Download Mapping the Horns (Ho) Locus in Sheep: A Further Locus

Mapping the Horns (Ho) Locus in Sheep:
A Further Locus Controlling Horn
Development in Domestic Animals
G. W. Montgomery, H. M. Henry, K. G. Dodds, A. E. Beattie, T. Wuliji, and
A. M. Crawford
The presence or absence of horns in Merino sheep is under the genetic control of
the autosomal Horns (Ho) locus. Sheep chromosome OOV1 is a candidate region
for the Ho locus because it shows conserved synteny with cattle chromosome
BBO1 where the cattle polled locus has been located. We demonstrate that the Ho
locus in sheep is excluded from sheep chromosome OOV1 and we identified linkage between the Ho locus and markers from sheep chromosome OOV10. These
data suggest that there are at least two loci affecting the presence or absence of
horns in sheep and cattle. The orthologous regions to OOV10 are likely to be on
cattle, human, and mouse chromosomes BBO12, HSA13, and MMU14.
From the AgResearch Molecular Biology Unit, Department of Biochemistry and Centre for Gene Research,
University of Otago, Dunedin, New Zealand (Montgomery, Henry, and Crawford), and AgResearch, Invernay
Agricultural Centre, Private Bag 50034, Mosgiel, New
Zealand (Dodds, Beattie, and Wuliji). This program
was supported by the Foundation for Research Science
and Technology through grants to AgResearch and a
grant from the New Zealand Lotteries Board.
Journal of Heredity 1996;87:358-363; 0022-1503/96/S5.00
358
Genes that regulate the development of
horns in bovid species show a number of
interesting features. The expression of
horns in some sheep strains has pleiotropic effects on wool fibers resulting in a
highly medullated hairlike fleece (Dry
1955). Alleles at the same locus control
sex-limited expression of horns or development of horns in both sexes (Dolling
1970). A dominant autosomal polled locus
that suppresses horn formation in goats of
both sexes is associated with sex reversal
and abnormal sexual development (Hamerton et al. 1969; Pailhoux et al. 1994).
Polled sheep are those that completely
lack horns and emerged in Britain during
the middle ages (Ryder 1983). Many
breeds are now polled, although a number
of breeds have retained horns, including
the Merino breed from Spain. Rams from
this breed normally have large coiled
horns (Figure 1). In crosses between
horned and polled Merino sheep, the phenotypes range from fully developed, large,
curly horns, firmly attached to the frontal
bone to polled or hornless animals with
no horn development and concave depressions in the skull bone at the site
(Dolling 1970). Sheep with intermediate
phenotypes have short knobs or scurs
that may or may not be attached to the
frontal bone. The presence of horns is
controlled by a single autosomal locus
[Horns (Ho); COGNOSAG 1989] with three
alleles (Dolling 1970). The three alleles at
the Ho locus are //op, «b% and HoM (Dolling 1970). Polledness is produced by the
allele Hor, which is incompletely dominant
to the other two alleles in rams and almost
completely dominant in ewes. Of the other
two alleles, Ho* produces horns in both
sexes and HoM produces sex-limited
horns. The growth of horns in rams can
be modified by a further autosomal locus
known as Scurs (Sc; COGNOSAG 1989). The
alleles interact with the Ho locus and result in the growth of scurs or aberrant
horns in rams.
A second locus is also known to produce horns in sheep (Dry 1955). Unlike
the Ho locus, the Halo hair (HH1) locus
(COGNOSAG 1989) has a marked effect on
fleece characteristics. Sheep carrying the
HH1N allele produce a high abundance of
halo hair on the backs of young lambs.
The adult mature fleece is highly medullated and is used as a specialty carpet
wool. Breeding experiments have failed to
separate the presence of horns from the
effects on the fleece (Dry 1955), and the
two effects are considered to be pleiotropic.
In Bos taunts cattle, the presence or absence of horns is controlled by an autosomal locus (polled) with two alleles
(Georges et al. 1993). A second locus, african horn, has been postulated in B. indicus breeds. It is epistatic to the polled locus and also has two alleles (Georges et
al. 1993). Expression of the african horn locus is sex limited. A scurred locus is also
present in cattle and this affects the expression of both the polled and african
horn loci, with differential expression in
males and females.
