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The ovine callipyge locus: a paradigm illustrating the
importance of non-Mendelian genetics in livestock
M Georges, N Cockett
To cite this version:
M Georges, N Cockett. The ovine callipyge locus: a paradigm illustrating the importance
of non-Mendelian genetics in livestock. Reproduction Nutrition Development, EDP Sciences,
1996, 36 (6), pp.651-657.
HAL Id: hal-00899932
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Submitted on 1 Jan 1996
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Review article
The ovine callipyge locus: a paradigm illustrating
the importance of non-Mendelian genetics
in livestock
M
1
Georges
N Cockett
2
Department of Genetics, Faculty of Veterinary Medicine, University of Liege (843),
20, boulevard de Colonster, 4000 Li6ge, Belgium;
2
Department of Animal, Dairy and Veterinary Sciences, College of Agriculture,
Utah State University, Logan, UT 84322-4700, USA
(Received
15 October
1996; accepted 17 October 1996)
inheritable muscular hypertrophy was recently described in sheep and shown to be
determined by the callipyge (CLPG) gene mapped to ovine chromosome 18. We demonstrate in this
work that the callipyge phenotype is characterized by a non-Mendelian inheritance pattern, referred to
as polar overdominance, in which only heterozygous individuals having inherited the CLPG mutation
from their sire express the phenotype. The possible role of parental imprinting in the determinism of polar
overdominance is envisaged.
Summary ― An
imprinting I domestic animals I callipyge
Résumé ― Le locus ovin Callipyge: un paradigme illustrant l’importance de la génétique non
mendélienne chez les animaux d’élevage. Une hypertrophie musculaire héritable a été décrite
récemment chez le mouton ; elle est déterminée par le gène Callipyge localisé sur le chromosome 18.
Nous montrons ici que le phénotype callipyge est caractérisé par un type d’hérédité non-mendélienne,
nommée surdominance polaire dans laquelle les individus hétérozygotes qui ont hérité la mutation CLPG
de leur père expriment le phénotype. Le rôle possible d’une empreinte parentale dans le déterminisme de la surdominance est envisagé.
empreinte parentale lanimaux domestiques /Callipyge
INTRODUCTION
By selecting the individuals exhibiting
properties as parents for next generations, animal breeders have been manipulating genes, albeit unwittingly, for centuries. During the latter half of this century,
the quantitative genetics theory allowed for
spectacular increases in genetic response
by modelling an individual’s phenotype as
the sum of an environmental and genetic
component. For most continuously distributed production traits, the genetic comdesired
was assumed to reflect the action
of a large number of polygenes or quantitative trait loci (QTL), each contributing modestly to the overall genetic variance in an
independent, additive and Mendelian way.
ponent
The emergence of genomics as a very
powerful discipline for genome analysis has
opened the possibility to dissect production
traits at the molecular level, that is, identify
the actual genes (both major genes and
QTL) underlying the observed genetic variation for the traits of interest. Not only should
the resulting information allow for the design
of
more
efficient marker-assisted selection
schemes, but it should lead to a better fundamental understanding of the so-called
black box, ie, the functioning of the network
of involved polygenes.
The first whole genome scans performed
during the last 5 years have already led to
the mapping of a number of production
genes (reviewed in Georges and Anderson,
1996), including major genes (such as the
polled and mh loci causing hornlessness
and double-muscling in cattle, the CRC and
RN genes affecting meat quality in pigs, the
FecB and callipyge loci underlying the
Booroola fecundity and a muscular hypertrophy in sheep), and QTL (including a QTL
affecting fatness in pigs and a QTL affecting
milk production in cattle).
Unexpectedly, detailed analysis of one
of these mapped production genes - the
ovine callipyge locus - revealed a nonMendelian inheritance mode, conflicting with
the traditional view of the Mendelian action
of the genes composing the ’black box’.
Likewise, preliminary evidence suggested
parent-of-origin effects for a QTL underlying trypanotolerance in a rodent model, suggesting that such a non-Mendelian mechanism might. not be that uncommon.
The objective of the present article is to
summarize the present knowledge about the
unusual inheritance pattern characterizing
the callipyge locus, referred to as polar overdominance (Cockett et al, 1996), as a
paradigm of non-Mendelian genetics that is
likely to be increasingly uncovered through
future genomic analysis of complex production traits in livestock and other organisms.
