vaccenic fatty acids content, whereas oleic and
muscle cholesterol content showed moderate to weak
BF with moderate heritabilities. Similarly, results regarding linoleic, arachidonic, n-3, and n-6 fatty acids
suggested a moderate genetic determinism, but only in
gluteus medius muscle. For the remaining traits (myristic and palmitoleic fatty acids in both muscles, along
with linoleic, arachidonic, n-3, and n-6 fatty acids in
the longissimus thoracis et lumborum muscle), no statistical evidence for genetic control was observed in this
study. As a whole, these results confirm the complexity
of lipid metabolism in pigs. Key words: Bayes factor, cholesterol, fatty acid, heritability, lipoprotein, meat quality
©2010 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2010. 88:2246–2254
doi:10.2527/jas.2009-2205 INTRODUCTION 1
This research has been funded by grants AGL2002-04271-C03
(Ministerio de Educación y Ciencia, Spain) and AGL2007-66707-C02
(Ministerio de Ciencia y Tecnología, Spain). The authors are indebted to Selección Batallé S.A. (Riudarenes, Spain) for providing
the animal material and for their cooperation in the experimental
protocol, and to D. Almuzara (Genètica i Millora Animal, IRTALleida, Lleida, Spain) for technical support. The research contract of
J. Casellas was partially financed by Spain’s Ministerio de Ciencia
e Innovación (programs Juan de la Cierva and José Castillejo). The
authors acknowledge the anonymous referees for their helpful comments on the manuscript.
Corresponding author: Joaquim.Casellas@uab.cat
Received June 8, 2009.
Accepted March 24, 2010. Lipid metabolism in pigs is an area of relevance in
genetics research because it is directly or indirectly
involved in human nutrition and health (Jarratt and
Mahaffie, 2002). The intramuscular fat (IMF) content and its fatty acid (FA) and cholesterol (CHOL)
composition show an impact on various traits related
with pork quality, such as technological attributes (i.e.,
firmness, storage stability), flavor, nutritive value, and
palatability (Cameron and Enser, 1991; Fischer, 2005;
Wood et al., 2008). Moreover, these meat quality traits
are also linked with serum lipid concentrations (Averette Gatlin et al., 2003), highlighting the complexity
of lipid metabolism. From a nutritional aspect, pork is
an important source of oleic and essential n-6 and n-3 2246
Downloaded from jas.fass.org by guest on July 16, 2011 Heritability of lipid profile in pigs FA, with their well-known influences on human health
(Tribole, 2006). The characteristics of fat associated to
meat are key factors for the production of high-quality
dry-cured hams, where IMF content and FA composition can affect the drying period, ripening, and flavor
of final products (Chizzolini et al., 1998; López-Bote,
1998). In spite of the relevance of pig meat composition
for humans, our knowledge about the genetic control
of pig serum and muscle lipid traits remains limited.
There are few estimates of the genetic parameters for
FA in pigs, mainly restricted to the most abundant FA
(Fernández et al., 2003; Suzuki et al., 2006).
The aim of this research was to characterize the heritability of 2 groups of relevant lipid-related traits in
pigs: the blood concentrations of CHOL, lipoproteins,
and triglycerides of young pigs at 2 ages, and the lipid
characteristics of the longissimus thoracis et lumborum
and gluteus medius muscles focused on IMF, CHOL
content, and FA profile at slaughter. These analyses
were performed on a Duroc population through the
simplified Bayes factor (BF) test developed by GarcíaCortés et al. (2001) and Varona et al. (2001). MATERIALS AND METHODS
All experimental procedures were approved by the
Animal Care and Use Committee of the Institut de Recerca i Tecnologia Agroalimentàries (http://www.irta.
es). Animal Material Source
This study was performed on a commercial Duroc
line (Selección Batallé SA, Riudarenes, Spain), which is
primarily used for producing dry-cured ham. Phenotypic data were collected from 370 castrated males generated between August, 2003 and May, 2005 in 4 contemporary batches (95, 114, 83, and 78 animals per batch),
under an experimental design of half-sib families. More
specifically, 370 purebred Duroc sows distributed in 3
farms were mated with 5 purebred Duroc boars, and 1
male offspring per litter was taken at random (49, 93,
81, 84, and 63 sons per sire). After weaning, castrated
animals born on the 3 farms (150, 153, and 55 animals per farm) were moved to the test station (IRTA
Pig Control Center, Monells, Spain), where they were
initially (transition period) housed with other animals
foreign to the experiment. Barrows from each contemporary batch were moved together to the fattening and
control units at 2 to 3 mo of age and randomly distributed over 10 pens (5 pens at each side of a central
corridor in the same barn) in groups of 8 to 12 animals.