Recently the polled locus in B. taurus
Figure 1. Expression of horns in (left) Merino ram, (center) Gl Merino X Romney ram, and (right) polled New Zealand Romney ewe
cattle was mapped to markers (GMPOLL-1
and GMPOLL-2) from cattle chromosome
BBO1 (Georges et al. 1993). There is extensive conservation of banding patterns between cattle and sheep chromosomes
(Hayes et al. 1991; Hediger et al. 1991),
and genetic linkage maps for cattle (Barendse et al. 1994; Bishop et al. 1994) and
sheep (Crawford et al. 1995) have recently
been published. The region of the sheep
genome with conserved synteny to cattle
chromosome BBO1 is sheep chromosome
OOVlq.
We have crossed horned Merino rams
with polled New Zealand Romney ewes.
The Gl crossbred rams were then backcrossed to Merino ewes to produce large
half-sib families. Growth and development
of horns is segregating in this flock and we
examined genetic markers from sheep
chromosome OOV1 to test whether the Ho
locus in Merino sheep is the same as the
polled locus in cattle. There was no evidence of genetic linkage to markers from
this region, including one of the markers
linked to the polled locus in cattle. We
screened markers from other chromosomes and identified linkage between the
Ho locus and markers from sheep chromosome OOV10.
Methods
Experiments were performed in accordance with the 1987 Animal Protection
(Codes of Ethical Conduct) Regulations of
New Zealand after approval was granted
by the Invermay Agricultural Centre Animal Ethics Committee.
Sheep Flocks for Linkage Analysis
All animals were run outdoors on pasture
at Tara Hills High Country Research Station, Omarama. Horned Merino rams were
mated with Romney ewes to generate Gl
Merino X Romney rams for studies on the
inheritance of wool characteristics. Four
Gl rams from different sires were mated
with Merino ewes and generated 183 backcross progeny born in 1992. Male lambs
were castrated at birth so that male and
female animals could be run in the same
flock to minimize environmental effects on
wool growth.
The 86 ewe and 97 wether Merino X
Romney hoggets were individually examined at 12 months and 15 months of age.
Phenotypes at the Ho locus were recorded
as depressions, bone knobs, scurs, or aberrant horns and assigned scores based
on the individual's sex (Dolling 1970). Genotypes (//ohl///ohl and //ohl///op) for ewe
hoggets were assigned from the phenotype scores (Dolling 1970).
Genotyping of Microsatellite Markers
Microsatellite markers were analyzed as
described previously (Montgomery et al.
1993). Each PCR reaction contained the
following constituents in a total volume of
10 ml: approximately 100 ng sheep DNA,
45 mM Tris-HCl pH 8.8, 11 mM (NH4)2SO4,
4.5 mM MgCl2, 6.7 mM B2-mercaptoethanol, 4.5 mM EDTA, 0.25 mM spermadine, 200
mg/ml BSA, 20 nM dNTPs, 400 nM CA
strand primer (unlabeled), and 20 nM GT
strand primer (5' end labeled with [732P]ATP using T4 polynucleotide kinase). Incubation conditions for the amplification
reaction were as follows: seven cycles of
denaturation at 94°C for 30 s, and a combined annealing and extension step of
63°C for 1 min (or stated annealing temperature, Table 2), followed by 20 cycles
of denaturation at 90°C for 30 s, annealing
and extension at 63°C for 1 min. The labeled PCR products were separated on denaturing sequencing gels and bands detected by autoradiography.
Data Analysis
For each family, progeny were genotyped
at a test locus, only if the sire was hetero-
zygous at that locus. All genotypes were
scored independently by two researchers
and genotypes were checked for consistency with the pedigree records. The results were then compared and any differences were identified and resolved. Twopoint LOD scores for linkage with the Ho
locus were calculated using maximum likelihood methods for half-sib pedigrees
(Dodds et al. 1993).
All other linkage analyses were performed with CR1-MAP (Lander and Green
1987). Data on linkage between markers in
this backcross flock were compared with
a previous map obtained from the analysis
of these markers in three-generation
sheep pedigrees (IMF; Crawford et al.
