doi:10.1093/humrep/dei067
Human Reproduction Vol.20, No.9 pp. 2476–2482, 2005
Advance Access publication May 5, 2005
Comparison of male and female meiotic segregation
patterns in translocation heterozygotes: a case study
in an animal model (Sus scrofa domestica L.)
A.Pinton1,5, T.Faraut2, M.Yerle2, J.Gruand3, F.Pellestor4 and A.Ducos1
1
UMR INRA-ENVT Cytogénétique des Populations Animales, 31076 Toulouse, 2Laboratoire de Génétique Cellulaire, INRA, 31326
Castanet Tolosan, 3Unité de sélection porcine SESP, INRA, 86480 Rouillé and 4CNRS, UPR 1142, 34396 Montpellier, France
5
To whom correspondence should be addressed at: UMR INRA-ENVT Cytogénétique des Populations Animales, Ecole Nationale
Vétérinaire de Toulouse, BP 87614, 23 chemin des Capelles, 31076 Toulouse Cedex 3, France. E-mail: [email protected]
BACKGROUND: The comparison of male and female meiotic segregation patterns for individuals carrying identical reciprocal translocations has been rarely reported in mammalian species. The main comparative study involving males and females with comparable genetic background has been performed in the mouse. Swine is another
relevant animal model species for meiotic studies. Here we present the segregation patterns determined for sows
carrying one of the two following reciprocal translocations: 38, XX, rcp(3;15)(q27;q13), and 38, XX,
rcp(12;14)(q13;q21). These segregation data were compared to those previously obtained for closely related boars
carrying the same balanced chromosomal rearrangements. METHODS: Dual colour in situ hybridization of whole
chromosome painting probes was carried out on metaphases of in vitro-matured oocytes II. Segregation results
were obtained for 118 and 206 metaphases II respectively for the two translocations. RESULTS: Significant differences between sexes were demonstrated for both rearrangements. For instance, for the 3/15 translocation, the chromosomally unbalanced gametes were of different origin: preponderance of the adjacent-I segregation in the male
(31.4%), and of the adjacent-II (14.3%) and 3:1 (14.3%) segregations in females. For the 12/14 translocation,
the proportion of balanced gametes was greater in males than in females (75.9 and 59.4% respectively).
CONCLUSION: This study is a new scientific contribution to compare the segregation patterns of male and female
carriers of identical chromosomal rearrangements. The results obtained are consistent with those previously
reported in mice. Hypotheses to interpret the observed differences between the two translocations, as well as
between the male and female segregation patterns, are formulated and discussed.
Key words: chromosome/meiosis/pig/reciprocal translocation/segregation
Introduction
Heterozygous carriers of constitutional structural chromosomal rearrangements (reciprocal translocations, inversions,
etc.) are likely to produce genetically unbalanced gametes
resulting in partial trisomy/monosomy in the embryo (Martin,
1988; Guttenbach et al., 1997; Morel et al., 2004). The main
consequences in humans are reproduction failures and the
birth of progeny with serious clinical defects. In livestock
species such as cattle or swine, numerous chromosomal
abnormalities have been identified due to the often considerable reduction in reproductive performance of carrier animals
or of their mates (reduced fertility and/or litter size) (Ducos
et al., 2002b).
Epidemiological studies in humans have frequently underlined the effect of sex of the carrier parent on the risk of
imbalance. Although the general picture of the differences
between male and female meiotic processes has been
known for a long time, the impact of these differences on
chromosome segregation remains unclear. In the case of reciprocal translocations, most imbalances at birth result from a
rearrangement carried by the mother (Boue and Gallano,
1984; Daniel et al., 1988). However, this apparently higher
predisposition of female meiosis than male meiosis to the
production of imbalance has not been conclusively established and certain studies suggest that such predisposition
would be limited to specific types of imbalance, notably the
3:1 (Mackie Ogilvie and Scriven, 2002). According to Faraut
et al. (2000), the observed sex-ratio deviation in the parental
origin of the chromosomal imbalances would not be due to a
predisposition of female meiosis but rather would result from
the male infertility frequently associated with reciprocal
translocations (Luciani and Guichaoua, 1990). Thus, it seems
important to be able to dissociate the different processes
(synapsis, formation of chiasmata, segregation during the first
and second meiotic divisions) when studying the meiotic
behaviour of chromosomes. Analysis of the differences
2476 q The Author 2005. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved.