&dquo;CALLIPYGE&dquo;: A MONOGENIC
MUSCULAR HYPERTROPHY MAPPING
TO OVINE CHROMOSOME 18
8
In 1983,
a
sheep breeder
in Oklahoma
(USA) described a ram, premonitorily called
Solid Gold, exhibiting an unusual muscularity transmitted to part of its descendants.
Systematic crosses performed between
male descendants of Solid Gold and normal ewes subsequently allowed for a rigorous characterization of this unusual phenotype (Jackson and Green, 1993; Jackson
et al 1993a,b), showing that the callipygous
animals were characterized by a generalized muscular hypertrophy (although manifesting itself primarily around the hind quarters; hence, its name) resulting in a 30%
increase in muscle mass when compared
to controls. Histological examination of
skeletal muscle tissue revealed an hypertrophy of all but the slow twitch oxidative
fibres. Interestingly, the callipyge phenotype manifested itself only after approximately 3 weeks of age. It should be noted
that some concerns have been raised about
the palatability of callipyge meat.
The same callipyge d x normal !9
revealed a sex-independent 1:11
segregation of the callipyge versus normal
phenotype, allowing Cockett et al (1994) to
postulate an underlying autosomal dominant mutation, referred to as callipyge
(CLPG). Linkage analysis performed in the
same pedigree material quickly positioned
the corresponding callipyge locus to the
subtelomeric region of ovine chromosome
18, at approximately 3 cM for the nearest
microsatellite markers: CSSM18 and
IDVGA30 (Cockett et al 1994, 1996). Linkage analysis in these matings were performed assuming full penetrance of the dominant CLPG mutation, ie, the corresponding
chromosome 18 locus accounted for all the
trait variance in these crosses. Lod score
values superior to 80 are presently obtained
from such crosses, proving the chromosomal location of the callipyge locus beyond
crosses
any doubt.
mal. The 51 offspring initially obtained from
such matings yielded a x
i value of 56.5 with
2
an associated P value of P < 0.0001. Since
then, these ratios have been confirmed in a
new crop of approximately 50 sheep.
The results obtained in both these
clearly pointed towards a non-conventional inheritance pattern of the callipyge
crosses
phenotype.
MARKER-ASSISTED SEGREGATION
ANALYSIS REVEALS POLAR
OVERDOMINANCE AT THE OVINE
CALLIPYGE LOCUS
To gain better understanding of the observed
inheritance pattern, all corresponding individuals were genotyped for the genetic markers known to flank the callipyge locus. This
marker-assisted segregation analysis
revealed a clear pattern.
lCLPG d
CLPG
First, although +
THE CALLIPYGE LOCUS EXHIBITS
A NON-MENDELIAN INHERITANCE
PATTERN
regate
types of matings yielded results conflicting with the simple model of an autosomal dominant mutation. Indeed, matings between callipygous
female descendants of Solid Gold (assumed
CLPG/CLPG’ genotype) and conventional
Subsequently, however,
two
(CLPGICLPG genotype) yielded noroffspring exclusively, pointing towards
non-equivalence of reciprocal crosses. At
present, more than 35 such offspring have
been generated, all of them being of conventional phenotype. Furthermore, matings
between callipygous male and female
descendants of the founder ram (assumed
+ genotype) resulted in 29%
CLPGICLPG
callipygous versus 71 % normal offspring
rams
mal
where a dominant mutation in such cross
would have yielded closer to the opposite
ratios, ie, 75% callipyge versus 25% nor-
x
CLPG/CLPGmatings generated only conventional offspring, the maternal 18 homologues were shown from marker data to segas
expected. However, offspring
inherited the CLPG mutation from
their dam (or CLPG+!Par)CLPG!Mar)) were
unexpectedly not expressing the phenotype,
contrary to individuals having inherited the
same mutation from their sire (or
having
CLPG!Par)lCLPG+!^’rar) individuals), clearly
pointing towards a parent-of-origin effect.
Genotyping the offspring of the second
+ d x
type of matings, ie, CLPG/CLPG
CLPG/CLPG! 9, revealed that CLPGIPar))
(Mat), CLPG+!Pat)/CLPG!n!at)
+
CLPG
and
at individuals generM
(
)/CLPG
P
(
CLPG
+
at )
ally expressed the same phenotypes as
observed in previous matings namely callipygous, normal and normal, respectively.