1995), by calculating likelihoods for a locus order defined by the IMF map, first
with distances unspecified and second
with distances set to those for the IMF
map. Twice the difference in lognormal
likelihoods for the two analyses was compared with a chi-square distribution with
degrees of freedom equal to the number
of intervals in the map. Multipoint LOD
scores (Ott 1991) were calculated by fixing
the Ho locus at various positions along the
map (with distances between markers set
to those of the IMF map), but no closer
than 0.01 cM to a mapped marker.
Results
Segregation of the Ho Locus in Merino
x Romney Sheep
In the Tara Hills Merino flock the rams develop large coiled horns. Ewes develop
knobs or scurs, but never full expression
of horns. Consequently, the main allele at
the Ho locus in this flock is the Hohl allele
resulting in expression of sex-limited
horns. Scurs are never seen on entire
males and it is unlikely that the analysis
of segregation at the Ho locus would be
complicated by the Sc locus.
Montgomery et al • Mapping the Horns [Ho) Locus in Sheep 3 5 9
Table 1. Segregation at the Horns (Ho) locus in
Merino x Romney backcross female sheep
Table 2. Description of microsatellite markers from sheep chromosomes 1 and 10
Locus name
Chromosome
location
No. of
alleles
Annealing
temperature
Source
Reference
BUI
OarAE57
RM65
BM6438
TGLA49
AGLA17
KAP8
MAF64
1LSTS004
INRA11
BM6506
MAF109
BM864
MAF4
BM1824
OarCP38
OarDB3
TGLA441
OarHH41
AGLA226
OarVH58
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
10
10
10
10
10
10
6
5
4
4
8
1
7
7
6
7
6
8
5
8
4
6
7
8
7
6
3
57
63
55
63
Tf>
TD
50
64
50
55
TD
58
55
65
63
55
63
TD
63
TD
63
Bovine
Ovine
Bovine
Bovine
Bovine
Bovine
Ovine
Ovine
Bovine
Bovine
Bovine
Ovine
Ovine
Ovine
Bovine
Ovine
Ovine
Bovine
Ovine
Bovine
Ovine
Bishop et al. 1994
Penty et al. 1993
Bishop et al. 1994
Bishop et al. 1994
Georges and Massey 1992
Georges and Massey 1992
Wood et al. 1992
Swarbrick et al. 1991
Bishop et al. 1994
Bishop et al. 1994
Bishop et al. 1994
Swarbrick and Crawford 1992
Bishop et al. 1994
Buchanan et al 1991
Bishop et al. 1994
Ede et al. 1995
Bancroft 1993
Georges and Massey 1992
Henry et al. 1993
Georges and Massey 1992
Pierson et al. 1993
Horns genotype
Family
Hd«IH<f
Ho»'/Ho»
4268
4269
4270
4271
14
8
9
12
10
16
7
10
The heads of each female offspring were examined. Animals with a depression or a bone knob in a depression
were assigned an Ho^'/Hcf genotype (Dolling 1970). Animals with a bone knob or scur and no depression were
assigned as Hox'/Ho1" genotype (Dolling 1970).
All of the Merino sires used to generate
the Gl rams had large well-developed
horns. The four Gl rams grew horns, but
these failed to develop fully into the large
curly horns characteristic of the Merino
breed and were classified as a large scur
or aberrant horn (Dolling 1970; Figure 1),
indicating that the Gl rams were heterozygous at the Ho locus.
Examination of the progeny of the four
Gl rams demonstrated that most male
progeny developed scurs. Homozygous
HoM/HoM males did not develop full horns
because they were castrated at birth. Consequently, it was not possible to differentiate between Hohl/Hoh> and HoM/Ho? male
progeny. Two of 97 male progeny were recorded as polled (Hop/Hor), indicating that
there is a low frequency of the Hop allele
in the Merino dams. The presence of the
Hop allele in the dams could introduce errors in the assignment of phenotypes at
the Ho locus. These errors would be expected to reduce the significance of any
linkage detected, but the low frequency of
polled males observed in the progeny of
these matings suggests that the effect of
these errors would be small. Male progeny
were excluded from subsequent linkage
analysis.
Female progeny were completely polled
or developed small scurs. The phenotypes
of the females closely followed the descriptions for crosses between horned
and polled Merino sheep (Dolling 1970).