Pigs were all raised under the same standard intensive management conditions and fed ad libitum up to
slaughter. During the first period of fattening (up to
90 kg of BW, around 150 d of age) barrows were fed a
standard diet with 18% protein, 3.8% fiber, 7.0% fat,
1.0% lysine, and 0.3% methionine (2,450 kcal/kg; DM
basis). In the second period of fattening (the last 30 to 2247 40 d before slaughter), animals were fed a standard diet
with 15.9% protein, 4.5% fiber, 5.2% fat, 0.7% lysine,
and 0.2% methionine (2,375 kcal/kg; DM basis). Pigs
were slaughtered at 122.5 kg ± 0.7 kg of BW and 193.7
d ± 0.4 d of age. All relevant data were recorded (e.g.,
farm of origin, batch and pen of fattening, along with
age, backfat thickness, and BW at blood collection and
at slaughter). Lipid Profile in Blood and Meat Samples
Blood samples (5 mL) were taken from a jugular vein
(Framstad et al., 1988) of each barrow at 45.0 ± 0.4
and 190.4 ± 0.4 d of age and appropriately processed
to measure total CHOL (CHOL45 and CHOL190),
low-density lipoproteins (LDL45 and LDL190), highdensity lipoproteins (HDL45 and HDL190), and triglyceride (TG45 and TG190) serum concentrations as
described by Gallardo et al. (2008). Venous blood was
collected into EDTA-containing TapVal tubes (Aquisel,
Barcelona, Spain). Blood samples were kept on ice until
centrifugation (10 min at 1,560 × g at 4°C), after which
serum was collected and stored at −80°C until analyzed.
Serum CHOL was measured by a CHOL oxidase-based
method employing CHOL esterase, CHOL oxidase, and
peroxidase enzymes (Richmond, 1992). Serum HDL
concentrations were determined by immune-inhibition
(Rafai and Warnick, 1994), whereas serum triglyceride (TG) concentration was quantified by means of a
glycerol kinase reaction with the method reported by
Fossati and Prencipe (1982). Finally, serum LDL concentration was calculated according to the equation reported by Friedewald et al. (1972).
Two 200-g samples were taken from the longissimus thoracis et lumborum and gluteus medius muscles
during carcass operations in the slaughterhouse (approximately 30 min after slaughter). After appropriate
processing, IMF content was determined by Near Infrared Transmittance (Infratec 1625, Tecator Hoganas,
Sweden), FA (C:12 to C:22 interval) composition was
analyzed by gas chromatography of methyl esters as
described in Mach et al. (2006), and muscle CHOL content (CHOLmusc) was measured following Cayuela et
al. (2003). All these quantifications were performed in
the Centre de Tecnologia dels Aliments of IRTA (Monells, Spain). Statistical Analyses
Linear mixed model analyses were performed on
CHOL, HDL, LDL, and TG serum concentrations (Table 1), as well as CHOLmusc and the percentage of
IMF and myristic, palmitic, palmitoleic, stearic, oleic,
cis-vaccenic, linoleic, and arachidonic FA in gluteus
medius and longissimus thoracis et lumborum (Table
2). Fatty acids with a less-than-1 average percentage in
both muscles were not included in the analyses due to
their biological relevance and because their small measures were close to the instrumentation error, but the Downloaded from jas.fass.org by guest on July 16, 2011 2248 Casellas et al. Table 1. Phenotypic summary of lipid serum traits recorded in Duroc pigs at 45 and
190 d of age
Trait Abbreviation At 45 d
At 190 d
Triglycerides, mg/dL Mean1 n SE CHOL45
1.31 1 Raw means from phenotypic data.
HDL = high-density lipoprotein.
LDL = low-density lipoprotein.