)/
t
a
P
Unexpectedly, however, CLPG(
CLPG!!’rar) offspring did not express the callipygous phenotype (fig 1 a). The inactive
CLPG!Mar) allele therefore seemed to dom-
inate the active CLPG(
) allele. The resultPat
ing segregation pattern, where only heterozygous individuals having inherited the
mutation from their sire express the phenotype, has been referred to as polar overdominance (Cockett et al, 1996).
To
verify whether the inactivation
of the
CLPG!Mar! allele (ie, the CLPG allele transmitted by the ewe) was reversible, normal
P
(
+
CLPG
CLPG(Mat)
at and
looking )/
CLPG!Par!lCLPG!Mar! rams were mated to
unrelated conventional CLPG
/CLPG
+
Callipygous offspring were indeed
generated from both types of matings,
demonstrating the reversible nature of the
maternal inactivation of the CLPG allele.
Moreover, the proportions of callipygous
versus conventional offspring as well as the
correlation with the segregation of chromosome 18 markers were in agreement with
the polar overdominance model. Indeed,
CLPG+!Pat)/CLPG!Mat) produced 50% cal)/
t
a
P
lipyge offspring (fig 1 b), while CLPG(
CLPG!Mar! rams yielded close to 100% calewes.
lipyge offspring (30/33, 91%).
DOES POLAR OVERDOMINANCE
INVOLVE PARENTAL IMPRINTING ?
To the best of our knowledge, only two other
phenotypes show polar overdominance: Pelement dependent hybrid dysgenesis in
Drosophila (Bregliano and Kidwell, 1983),
and an early embryonic lethality referred to
as polar lethality in DDK mice (Wakasugi,
1974). Hybrid dysgenesis is well understood
and due to P-element transposition in the
germline of offspring from paternal (P) males
x maternal (M) females crosses. This zygotic
mechanism seems, however, difficult to reconcile with the muscle-specific expression of
the callipyge phenotype. The molecular
mechanisms underlying polar lethality are
yet unknown, but imprinting has been
invoked as a possible cause (Baldacci et
al, 1992; Sapienza et al, 1992).
The conventional
phenotype of homozy-
gous CLPGICLPG individuals, however, is
difficult to reconcile with the allele-specific
transcriptional silencing observed for all
known imprinted genes (reviewed in Efstradiatis, 1994). A number of molecular models
based on conventional parental imprinting
can, however, be envisaged to account for
the observed segregation pattern (Cockett et
al 1996). It could be postulated that the callipyge mutation switches the expression pattern of the corresponding imprinted callipyge
gene from paternal to maternal.
)/CLPG individuals would then be
t
a
P
CLPG(
+
the only genotype with two transcriptionally
silent copies of the gene, which would
explain their unique phenotype (fig 2a). An
alternative model considers two closely
linked genes, one of which is paternally
expressed and coded for a trans-acting suppressor of the other one. The CLPG muta-
tion would be a deletion of both genes,
resulting in the expression of the otherwise
suppressed gene in CLPG!Par!lCLPG+ individuals only (fig 2b).
Interestingly, the homologous chromosomal regions in humans (chromosome 14)
and mice (chromosome 12) are known to
harbour imprinted regions (Temple et al
1991; Wang et al 1991; Cattanach and
Raspberry, 1993; Beechey and Cattanach,
1994). Moreover, the phenotypic manifestation of this gene seems to be in agreement with the predictions made from evolutionary considerations by Moore and Haig
(1991namely that imprinting is expected at
loci that influence how much an offspring
receives at the expense of its mother (including genes that affect postnatal growth rate),
with the paternal allele stimulating growth
and the maternal one opposing it. It is noteworthy that, from the point of view of population genetics, dominant negative imprint-
ing may generate balanced polymorphism at
corresponding loci.
the
SHORTCOMINGS OF THE POLAR
OVERDOMINANCE MODEL
Whereas the polar overdominance model
explains the majority of our observations,
some of the inconsistencies between the
phenotype and the callipyge genotype as
inferred from marker data remain puzzling
(see fig 1 and Cockett et al, 1996).
While at this point we cannot exclude
that this involves trivial phenotypic misclassifications, this hypothesis is hardly convincing as a recombination rate as low as
6% was found with the closest microsatellite
marker for more than 600 offspring issued
x
from
)ICLPG(mat)
t
a
P
CLPG(
6
CLPG(Pal)/CLPG(Mal) q mating, putting an
upper limit of 6% misclassification in these
crosses.