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Comparison of male and female meiotic segregation
between the meiotic products of females and males for a
given chromosomal rearrangement could provide clues as to
the respective roles of the mechanisms in operation.
Numerous experimental meiotic segregation studies have
been carried out in males using the ‘spermFISH’ technique
(fluorescent in situ hybridization on decondensed sperm
nuclei) (Guttenbach et al., 1997). In contrast, the limited
accessibility of human biological material has restricted the
study of female meiosis. The precise determination of female
segregation patterns in carriers of structural chromosomal
rearrangements has been rarely reported. The development of
preimplantation genetic diagnosis (PGD) has brought forth
new procedures for chromosomal segregation studies, such as
polar body chromosome painting analysis or comparative
genomic hybridization assays on biopsied blastomeres (e.g.
Munné et al., 1998; Wells and Delhanty, 2000). However,
the cytogenetic analysis of female gametes and embryos
remains extremely difficult in humans for ethical reasons.
Consequently, recourse to an animal model for the study of
female meiosis is of great interest. The main comparative
study of segregation products in male and female carriers of
identical reciprocal translocations has been performed in the
mouse, using analyses of metaphases I and II of spermatocytes and oocytes (Tease, 1998). The karyotypic structure of
the domestic pig (Sus scrofa domestica L.) being more similar to human than that of the mouse, this species could
provide another relevant animal model for studying the
mechanisms of meiosis in the presence of chromosomal
rearrangements. The females of this species are relatively
prolific (12 progeny per litter on average). The number of
oocytes (embryos) that can be analysed per female is therefore relatively high. In addition, the generation interval is
relatively short (, 2 years), and the experimental production
of individuals with particular karyotypes is possible at
reasonable expense. Finally, a programme for systematic control of the karyotypes of individuals destined for reproduction
exists in this species, which permits the identification of new
chromosomal rearrangements (5 –15 per year; incidence in
the order of 0.4%; Ducos et al., 2002b).
Here, we investigate the segregation patterns determined
from metaphase II oocyte samples obtained from sows heterozygotes for one of the two following reciprocal translocations:
38, XX, rcp(3;15)(q27;q13) and 38, XX, rcp(12;14)(q13;q21).
These data were compared with the segregation patterns previously determined in boars carrying the same chromosomal
rearrangements (Pinton et al., 2004). Hypotheses to interpret
the observed differences between the two translocations, as
well as between the male and female segregation patterns are
formulated and discussed.
Materials and methods
Animal material
The two reciprocal translocations were initially identified within the
national systematic control programme of young pedigree boars destined for use in artificial insemination centres (Ducos et al., 2002a).
The first was demonstrated in a Large White male and involved
chromosomes 3 and 15: rcp(3;15)(q27;q13) (Figure 1). The second
Figure 1. Metaphase of the boar heterozygote for the 3/15 translocation after painting with the chromosome 3 (green) and 15 (red)
probes. Bar = 10 mm.
Figure 2. Metaphase of the boar heterozygote for the 12/14 translocation after painting with the chromosome 12 (red) and 14 (green)
probes. Bar = 10 mm.
was identified in a boar belonging to a Duroc/Large White composite line and involved chromosomes 12 and 14: rcp(12;14)(q13;q21)
(Figure 2). The pachytene diagrams for these two rearrangements
are given in Figure 3.
Experimental inseminations were carried out, with each boar, to
obtain daughters that were heterozygous carriers of the rearrangement from the father. These carrier daughters were in turn crossed
with a boar having a normal karyotype. Thirteen female heterozygous carriers of the first rearrangement (daughters and granddaughters of the translocated boar) and 29 of the second were slaughtered
to collect the oocytes. The granddaughters were 8 – 12 months old
(slaughtered after detection of the first heat), and their mothers
20 – 24 months old. No special hormone treatment was administered
to these animals.