2 sum of n-3 and n-6 FA were also analyzed given their
implications on human health (Table 2). Serum concentrations were log-transformed to correct departures
from normality as in Gallardo et al. (2008), whereas the
remaining traits did not show relevant departures from
normality (Gallardo et al., 2008). Nongenetic sources
of variation were preliminarily evaluated with the Generalized Linear Models procedure (SAS Institute Inc.,
Cary, NC), leading to the following models for Yij = Bi + IMFj + eij d) IMF,
Yij = Bi + BTj + eij and e) CHOLmusc,
Yij = Bi + eij , a) serum lipid concentration traits at 45 d of age, where Y was the phenotypic record, Bi was the fattening contemporary batch, Fj was the farm of origin, covj
was a different covariate depending on the trait (BW
for CHOL, HDL, and LDL, and age at blood collection
for TG), and IMFj and BTj were continuous covariates with the percentage of IMF and backfat thickness
(mm) fat at slaughter, respectively, and eij was the residual term.
The statistical relevance of the heritability parameter
was evaluated through the BF described by García- Yijk = Bi + Fj + eijk b) serum lipid concentrations at 190 d of age,
Yij = Bi + cov j + eij c) FA, Table 2. Phenotypic summary of intramuscular fat and fatty acid (FA) composition in longissimus thoracis et
lumborum and gluteus medius muscles of Duroc pigs
Trait Abbreviation Intramuscular fat, %
CHOL2 content, mg/g
Myristic FA, %
Palmitic FA, %
Palmitoleic FA, %
Stearic FA, %
Oleic FA, %
Cis-vaccenic FA, %
Linoleic FA, %
Arachidonic FA, %
n-6 FA, %
n-3 FA, % IMF
n-3 Gluteus medius n Mean1 SE Mean SE 321
0.02 1 Raw means from phenotypic data.
CHOL = cholesterol. 2 Downloaded from jas.fass.org by guest on July 16, 2011 2249 Heritability of lipid profile in pigs Table 3. Bayes factor and heritability estimates for lipid serum traits in Duroc pigs at
45 and 190 d of age
TG190 Mean Mode PSD HPD95 3.1
Total cholesterol (CHOL), low-density lipoproteins (LDL), high-density lipoproteins (HDL), and triglyceride (TG) serum concentrations at 45 d (e.g., CHOL45) and 190 d of age (e.g., CHOL190).
Bayes factor of the model with additive polygenic effects against the same model without additive polygenic
effects following García-Cortés et al. (2001).
The posterior distribution of heritability was characterized by several basic statistics: mean, mode, posterior
SD (PSD), and highest posterior density region at 95% (HPD95). Cortés et al. (2001) and Varona et al. (2001; see Appendix for a comprehensive description of this BF approach). Note that BF is the ratio of the posterior
probabilities of 2 competing models, taking any positive value between >0 and +¥. In this case, a linear
mixed model with additive polygenic effects (numerator model) was compared against a model without additive polygenic effects (denominator model), where
greater-than-1 BF favored the numerator model and
less-than-1 BF favored the denominator model. In this
report, the BF results were discussed within the context of the Jeffreys (1984) discrete scale of evidences.
This scale classifies the BF according to 6 different levels of evidence for the numerator model, objectively
classifying the BF as denominator model supported,
not worth more than a bare mention, substantial evidence, strong evidence, very strong evidence, and decisive evidence (see Appendix). From now on, this terminology will be systematically used when referring to the
BF. For each analysis, a unique chain with 25,000 iterations was launched, after discarding the first 5,000 elements as burn-in. Convergence was evaluated by visual
inspection after plotting the Markov chain Monte Carlo
sampled values for all variance components across iterations. Moreover, the Raftery and Lewis (1992) approach was used to objectively evaluate the convergence
and length of the burn-in period on the variance component parameters. For all traits and variances, convergence was guaranteed with less than 500 iterations. RESULTS
The average phenotypic values for CHOL, HDL,
LDL, and TG in 45- and 190-d-old Duroc barrows are
shown in Table 1. Note that TG190 concentrations were
approximately 1.2 times greater than TG45 whereas CHOL190, HDL190, and LDL190 serum concentrations
were approximately 1.7 times greater that their corresponding estimates at 45 d of age. Longissimus thoracis et lumborum and gluteus medius muscles showed
relevant differences (P < 0.05) in terms of IMF and
CHOLmusc, with values of 3.84 ± 0.08% vs. 5.17 ±
0.08%, and 58.60 ± 0.52 mg/g vs. 64.65 ± 0.61 mg/g,
respectively (Table 2). As a whole, oleic (n-9; ~34%),
palmitic (~23%), and linoleic (~15%) acids were the
most abundant FA, accounting for almost 75% of the
overall FA amount. Genetic Determinism for Serum Lipid
and Meat Composition Traits
Results based on the BF test for heritability estimation are shown in Tables 3 and 4. Bayes factors between models with and without a genetic component
gave values greater than 1 for all analyzed serum lipid
concentrations (Table 3), providing evidence that the
model was more predictive when polygenic additive
effects were included. The greatest BF were reached
by serum lipid concentrations at 190 d, where LDL190
and TG190 showed strong evidence (BF >10) and
CHOL190 showed very strong evidence (BF >31.62)
of genetic determinism according to the Jeffreys (1984)
scale. Posterior estimates of heritabilities (mean and
mode) reflected medium values (from 0.18 to 0.48) for
all traits analyzed (Table 3). However, it should be noted that the large SD associated to the limited sample
size of our experimental population led to large highest posterior density regions at 95% (HPD95), which
included values near to zero for HDL45, LDL45, and
HDL190 heritabilities and lesser BF.