Genotypic misclassification due to undetected recombinations could account for
some of the observed discrepancies. This
will be verified by typing the corresponding
families with additional genetic markers in
the callipyge region when they become
available in order to detect and characterize
previously undetected recombinations.
The observed inconsistencies, however,
to fit two distinct patterns. First, we
observe a number of CLPG!PAT!lcIpglMar!
individuals issued from d clpg(Pat)
/
) xCLPG/CLPG matings and
l
a
M
CLPG(
d CLPGlCLPGx !CLPG/CLPG matings,
which are normal in appearance contrary
to what would be predicted under the polar
overdominance hypothesis. Assuming that
parental imprinting occurs at the callipyge
locus, the four individuals with normal phenotype, although having inherited the CLPG
mutation from their clpg(
) or
l
)/CLPG(Ma
Pal
) sire, might be due to
Mat
)/CLPG(
Pat
CLPG(
incomplete erasure of the grand-maternal
seem
imprint. The capacity to erase the maternal
imprint could itself be under genetic control
of modifier ’imprintor’ loci either in the sire
(Sapienza, 1990; Forejt and Gregorova,
1992) or transmitted by the ewes (Sapienza
et al 1989; Allen et al, 1990). It would be of
interest to determine the grand-parental origin of the CLPG mutation for the three phenotypically normal offspring of the
) sires, predicted to be
Mat
)/CLPG(
Pat
CLPG(
grand-maternal under the hypothesis of
incomplete imprint erasure. As they become
available, additional markers will be typed on
the corresponding pedigrees in order to
resolve this issue. Moreover, when mated to
CLPG/CLPG ewes these normal looking
CLPG!Pat)lclpg!Mar) rams should yield offspring segregating for the callipyge phenotype in linkage with chromosome 18. Offspring from such offspring will be generated
in order to verify this prediction.
Second and more intriguing, a few
CLPG/CLPG individuals issued from d
pgIPat) x CLPG/CLPG matI
c
IMat)
/CLPG
ings are callipygous in appearance while they
are obviously expected to be of conventional
phenotype. As the segregation of the callipyge locus accounted for virtually all the trait
variance in the d
)/CLPG(Mat) x
t
a
P
CLPG(
CLPG(Pat)/CLPG(Mat)matings (Cockett et
al, 1994), a two-locus model is difficult to fit to
the data. It could be postulated either a transposition of the callipyge locus in some
CLPG!Par)lCLPG!Mat) sires or the conversion
of the paternal CLPG!Pat) allele by its
CLPG!Mar) homologue, possibly by a transsensing effect (Tartof and Henikoff, 1991).
The latter hypothesis will first be tested by
producing offspring from such callipygous
clpglclpg rams mated to normal clpglclpg
ewes. Segregation of the callipygous phenotype in such pedigrees in linkage with the
chromosome 18 callipyge locus would point
towards a conversion event. Segregation of
the callipygous phenotype in the absence of
linkage with chromosome 18 would point
towards the transposition hypothesis. A new
would then have to be under-
genome
scan
taken to
identify the new locus.
CONCLUSION
unravelling of the polar overdominance
characterizing the ovine callipyge locus
clearly illustrates the importance of dissecting production traits into their
’Mendelian’ (or not-so-Mendelian) components using the new genomic techniques.
Elucidating the molecular mechanisms
underlying polar overdominance is of fundamental interest. It might help to explain
complex inheritance patterns observed in
other organisms, including humans. Already,
linkage analysis performed under the polar
overdominance model might uncover previously undetected causative loci. In addition, this work demonstrates how a genomic
analysis of a production gene might have
immediate implications for breeding pro-
The
grams. It is evident that conventional selection programs could not deal appropriately
with genes exhibiting polar overdominance.
For instance, it would be impossible to fix
the callipyge phenotype by selecting hypertrophied parents in subsequent generations.
From the point of view of population genetics, polar overdominance generates balanced polymorphism at the corresponding
loci. Based on our model, however, we could
predict that non-expressing CLPG!Par!!
t) males mated to cIpg(Pat)/clpg(Mat)
Ma
CLPG(
females might produce 100% callipyge offspring, which would be the breeding scheme
of choice to optimally exploit the callipyge
entity.
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