All four Gl sires had both polled and scurred female progeny (Table 1), demonstrating that all were heterozygous for Hohl/Hop
alleles. Distributions of polled and scurred
progeny in the four families were consistent with Mendelian segregation of the Ho*
allele; across all families there were 43
polled progeny (HoMIHop) and 43 progeny
(//o/i////ohl) that developed scurs.
Exclusion of the Ho Locus From Sheep
Chromosome OOVI
Markers from sheep chromosome OOV1
(Table 2) were analyzed in the Merino
3 6 0 The Journal of Heredity 1996.87(5)
- TD = touchdown PCR procedure: 95°C, 45 sec; 60°C, 1 min; x 3 cycles/95°C, 45 sec; 57°C, 1 min; x 3 cycles/95°C,
45 sec; 54°C, 1 min; x 3 cycles/95°C, 45 sec; 51°C, 1 min; x 3 cycles/95°C, 45 sec; 48°C, 1 min; x 20 cycles.
backcross pedigrees. There are 10 markers in common on the sheep and cattle
linkage maps (Figure 2). The markers
linked to the polled locus in cattle are
AGLA17 (GMPOLL-2) and TGLA49 (GMPOLL1; Georges M, personal communication).
All the markers were informative in at
least one of the families with the exception of AGLA17. AGLA17 was monomorphic in sheep and could not be placed on
the map. Analysis of cosegregation of the
Ho phenotype and individual markers, including TGLA49, showed no evidence of
linkage. A multipoint linkage analysis with
all 14 markers demonstrated that the Ho
locus was excluded from the region of
sheep chromosome OOV1 covered by this
linkage group with a maximum LOD score
-4.7 across the linkage group. The maximum LOD score around the likely location
of the cattle polled locus, between markers BM6348 and ILSTS004 (Figure 2), was
-43.0.
Location of the Ho Locus in Sheep
A further 34 markers from across the
sheep genome were screened in the Merino backcross pedigrees. Linkage to the Ho
phenotype was first detected with the microsatellite marker OarVH58. Three of the
four sires were heterozygous for alleles at
the OarVH58 locus and there were no recombinants between OarVH58 and the Ho
phenotype (Table 3). For marker OarVH58,
the allele segregating with the Hcf allele
was the same in three of the families. However, these alleles were not fixed in the parental breeds and estimates of the allele
frequencies at the OarVH58 locus in 40 unrelated Merino and 40 unrelated Romney
sheep demonstrated that the OarVH58 allele associated with the Hop allele from
Romney sheep was present in the Merino
breed at a frequency of 0.51. The OarVH58
allele associated with the Hohl allele from
Merino sheep was present in the Romney
breed at a frequency of 0.27.
The microsatellite OarVH58 belongs to
a linkage group from sheep chromosome
OOV10 (Figure 3; Crawford et al. 1995). Six
markers from this linkage group were analyzed in the flock (Table 2). The Ho.phenotype showed significant linkage to two
additional markers from this linkage group
(Table 3). OarHH41 was linked to the Ho
phenotype with a maximum LOD score of
10.5 at a distance of 8 cM. AGLA226 was
linked to the Ho phenotype with a maximum LOD score of 5.3 with no recombinants.
Data on linkage between the markers on
chromosome OOV10 were compared with
the analysis of these markers in three-generation sheep pedigrees (Crawford et al.
1995). There were no differences between
the maps generated from these and the
IMF pedigrees. The position of the Ho locus was estimated to be close to AGLA226
(Figure 4). The one LOD support interval
(region where the LOD was within one of
the maximum LOD) for the location of the
Ho locus extended from 63 cM to beyond
OarVH58 (Figure 4).