Focusing on meat quality traits, BF estimates for the
genetic background of the muscular fat content and
composition showed a consistent pattern across muscles Downloaded from jas.fass.org by guest on July 16, 2011 2250 Casellas et al. Table 4. Bayes factor and heritability for the intramuscular fat and fatty acid (FA) composition of 2 muscles in
factor1 Mode PSD 0.55
2.0 Longissimus thoracis et lumborum muscle
Intramuscular fat, %
CHOL3 content, mg/g
Myristic FA, %
Palmitic FA, %
Palmitoleic FA, %
Stearic FA, %
Oleic FA, %
Cis-vaccenic FA, %
Linoleic FA, %
Arachidonic FA, %
n-6 FA, %
n-3 FA, %
Gluteus medius muscle
Intramuscular fat, %
CHOL content, mg/g
Myristic FA, %
Palmitic FA, %
Palmitoleic FA, %
Stearic FA, %
Oleic FA, %
Cis-vaccenic FA, %
Linoleic FA, %
Arachidonic FA, %
n-6 FA, %
n-3 FA, % Mean 1,152.3
0.4 Trait 0.47
0.50 HPD95 1
Bayes factor of the model with additive polygenic effects against the same model without additive polygenic effects following García-Cortés
et al. (2001).
The posterior distribution of heritability was characterized by several basic statistics: mean, mode, posterior SD (PSD), and highest posterior
density region at 95% (HPD95).
CHOL = cholesterol. (Table 4). The additive genetic variability for the percentage of IMF was clearly demonstrated with a BF of
1,152.3 and 992.9 for longissimus thoracis et lumborum
and gluteus medius muscles, respectively. Additionally,
this trait reached the greatest heritability estimates,
with modal values around 0.6 in both muscles. Although the small sample size led to wide HPD95, they
were far apart from the null estimate, starting around
0.18 (Table 4). Also CHOLmusc showed inheritable
patterns in both muscles, but BF values provided less
relevant evidences of genetic determinism with modal
heritabilities centered around approximately 0.20.
The additive genetic background for the different FA
was not homogeneous. The percentage of stearic FA was
very heritable in both muscles (0.53 and 0.33 in gluteus
medius and longissimus thoracis and lumborum, respectively), providing decisive evidence (BF >100) on the
basis of the Jeffreys (1984) scale. Also the cis-vaccenic
acid content provided decisive (BF = 566.6; h2 = 0.38)
and very strong (BF = 37.1; h2 = 0.32) evidence of
an additive genetic background in gluteus medius and
longissimus thoracis et lumborum muscles, respectively.
The palmitic FA revealed strong (longissimus thoracis
et lumborum; h2 = 0.30) and very strong (gluteus me- dius; h2 = 0.30) evidence of additive genetic variance.