In vitro maturation of oocytes
The metaphase II oocytes were obtained after in vitro maturation of
oocytes (Marchal et al., 2001). Briefly, the oocytes were removed
from ovaries obtained from the abattoir. The cumulus– oocyte complexes (COC) from follicles of 3– 6 mm were initially washed four
times in phosphate-buffered saline þ0.5 mg/ml of bovine serum
albumin (Sigma, St Louis, MO, USA) þ 50 mg/ml of Gentamycin
(Sigma), then cultured in 500 ml of maturation medium composed of
culture medium 199 (Sigma) supplemented with epidermal growth
factor (10 ng/ml final; Sigma) and Cysteamine (570 mmol/l final;
Sigma). The duration of maturation was fixed at 44 h at 398C in a
5% CO2 oven.
2477
A.Pinton et al.
Figure 3. Schematic representations of the pachytene quadrivalents.
(A) 3/15 translocation: chromosome 3 is shown in black and
chromosome 15 is hatched. (B) 12/14 translocation: chromosome 12
is shown in black and chromosome 14 is hatched.
Harvesting of metaphase II oocytes
After maturation, the oocytes were mechanically cleaned of cumulus
cells (by sucking and blowing with a micropipette). Maturation was
determined by the presence of polar bodies at the oocyte periphery.
The mature oocytes were then spread on slides as described by
Tarkowski (1966).
Chromosome painting on metaphase II oocytes and lymphocytes
Two types of painting probes were used. For chromosomes 3, 12
and 14, the probes were produced from flow-sorted chromosomes
amplified using priming authorizing random mismatches (PARM) –
PCR (Schmitz et al., 1992; Yerle et al., 1993). The probe for
chromosome 15 was produced by chromosomal microdissection followed by degenerate oligonucleotide-primed (DOP) – PCR amplification (Telenius et al., 1992) of the microdissected material (Pinton
et al., 2003). Probe labelling was carried out either by PARM – PCR
or DOP – PCR using 2 ml of amplified chromosomal products. For
this, 50 ml of a reaction mixture consisting of: Taq 1 £ buffer; 2 IU
AmpliTaq (Applied Biosystems, Foster City, CA, USA); MgCl2
2 mmol/l; 0.2 mmol/l of each dAGC/TP; dTTP 20 mmol/l; biotin-16dUTP or digoxigenin-11-dUTP (Roche Applied Science, Meylan,
38, France) 100 mmol/l; 50 pmol of primers: (GAG)7 for PARM
PCR or 50 -CCGACTCGAGNNNNNNTGTGG-30 for DOP PCR;
and H2O quantity sufficient for (q.s.f.) were added to 2 ml of amplified chromosomal products. Twenty PCR cycles were performed
(568C, 1 min; 728C, 1 min; 948C, 1 min) followed by a terminal
elongation (728C, 5 min). Once mixed (i.e. either chromosomes 3
and 15 or chromosomes 12 and 14) and purified by G50 column
chromatography, the PCR products were precipitated in the presence
of 15 mg of porcine competitor DNA (Hybloc Competitor DNA;
Applied Genetics Laboratories, Melbourne, FL, USA). The probes
were then re-suspended in 25 ml of conventional hybridization
solution and 5 ml were placed (under a 22 mm £ 22 mm cover slip)
on each preparation that had been previously denaturated (in 70%
formamide at 708C for 2 min) and treated with proteinase K for
3 min. The biotin-labelled probes were revealed using a red Alexa
594 fluorochrome (Molecular Probes, Eugene, OR, USA), and the
digoxigenin-labelled probes using a green Alexa 488 fluorochrome
(Molecular Probes) as in the protocol described by Pinton et al.,
(2003).
Only the metaphase II oocytes showing clear and homogeneous
hybridization signals were further considered in this study. The presence of differentially labelled chromatids for the same chromosome
allowed the detection of interstitial crossing-over.
The same sets of probes and hybridization protocols were used
for chromosome painting on metaphases of the boars obtained from
lymphocyte cultures (Ducos et al., 2002b; Pinton et al., 2003)
(Figures 1 and 2).
Sperm preparation and analysis
The classical methods used to determine the male segregation patterns (fluorescent in situ hybridization on decondensed sperm heads:
‘spermFISH’) have been presented in detail by Pinton et al. (2004).
Data analysis
A conventional 2 £ 2 x2-test with Yates’ correction (Dagnélie,
1975) was used to compare the proportions of the different segregation products (comparison between translocations, for both males
and females, as well as comparison between males and females carrying the same translocation). P , 0.05 was considered statistically
significant.