Discussion
Analysis of a sheep backcross population
from the Merino x Romney sires shows
Sheep
Cattle
Chromosome 1 & 3
(Bishop et al 1994)
Chromosome 1
3.3 cM
9.4 cM
19.6 cM
1.2 cM
3.0 cM
BL41
i— B L 4 1
OarAE57
-
0
E
5.0 cM
BM6438
TGLA49
KAP8
-
MAF64
12.4 cM
-
ILSTS004
FCGR2
RM065
RM065
12.9 cM
Chromosome 1
(Barendse et al 1994)
,SOD1
'TGLA49
BM6438
6.4 cM
AGLA17
20cM
— RM095
23.5 cM
RM095
29 cM
10.2 cM
36.7 cM
5.3 cM
ILSTS004
BM4307
21 cM
17.4 cM
-
INRA11
~ OarDB6
-CRYG8
BM6506
-BM6506
BR2724
BM864
10.2 cM
- URB038
TGLA130
MAF109
CSSM32
BM864
30.7 cM
- CSSM32
9.8 cM
12.0 cM
-
26 cM
18.0 cM
-TGLA415
6.8 CM
17 cM
13.5 cM
17.4 cM
2.8 CM
-UMPS
INRA11
BM1312
9.1 cM
8.5 cM
- TGLA57
6.9 cM
8.6 cM
6.5 cM
_ MAF4
-BM1824
-TF
24 cM
BL28
- CSSM19
BM1824
TF
MAF46
10 cM
MAF46
CRYA1
12 cM
-GMBT7
20.3 CM
BM3205
11 cM
— CSRD1613
Figure 2. A comparison of the linkage maps from sheep chromosome 1 (Crawford et al. 1995) with the published maps for cattle chromosome 1 and a portion of cattle
chromosome 3 close to the centromere (Barendse et al. 1994; Bishop et al. 1994). Markers in common between the sheep and cattle chromosomes are shown in bold type
and markers in common between the published cattle maps are shown in italics.
segregation for the Ho locus in the female
progeny. We have demonstrated that this
locus is not linked to markers from sheep
chromosome OOV1, including the marker
TGLA49, linked to the polled locus in B.
taurus cattle (Georges et al. 1993). The Ho
locus was linked to the markers OarHH41,
OarVH58, and AGLA226 on sheep chromosome OOV10. The orientation of the
linkage and physical maps for sheep chromosome 10 have not been defined, but
linkage between OarHH41 and RP11 suggests that the Ho locus maps toward the
telomere of this chromosome (Crawford et
al. 1994).
One explanation for these results is that
the polled condition in sheep and B. taurus
cattle results from a mutation in the same
gene, but that this locus is located on
chromosome BBO1 in cattle and chromosome OOV10 in sheep. Sheep chromosome OOV1 is a metacentric chromosome
formed from a Robertsonian fusion of ancestral chromosomes making up chromosomes BBO1 and BBO3 in the cattle karyotype (Hediger et al. 1991). The polled gene
in cattle maps approximately 17 cM from
TGLA49. If polled is proximal to TGLA49 it
could lie close to the centromere (Georges
et al. 1993). It is possible that an additional translocation event during evolution of
the ovine metacentric chromosome OOV1
could have relocated the orthologous locus in sheep to OOV10. Comparison of the
linkage maps of sheep and cattle (Bishop
et al. 1994; Crawford et al. 1995) reveals at
least one chromosomal rearrangement between the species.
An alternative explanation is that loci
Montgomery et al • Mapping the Horns (Ho) Locus in Sheep 3 6 1
mal female (Hamerton et al. 1969). Histologically the gonads of intersex animals
were testicular in appearance, but germ
cells were never seen in the seminiferous
tubules after birth (Hamerton et al. 1969).
Y-specific sequences including SRY (sexdetermining region Y chromosome) were
not detected in 60.XX pseudohermaphrodite goats (Pailhoux et al. 1994). The
polled genes in sheep and cattle are not
associated with abnormal sexual development. The "polled" locus in goats maps
to the distal end of goat chromosome CHU
(Vaiman et al. 1996) and appears to be a
different gene than the polled locus in cattle and the Horns locus in sheep.
The linkage maps for OOV10 and BBO12
have few markers in common (Bishop et
al. 1994; Crawford et al. 1995). Erythrocyte
antigen B (EAB) has been mapped to
OOV10 (Crawford et al. 1995) and BBO12
(Bishop et al. 1994). Coagulation factor X
(F10) has been mapped to OOV10q33-qter
by in situ hybridization (Pearce et al.
1994) and to BBO12 by somatic cell hybrid
analysis (Fries et al. 1993). The retinoblastoma gene (RBI) has been mapped to
sheep chromosome OOV10, to cattle chromosome BBO12, and goat chromosome 12
(Hayes et al. 1993). The orthologous
genes for RBI have been mapped to human chromosome HSA13 and mouse chromosome MMU14.