Heritability estimates for those FA with relevant BF
were moderate, with modal estimates ranging between
0.29 (cis-vaccenic) and 0.53 (stearic). For the oleic acid
content a small but BF >1 was observed. In the light
of these results, and despite the medium values (from
0.28 to 0.32) obtained for the posterior mode of the
heritability, it is difficult to conclude about the genetic
determinism of oleic content. Linoleic and arachidonic
FA revealed moderate BF (BF <10) in gluteus medius,
suggesting a certain level of genetic control. The remaining FA scored nonrelevant BF, failing to provide
evidence about polygenic additive genetic effects (Table
4). Note that n-3 and n-6 FA in longissimus thoracis et
lumborum were included in this last group of FA because BF failed to identify evidence of genetic control. DISCUSSION
Pork is one of the most consumed meats in Western cultures, with 35 kg/(habitant/yr) in the European
Union, 30 kg/(habitant/yr) in the United States, 27 Downloaded from jas.fass.org by guest on July 16, 2011 Heritability of lipid profile in pigs kg/(habitant/yr) in Canada and 21 kg/(habitant/yr)
in Australia. Moreover, pork is also relevant for Eastern countries like China [35 kg/(habitant/yr)] or Japan
[18 kg/(habitant/yr)], and has an average worldwide
consumption of 15 kg per habitant and year (Food
and Agriculture Organization of the United Nations;
http://www.fao.org). Within this context, concern
about pork quality increased during last decades (Tarrant, 1998) concerning sensorial, nutritional, and technological variables. In our sample of Duroc barrows,
longissimus thoracis et lumborum and gluteus medius
muscles showed substantial differences in terms of IMF
and CHOLmusc, with gluteus medius being fatter and
with a greater CHOL content, as previously reported
by Fiedler et al. (2003) and Kim et al. (2008). Apart
from their incidence on quality of fresh meat, IMF
content becomes a variable of paramount importance
in the production of dry-cured products, where an increased IMF content has a key role in flavor and slow
dehydration during the curing process (Ruiz-Carrascal
et al., 2000).
Despite differences in fat content, longissimus thoracis et lumborum and gluteus medius muscles are a relevant source of oleic (34.61 and 34.85%, respectively),
palmitic (23.42 and 23.25%, respectively), and linoleic
(14.44 and 15.11%, respectively) FA, without significant departures across muscles in terms of percentage.
Although some metabolic differences were described
previously between longissimus thoracis et lumborum
and gluteus medius muscles (Mora et al., 2008), fat
composition differences between both muscles focused
on less abundant FA. It is important to highlight the
significant increase in n-3 FA found in gluteus medius.
Given that FA profile is largely determined by genotype
and diet, our estimates showed substantial differences
when compared with those provided by other authors
(e.g., Cameron and Enser, 1991; Suzuki et al., 2006;
Zhang et al., 2007), although the most abundant FA
yielded values that agreed with previous studies in pigs
(Cameron et al., 2000; Tejeda et al., 2002). Heritabilities for Serum Lipid and Meat
The heritability of serum CHOL in swine in the current study was medium-to-large and comparable with
that in humans (Feitosa et al., 2005). This result agreed
with estimates previously reported in other pig populations (Rothschild and Chapman, 1976; Pond et al.,
1986; Pond and Mersmann, 1996) and also with swine
selection experiments with effective changes on serum
CHOL (Pond et al., 1993; Young et al., 1993). Conversely, heritabilities for TG concentrations at both
ages were also large but less than those reported by
Pond et al. (1986). Heritabilities for HDL and LDL
are, to the best of our knowledge, the first reported for
pigs, with moderate values agreeing with the previous
values reported in humans (Kaess et al., 2008). Even in 2251 humans, little information exists about the inheritance
aspects of HDL- and LDL-bound CHOL concentrations, although their genetic background at older ages
is undoubted (Heller et al., 1993; Bosse et al., 2004;
Kaess et al., 2008). As a whole, our heritability estimates and the corresponding BF provide evidence of
the existence of genetic factors controlling serum lipid
concentrations at 190 d of age, but these were somewhat weaker for HDL90. Similar conclusions might be
reached for serum lipid concentrations at 45 d of age,
although the smaller BF and the wide HPD95 advocate
for a cautious interpretation of these results. In any
case, this genetic component agreed well with previous
results demonstrating that serum lipid QTL segregate
in this population (Gallardo et al., 2008), thus corroborating that pigs are a good model for studying genetic
pathways involved in lipid metabolism.