Results
3/15 translocation
A total of 118 metaphase II oocytes were analysed by FISH
after in vitro maturation and spreading. Twenty-seven out of
the 118 were diploids (i.e. 22.9%). Some representative
pictures of the main meiosis I products are presented in
Figure 4. The relative proportions of the different female
segregation products are indicated in Table I. The results
obtained for the boar using spermFISH (3000 sperm studied:
Pinton et al., 2004) are given for comparison in Table III.
Analysis of the metaphase II oocyte sample revealed a
preponderance of products derived from simple alternate
segregation (64.8%). Interstitial crossing-over was detected
in 5.5% of the oocytes analysed (alternate/adjacent-I
Figure 4. Examples of metaphase II in oocytes of sows carrying the 3/15 translocation after painting with the chromosome 3 (red) and
chromosome 15 (green) probes. The occurrence of an interstitial crossing-over (alternate/adjacent-I product) is revealed by the presence of a
small red signal on chromosome 15 (green). Bar = 10 mm.
2478
Comparison of male and female meiotic segregation
12/14 translocation
Table I. Female segregation pattern for the 3/15 translocation
Segregation mechanism
No. of cells
(metaphase II oocytes)
%
Alternate (simple)
Adjacent-I (simple)
Alternate/adjacent-I
Including COa on chromosome 3
Including COb on chromosome 15
Adjacent-II
3:1
Total
59
1
5
3
2
13
13
91
64.8
1.1
5.5
3.3
2.2
14.3
14.3
100
The products were assigned to a segregation type, depending on whether
they arose from alternate, adjacent-I, adjacent-II and 3:1 orientations, or
were uninformative (alternate/adjacent-I) because of the occurrence of an
interstitial crossing-over.
a
Occurrence of a crossing-over (CO) in the interstitial region of
chromosome 3.
b
Occurrence of a crossing-over in the interstitial region of chromosome 15.
segregation) (Table I), 3.3% of which were located on segment IV (chromosome 3) and 2.2% on segment II (chromosome 15) (Figure 3A). The estimated total percentage of
balanced gametes was 67.6% (corresponding to the proportion of gametes derived from simple alternate segregation
plus half of the 5.5% derived from alternate/adjacent-I segregation). The chromosomally unbalanced gametes are mainly
derived from the adjacent-II and 3:1 segregations, in identical
proportions (14.3%).
Comparison of these results with those previously obtained
in a male carrier of the same rearrangement reveals important
differences in the segregation profiles between the two sexes
(Table III). The overall percentage of balanced gametes is
higher in females (67.6%) than in the male (52.2%,
P , 0.004). The modes of production of the chromosomally
unbalanced gametes are also different. In the male, these
unbalanced gametes are mainly produced by adjacent-I segregation (31.4%), whereas they are mainly derived from adjacent-II and 3:1 segregations in females: 14.3% for each,
compared with 3.8% = 1.1% þ (0.5 £ 5.5%) for the adjacent-I segregation. The percentages of the 3:1 segregation
products did not differ significantly between the males
(13.50%) and females (14.3%; P . 0.5).
Finally, no diploid spermatozoid were detected in the boar
semen, whereas 22.9% of the metaphase II oocytes were
diploids.
A total of 206 metaphase II oocytes were analysed by FISH
after in vitro maturation and spreading; of these, 11 (5.3%)
were diploids. Some representative images of the main meiosis I products are presented in Figure 5. The relative proportions of the different female segregation products are
indicated in Table II. As for the previous translocation, the
results obtained in the boar using spermFISH (3006 sperm
studied: Pinton et al., 2004) are given for comparison in
Table III.
Large differences can be observed between this and the
previous translocation. The proportion of metaphase II
oocytes derived from simple alternate segregation is much
lower (40.5% compared with 64.8%; P , 0.001). In contrast,
formation of interstitial crossing-over was detected much
more frequently (37.9% compared with 5.5%; P , 0.001).
Most of these (37.4%) are located on segment II (chromosome 14) with a much lower percentage (0.5%) on segment
IV (chromosome 12) (Figure 3B). The percentages of
gametes derived from adjacent-II and 3:1 segregations were
very low (2.1% for each category) and much lower than
those estimated for the previous translocation (14.3% for
each category; P , 0.001). Overall, the proportions of
balanced gametes were relatively similar for both translocations (67.6% for the 3/15 translocation compared with 59.4%
for the 12/14 translocation; P . 0.27).