Table 3. Pairwise linkage data to the Horns (Ho) phenotype
Recombination fraction
Locus
N
0.01
0.05
0.10
0.15
0.20
0.30
0.40
2^
e
OarVH58
AGLA226
OarHH41
TGLA441
0arDB3
OarCP38
60
21
68
60
39
60
10.86
5.21
7.84
-29.82
-15.87
-26.62
10.16
4.86
10.27
-14.36
-7.76
-13.69
9.24
4.41
10.38
-8.24
-4.51
-8.42
8.26
3.93
9.79
-5.04
-2.78
-5.54
7.21
3.43
8.88
-3.05
-1.66
-3.67
4.89
2.34
6.46
-0.90
-0.46
-142
2.23
1.11
3.25
-0.12
-0.59
-0.33
11.03
5.30
10.49
0.00
0.00
0.00
0.00
0.00
0.08
0.50
0.50
0.50
Lod scores for linkage were calculated at different recombination fractions using maximum likelihood methods for
half-sib pedigrees (Dodds et al. 1993). The number of progeny scored for each comparison is given by N.
mapped to BBO1 and OOV10 are not orthologous genes. There are at least two
loci affecting the presence or absence of
horns in sheep. In addition to the Ho locus
studied in these pedigrees, the HH1 locus
causes the development of horns and has
marked pleiotropic effects on fleece characteristics (Dry 1955). The Ho locus is not
associated with any known effects on the
fleece and is thought to be a different locus (Dolling 1970; Dry 1955). In B. indicus
cattle, the african horn locus is epistatic to
the polled locus with sex-limited expression (Georges et al. 1993). Since the expression of the major allele at the Ho locus
in sheep shows sex-limited expression,
and the cattle polled locus and sheep Ho
locus map to different chromosomes, the
sheep Ho locus may be orthologous to the
african horn locus. The cattle homologue
of sheep chromosome OOV10 is BBO12
(Hediger et al. 1991) and the african horn
locus may be located on this chromosome. Similarly the HH1 locus in sheep
may be orthologous to the polled locus in
cattle and sheep chromosome OOV1 is a
potential candidate region for this gene.
The marker TGLA49 is located close to a
cluster of keratin genes on chromosome
OOV1 (Crawford et al. 1995; Wood et al.
1992).
The relationship of the polled genes in
sheep and cattle to those in other ungulates is not known. In goats, the autosomal
dominant polled locus has complete penetrance in both sexes and is associated
with abnormal sexual development (Hamerton et al. 1969). Genetic females (60,XX)
that were homozygous for the polled (PP)
locus demonstrate a range of phenotypes
from almost normal male to almost nor-
OarCP38
7.1 CM
20-1
OarDB3
5.3 CM
TGLA441
15.6CM
10-
-
EAB
O
O
0-
27.2 CM
_ OarHH41
-10-lt
I
o\
18.7 CM
oo
— AGLA226
OarVH58
CO
Q_
O
3
Figure 3. The current linkage map for sheep chromosome 10 (Crawford et al. 1995) including microsatellite markers used in the linkage study with the Ho
locus.
3 6 2 The Journal of Heredity 199687(5)
I ,
I 10 I
8
Q
ra
O
• 1 1 1
20
30
40
i
I I | M
50
3
Distance (Kosambi cM)
60
70
| (80
<5
O
Figure 4. A mutilocus linkage analysis with the Horns (Ho) locus and markers from sheep chromosome 10 in
female progeny of Merino x Romney backcross pedigrees. The recombination frequencies between markers were
set from analysis in independent three-generation pedigrees and converted to centimorgan using Kosambi's mapping function. The nonshaded area corresponds to positions where the LOD score is within one unit of the maximum (one LOD support interval; Ott 1991).
Further analyses with more animals and
additional markers from this region, including orthologous genes from other species, will be required to locate the Ho locus more precisely. However, linked markers identified in this study, used in conjunction with breeding records, would
provide a DNA test for carriers of alleles
at the Ho locus.
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Received May 17, 1995
Accepted January 19, 1996
Corresponding Editor: Stephen J. O'Brien
Montgomery et al • Mapping the Horns {Ho) Locus in Sheep 3 6 3