As previously mentioned, the IMF content has an
important impact on various features of the sensory
(Fernandez et al., 2000) and technological quality of
pork meat (Ruiz-Carrascal et al., 2000). Heritabilities
of IMF for gluteus medius and longissimus thoracis et
lumborum muscles were included in the wide range
of estimates reported in the literature for commercial
breeds (from 0.26 to 0.86; Sellier, 1998). More specifically, our estimates match up with those obtained by
Solanes et al. (2009) in another sample of the same
commercial Duroc line (h2 = 0.57). In any case, the
genetic background for IMF was clearly demonstrated,
providing enough genetic variability for selection programs focused on meat-quality traits, as demonstrated
by Suzuki et al. (2005) and Schwab et al. (2009) in
other swine populations.
Fatty acid profile has profound effects on meat quality because it determines the nutritional value, shelf life,
and processing characteristics of meat (Sheard et al.,
2000). Fatty acid composition also influences the firmness/oiliness of adipose tissue and the oxidative stability of muscle, which in turn affects flavor and muscle
color (Wood et al., 2008). Several authors (e.g., Cameron et al., 2000; Nguyen et al., 2003) pointed out that
the influence of nutrition is stronger than genetic effects
on the FA composition of adipose tissues and also IMF,
but few reports have addressed genetic parameter estimates of FA profile of IMF to corroborate this point.
In the present study, all animals were subjected to the
same dietary conditions, so no differences in the FA
deposition resulting from differences in dietary FA were
expected. Under these circumstances, some FA showed
strong and moderate evidences of genetic determinism,
whereas the BF failed to provide statistical evidences of
additive polygenic effects in the remaining FA.
The existence of additive genetic variability for the
main SFA present in IMF and palmitic and stearic acids was beyond any doubt in longissimus thoracis et
lumborum and gluteus medius muscles. Heritability
estimates obtained for these FA matched previous estimates by Suzuki et al. (2006) in another Duroc population. In close resemblance with our data, Fernández et Downloaded from jas.fass.org by guest on July 16, 2011 2252 Casellas et al. al. (2003) showed that the greatest heritability content
of subcutaneous fat in the Iberian breed corresponded
to stearic (0.41) and palmitic (0.38) FA. Sellier (1998)
reported mean heritability estimates of 0.51 (0.42 to
0.57) for stearic FA of subcutaneous fat. On the other hand, genetic influences for the myristic FA were
not observed in our study, disagreeing with the weak,
but significant heritability reported by Suzuki et al.
In nonruminant animals, dietary FA may be oxidized
or deposited in fat tissues, but there is also de novo
synthesis of FA from acetyl-CoA derived from carbohydrate or protein breakdown or both (Acheson et al.,
1988). In that way, palmitic, stearic and oleic acids are
present in the meat composition (Fischer, 2005). According with results obtained in this and in previous
studies, the genetic determinism of palmitic and stearic
acids content in IMF is largely demonstrated, but the
evidence for oleic FA is far more elusive. Medium to high
heritabilities had been obtained for the oleic content of
several fat tissues (from 0.26 to 0.44) by Fernández et
al. (2003) and Suzuki et al. (2006) in Iberian and Duroc
populations, respectively. In the present study, similar
but not conclusive results regarding heritability of IMF
oleic content were obtained in both analyzed muscles.
Concerning other relevant MUFA, the cis-vaccenic FA
showed polygenic genetic control in longissimus thoracis et lumborum and gluteus medius muscles, with
posterior mean and mode estimates ranging from 0.29
to 0.41, which are the first estimates obtained in pigs
for these traits.
Opposite to the previously mentioned FA, the n-3
and n-6 PUFA are labeled as essential FA because the
biosynthetic pathway for n-3 and n-6 FA does not hold
in mammalian cells and they cannot be synthesized de
novo in pigs (El-Badry et al., 2007). These FA are involved in multiple metabolic routes with direct incidence
on human and animal health (Van Oeckel et al., 1997;
Trivedi, 2006). As they can be obtained only from diet,
several authors (e.g., Nguyen et al., 2003) confirmed
that the percentage of essential FA in the subcutaneous
and intramuscular fat of pigs is directly related to the
percentage of these FA in the dietary fat. According to
this statement, we would not expect to find evidence
of genetic variability on n-3 and n-6 FA content. Our
results corroborated this hypothesis within the context
of the longissimus thoracis et lumborum muscle, with
less-than-1 BF for both groups of essential FA, as well
as for the majority n-6 FA (linoleic and arachidonic
FA), which were analyzed separately. Nevertheless, a
relevant genetic determinism with moderate heritabilities was suggested for linoleic and arachidonic n-6 FA in
the gluteus medius muscle. Besides, Suzuki et al. (2006)
obtained a relevant heritability for the linoleic content
in several fat tissues of pig including IMF, and previous
works showed heritabilities from 0.47 to 0.70 for linoleic
acid of subcutaneous fat (Sellier, 1998). Evidence of
genetic determinism were also found for n-3 FA in the gluteus medius muscle. Note that these heritabilities
for n-3 FA agreed with the one reported by Greeff et
al. (2006). Given that n-3 FA cannot be synthesized de
novo by pigs, this genetic determinism must be related
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Our results confirm the complex genetic control of
FA and relevant influences of environmental factors.