As with the previous translocation, large differences
were observed between the meiotic segregation profiles of
the two sexes (Table III). The total proportion of balanced
gametes was lower in the females than in the males
(59.4% = 40.5% þ 0.5 £ 37.9%, compared with 75.9%;
P , 0.001). The adjacent-I segregation was the main mode of
production of unbalanced gametes in both sexes, but was more
frequent in the females (36.4% = 17.4 þ 0.5 £ 37.9%) than in
the males (14.9%; P , 0.001). In contrast, the estimated percentage of adjacent-II segregation products was higher in the
males (5.7%) than in females (2.1%; P , 0.05). The percentages of the 3:1 segregation products did not differ significantly
between the male (3.5%) and females (2.1%) (P . 0.38).
A single diploid spermatozoid had been identified out of the
3006 analysed in the semen of a boar carrier of translocation
12/14. The estimated percentage was higher (5.3%) in the
oocyte samples.
Figure 5. Examples of metaphase II in oocytes of sows carrying the 12/14 translocation after painting with the chromosome 12 (red) and
chromosome 14 (green) probes. The occurrence of an interstitial crossing-over (alternate/adjacent-I product) is revealed by the presence of a
small red signal on chromosome 14 (green). Bar = 10 mm.
2479
A.Pinton et al.
Table II. Female segregation pattern for the 12/14 translocation
Segregation mechanism
No. of cells
(metaphase II oocytes)
%
Alternate (simple)
Adjacent-I (simple)
Alternate/adjacent-I
Including COa on chromosome 12
Including COb on chromosome 14
Adjacent-II
3:1
Total
79
34
74
1
73
4
4
195
40.5
17.4
37.9
0.5
37.4
2.1
2.1
100
The products were assigned to a segregation type depending on whether
they arose from alternate, adjacent-I, adjacent-II and 3:1 orientations, or
were uninformative (alternate/adjacent-I) because of the occurrence of an
interstitial crossing-over.
a
Occurrence of a crossing-over (CO) in the interstitial region of
chromosome 12.
b
Occurrence of a crossing-over in the interstitial region of chromosome 14.
Table III. Comparison of male and female segregation patterns
Segregation products
Translocation
3/15
Alternateb
Adjacent-Ib
Adjacent-II
3:1
12/14
a
Female
Male
Female
Malea
67.6
3.8
14.3
14.3
52.2
31.4
2.9
13.5
59.4
36.4
2.1
2.1
75.9
14.9
5.7
3.5
Values are percentages.
a
Data from Pinton et al. (2004)
b
The gametic products originating from alternate and adjacent-I segregations
cannot be fully distinguished for the males. Therefore, the segregation
products indicated ‘alternate’ correspond to balanced sperm originating from
simple alternate segregation, or from adjacent-I segregation with interstitial
crossing-over. Those indicated ‘adjacent-I’ correspond to unbalanced sperm
originating from simple adjacent-I segregation, or from alternate segregation
with interstitial crossing-over.
Discussion
Comparison of the segregation patterns between
translocations
The differences between the segregation profiles for the two
reciprocal translocations studied are significant, in both males
and females. Hypotheses for the origin of these differences
may be inferred from the analysis of the pachytene diagrams.
As initially suggested by Jalbert et al. (1980), an asymmetrical pachytene diagram, i.e. presence of a short (translocated)
chromosome, could lead to the formation of a III þ I configuration in prophase I (i.e. one trivalent þ one univalent),
due to a failure either in the pairing process or in the chiasma
formation on this short chromosome, which would increase
the risk of 3:1 segregation. Such a III þ I configuration is
expected for the 3/15 translocation (Figure 3A). Conversely,
in the case of the 12/14 translocation, analysis of the pachytene diagram reveals relatively large centric and translocated
segments (Figure 3B), compatible with a ring configuration
in prophase I. With this type of configuration, 2:2
segregations (alternate or adjacent) would be most common
(Goldman and Hultén, 1993a,b). The results obtained for the
two translocations studied (higher proportion of 3:1
2480
segregation products in the case of the 3/15 translocation)
conform with the model of Jalbert et al. (1980) and the
results of Goldman and Hultén (1993a,b). However, even for
the 3/15 translocation, the relative proportions of 3:1 segregation products remain quite low (, 15%). This is in agreement with earlier results showing that the small size of the
chromosomal segments is not necessarily a limiting factor to
produce a ring configuration during meiosis I. Indeed, even
in the cases with very short pairing segments, the incidence
of 3:1 segregations can be low (Templado et al., 1990;
Oliver-Bonet et al., 2004). One possible explanation would
be that the small physical size of some translocated chromosomal segments could be partially overcome by a decrease in
interference along the chromosomes involved in the translocation (Arana et al., 1980).
In the case of the 12/14 translocation, the frequency of
interstitial crossing-over is higher on chromosome 14 than on
chromosome 12. For the 3/15 translocation, this frequency is
highest on chromosome 3. These results are consistent with
the hypothesis of a positive correlation between the size of
the chromosomal segments and the probability of crossingover (Figure 3).
Comparison of the segregation patterns between sexes
The 11q;22q translocation seems to be the only recurrent
reciprocal translocation in humans. Preimplantation genetic
diagnosis was carried out for several couples carrying this
particular chromosomal rearrangement. To our knowledge,
this is the unique source of data allowing a direct comparison
of segregation patterns between males and females carrying
an identical reciprocal translocation in humans (Van Assche
et al., 1999; Mackie Ogilvie and Scriven, 2002). However,
due to the very low number of embryos studied, the significance of these results is rather limited. Still in humans, it
would be worthwhile to consider PGD data on female and
male translocation carriers even though the comparison
relates to different translocations. In a first review of 35
cases of PGD of translocations with several methods, Munné
et al. (2000) found no differences in the rates of chromosome
abnormalities between male and female carriers. So far there
have been close to 500 PGD cycles for translocations performed worldwide (Verlinsky et al., 2004), and the initial
conclusions seem to be confirmed. This would indicate that,
on average, male and female meioses generate comparable
proportions of unbalanced gametes in the case of reciprocal
translocations. However, such data should be considered cautiously because patients undergoing PGD usually have a
history of recurrent miscarriage which could bias the results
obtained.
The most complete and important study aimed at comparing segregation patterns for males and females carrying
identical reciprocal translocations has been carried out in
the mouse (Tease, 1998). Very different segregation patterns
between males and females were demonstrated in this study.
Our results, obtained in another mammalian species, also
reveal marked differences between the two sexes, for the two
translocations studied. In the reciprocal translocation
rcp(12;14)(q13;q21) for instance, the proportion of products
Comparison of male and female meiotic segregation
derived from alternate segregation (i.e.chromosomally
balanced) was significantly higher in the male than in
females. For translocation, rcp(3;15)(q27;q13), the production
of unbalanced gametes was very different between the two
sexes, with a significantly higher proportion of products
derived from adjacent-I segregation in males than in females.
These results confirm those previously reported by Tease
(1998) in the mouse. They may partially be explained by
differences in the frequency and localization of chiasmata
(crossing-over) between the two sexes. Indeed, both parameters determine the meiotic configurations (ring, chain
etc.) which form during prophase of meiosis I, as well as the
orientation of multivalents, and consequently affect the types
of segregation which occur, as already mentioned (Goldman
and Hultén, 1993a,b; Tease, 1996, 1998). As postulated by
McClintock (1945), the presence of an interstitial chiasma
might have a direct effect on the percentage of products
derived from adjacent-II segregation (Faraut et al., 2000).
Such an event favours the co-orientation of homologous centromeres to the opposite poles, thus limiting the probability
of adjacent-II segregation. Globally, the frequency of
adjacent-II segregations is inversely proportional to the frequency of crossing-over on interstitial segments (Rickards,
1983). In our study, the translocation 12/14 exhibits the highest frequency of interstitial crossing-over (37.9%, compared
with 5.5% for the 3/15 translocation; P , 0.001; Tables I
and II). Therefore it is logical that the lowest proportion of
adjacent-II segregation is observed for this translocation
(2.1%, compared with 14.3% for the 3/15 translocation;
P , 0.001). Genetic mapping data (e.g. Broman et al., 1998)
and localizations of chiasmata by immunohistochemistry in
humans (Barlow and Hulten, 1998) and mice (Anderson
et al., 1999; Froenicke et al., 2002), have indicated that the
frequency of crossing-over is higher and their distribution
more interstitial in females than in males. A lower proportion
of adjacent-II segregation products would therefore be
expected in females than in males. This was observed in the
case of the 12/14 translocation. In contrast, a reverse distribution was observed for the 3/15 translocation.