The existence of an important genetic determinism affecting nonessential FA deposition has been confirmed,
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J. Anim. Sci. 85:583–591. Downloaded from jas.fass.org by guest on July 16, 2011 2254 Casellas et al. APPENDIX
Bayes Factor for Testing the Additive
Polygenic Background of Quantitative Traits
Heritability was evaluated by calculating the Bayes
factor (Kass and Raftery, 1995) of a model accounting
for infinitesimal polygenic effects, y = Xb + Za + e,
against a reduced model without genetic effects, Smith, 1990), with the exception of h2, which required
a Metropolis-Hastings step (Hastings, 1970). The Bayes
factor was calculated from the Markov chain Monte
Carlo sampling by averaging the full conditional densities of each cycle at h2 = 0 (see Varona et al., 2001 for
a detailed description of this methodology). The Bayes
factor provides the ratio of posterior probabilities between the 2 tested models. A Bayes factor >1 suggests
a relevant genetic background for the analyzed trait,
whereas a Bayes factor <1 shows that the model without genetic effects is more probable. Jeffreys (1984) Scale of Evidence
for Bayes Factors y = Xb + e,
by applying the García-Cortés et al. (2001) and Varona et al. (2001) method. Note that y was the vector
of phenotypic data, e was the vector of residuals, β
was the vector storing the systematic effects described
above, a was the vector of additive genetic effects, and
X and Z were appropriate incidence matrices. Following Varona et al. (2001), only the analysis of the most
complex model is required to calculate the Bayes factor, after reparameterizing it as The BF is the ratio of posterior probabilities between
2 competing models placed as numerator and denominator in the ratio. Following Jeffreys (1984), the BF
can be classified according to 6 levels of evidence:
BF < 1: denominator model supported
(<0 deciban; dB);
1 < BF < 3.16: not worth more than a bare mention
(0 to 5 dB); y = Xb + e*, where e* = Za + e is assumed to follow a multivariate
normal distribution with mean 0 and variance
V = sp éê ZAZ ' h 2 + I(1 - h 2 )ùú . Note that A was the nuë
merator relationship matrix between individuals, I was
an identity matrix with dimensions equal to the num2
ber of data, sp was the phenotypic variance, sa was
the additive genetic variance, and h 2 = sa sp . Under a standard Bayesian development, the posterior probability of all the parameters in model was proportional
p ( b, s , h y) µ p ( y
p ( 2 ) 2
b, sp , h 2 p (b ) p (sp ) p (h 2 ) , ) 2
where p y b, sp , h 2 ~ N (Xb, V ) and the remaining a priori distributions were assumed flat as in Varona et
al. (2001). Random samples from all unknowns in model were obtained by Gibbs sampling (Gelfand and 3.16 < BF < 10: substantial evidence favoring
the numerator model (5 to 10 dB);
10 < BF < 31.62: strong evidence favoring
the numerator model (10 to 15 dB);
31.62 < BF < 100: very strong evidence favoring
the numerator model (15 to 20 dB); and
BF > 100: decisive evidence favoring
the numerator model (>20 dB).
Note that these specific cutoff values were defined on
the basis of a 5-unit dB increase, a base-10 logarithmic
unit that measures information and entropy (Good,
1979). The BF can be transformed to a dB value (d ) by
applying d = 10
(d ). BF Downloaded from jas.fass.org by guest on July 16, 2011 10 , and inversely, BF = 10 × log10 References This article cites 50 articles, 17 of which you can access for free at:
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