Finally it can be noted that the proportions of 3:1 segregation products in males and females for both translocations
are not significantly different. These results are consistent
with those obtained by Tease (1998) in mice but disagree
with the hypothesis of a strong predisposition of the female
meiosis to 3:1 segregation (Faraut et al., 2000; Mackie
Ogilvie and Scriven, 2002). For the 12/14 translocation, as
already mentioned, the symmetrical pachytene configuration
(long pairing segments) as well as the relatively high proportion of interstitial chiasmatas (37.9%) clearly favours the
occurrence of 2:2 segregations. This leads to a very low proportion of 3:1 segregation products, even in the females, and
can explain the lack of difference between the two sexes. In
constrast, in the 3/15 translocation, a higher proportion of
III þ I configuration in prophase I is expected. In such a
situation, a non-negligible proportion of 3:1 segregation products should be observed. Moreover, this proportion should
be higher in females than in males because of the apparent
predisposition of female meiosis to produce this kind of
imbalance (lower meiotic quality controls: Hunt and Hassold,
2002). However, a high chiasma frequency in the translocated and interstitial segments may incline the prophase I
meiotic configurations towards chain-IV and ring configurations instead of the expected III þ I, and therefore may
incline the segregation towards 2:2 segregations instead of
3:1. The chiasma frequency being more important in females
than in males (especially in the interstitial segments), this
could partially compensate the basic predisposition of the
female meiosis to produce 3:1 segregation products, and partially explain the low difference between the two sexes.
Another possible explanation could be the relatively young
age of the sows used in our study. Indeed, the data showing
that human female meiosis appears highly error-prone (postnatal and gametic studies) were obtained on adult women
whose oocytes were arrested in prophase I for a very long
period (generally . 20 years). This is not the case of the
females used in our study, the majority of which have been
slaughtered just after puberty before 1 year of age.
The estimated frequency of diploid oocytes was relatively
high in the case of the 3/15 translocation (22.9%). Although
this value is much higher than that of the 12/14 translocation
(5.3%; P , 0.001) or than several results obtained in humans
(e.g. 3.5%, Plachot, 2001; 5.4%, Pellestor et al., 2002) it
is nevertheless consistent with earlier results obtained by
Vozdova et al. (2001) and Sosnowski et al. (2003) in sows
with normal karyotypes (27.7 and 12.8% respectively). Otherwise, the higher diploidy rate observed in females compared
to males for both translocations suggests a more stringent
meiotic quality control affecting spermatocytes as compared
with oocytes during the meiotic process (Hunt and Hassold,
2002).
In conclusion, this study is a new scientific contribution to
compare the segregation profiles of male and female carriers
of identical chromosomal rearrangements, a topic that has
been rarely investigated to date. This approach could be
extended to an analysis of other translocations in pigs, even
of other types of chromosomal rearrangements (such as
inversions). An interesting complement to this work could be
provided by new immunohistochemical techniques, based on
the use of anti-MLH1 and anti-SCP3 antibodies, which allow
an accurate determination of the distribution of crossing-over
in oocytes and spermatocytes (Baker et al., 1996). This
would enable the effects of the rearrangements on chiasma
distribution in the two sexes to be analysed and compared.
Also, complementary studies difficult to carry out in humans
could be envisaged in pigs, such as the analysis of the effect
of age on male and female segregation profiles in the presence of a chromosomal rearrangement, or the investigation
of inter-individual variability of male segregation profiles.
Acknowledgements
We thank the reviewers for their helpful comments on the manuscript. We thank the pig selection organizations and breeders for
their assistance in animal reproduction, rearing and sampling, and
are grateful to Pascal Mermillod (Oocytes and Development team at
the station of Physiology of Reproduction and Behaviour at INRA
Nouzilly) for training in the in vitro maturation of oocytes
technique.
2481
A.Pinton et al.
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Submitted on February 7, 2005; resubmitted on April 1, 2005; accepted on
April 11, 2005