129
Development 103, 129-136 (1988)
Printed in Great Britain © The Company of Biologists Limited 1988
Two maternally derived X chromosomes contribute to parthenogenetic
inviability
JEFF R. MANN1* and ROBIN H. LOVELL-BADGE2
'Murdoch Institute for Research into Birth Defects, Royal Children's Hospital, Flemington Road, Parkville, Victoria 3052, Australia
MRC Mammalian Development Unit, Wolfson House (University College London), 4 Stephenson Way, London NW1 2HE, UK
2
^Present address: Department of Cell and Develomental Biology, Roche Institute of Molecular Biology, Nutley, New Jersey, 07110.
USA
Summary
In certain extraembryonic tissues of normal female
mouse conceptuses, X-chromosome-dosage compensation is achieved by preferential inactivation of the
paternally derived X. Diploid parthenogenones have
two maternally derived X chromosomes, hence this
mechanism cannot operate. To examine whether this
contributes to the inviability of parthenogenones, XO
and XX parthenogenetic eggs were constructed by
pronuclear transplantation and their development
assessed after transfer to pseudopregnant recipients.
In one series of experiments, the frequency of postimplantation development of XO parthenogenones
was much higher than that of their XX counterparts.
This result is consistent with the possibility that two
maternally derived X chromosomes can contribute to
parthenogenetic inviability at or very soon after implantation. However, both XO and XX parthenogenones showed similar developmental abnormalities
at the postimplantation stage, demonstrating that
parthenogenetic inviability is ultimately determined
by the possession of two sets of maternally derived
autosomes.
Introduction
ential gene modifications or 'imprinting' presumably
imparted during gametogenesis (Cattanach & Kirk,
1985). These modifications could involve DNA methylation (Swain et al. 1987; Reik et al. 1987; Sapienza et
al. 1987).
It is not known at how many loci imprinting is
important for development, or the time at which each
becomes critical. The only mechanism involving
imprinting known to occur around the time of implantation is the preferential inactivation of the paternally
derived X chromosome, X p , in the trophectoderm
and primitive endoderm of XX blastocysts (Takagi et
al. 1978; Frels et al. 1979; Frels & Chapman, 1980;
Papaioannou & West, 1981; Harper et al. 1982). This
mechanism cannot operate in parthenogenones or
gynogenones which have two maternally derived X
chromosomes, XM, and therefore may determine the
characteristically poor development of trophoblast in
postimplantation-stage parthenogenones and gynogenones (Endo & Takagi, 1981; Surani et al. 1984,
1987). This poor development of trophoblast has
been shown to impair significantly the development
Diploid parthenogenetic 1-cell mouse eggs possess
two maternally derived haploid pronuclei. This is also
the case in diploid gynogenetic 1-cell eggs (fertilized
eggs in which there is no contribution of paternal
chromosomes). Both these types of eggs can undergo
a high frequency of development to the blastocyst
stage and the majority undergoes implantation. However, only a few develop to postimplantation stages,
reaching at most about the 25-somite forelimb-bud
stage (Kaufman & Gardner, 1974; Kaufman et al.
1977; Surani & Barton, 1983). Their abnormalities
result from the presence of two maternally derived
genomes (Mann & Lovell-Badge, 1984; Surani et al.
1984; McGrath & Solter, 1984), and parthenogenones
and gynogenones are therefore potentially subject to
aH deleterious effects involving maternal duplication/
paternal deficiency of certain chromosome regions
(Searle & Beechey, 1978). In these regions, there
appears to be a differential expression of maternal
and paternal gene homologues, determined by differ-
Key words: X chromosome, maternally derived X
chromosome, parthenogenones, mouse embryo, dosage
compensation, preferential inactivation, inviability.
130
J. R. Mann and R. H. Lovell-Badge
of the embryo-proper (Barton et al. 1985).
To investigate whether two XM chromosomes contribute to the inviability of gynogenones, we previously studied the developmental potential of X M O
gynogenetic eggs (Mann & Lovell-Badge, 1987). In
these eggs, X-dosage-related functions should be
essentially normal, as X M O fertilized eggs are viable.
Thus, any developmental abnormalities in X M O
gynogenones could be attributed to the lack of
paternally derived autosomes, A p . XO gynogenetic
eggs showed the same mortality at or very soon after
implantation as did their XX counterparts, showing
that a lack of A F chromosomes was sufficient to cause
gynogenetic inviability at this stage. Nevertheless, as
suggested, these results could not preclude the possibility that the presence of two XM chromosomes
might also have a deleterious effect. Furthermore,
postimplantation development of XO or XX gynogenones was not obtained. Therefore it was not
possible to determine whether normal X dosage could
result in an improved development of gynogenetic
trophoblast.
We have now extended these studies to XO and
XX parthenogenetic eggs. Postimplantation development was obtained from some XO and XX eggs,
allowing for a comparison of their developmental
potential during embryonic development.
Materials and methods
Females more than 8 weeks of age were superovulated by
intraperitoneal injection of 5i.u. pregnant mare serum
gonadotrophin (Folligon; Intervet) followed about 48 h
later with 5i.u. human chorionic gonadotrophin, hCG
(Chorulon; Intervet). Ethanol-induced parthenogenetic activation of cumulus-denuded eggs was carried out 16-17 h
after hCG injection (Barton et al. 1985) to produce haploid
parthenogenetic 1-cell eggs (one pronucleus, 2nd polar
body). The frequency of activation was typically greater
than 80%. Eggs were cultured in drops of medium M16
(Whittingham, 1971) under light paraffin oil (British Drug
Houses) at 37°C in 5 % CO2 in air. Pronuclei were
transplanted according to McGrath & Solter (1983) as
modified slightly by Mann & Lovell-Badge (1987). The
procedure involves fusion of an egg fragment, consisting of
a pronucleus surrounded by a small amount of cytoplasm
and plasma membrane, with an egg utilizing Sendai virus
plasma membrane fusion. Eggs were transferred at the
2-cell stage to the oviducts of recipients on day i of
pseudopregnancy (day of a vaginal plug after mating to
vasectomized males of proven sterility). Phosphoglycerate
kinase-1, PGK-1, electrophoresis was carried out according
to Biicher et al. (1980). The enzyme was localized using the
visible tetrazolium-linked staining procedure of Oelshlegel
& Brewer (1972).
Source of O eggs
Females [hereafter termed In(X)/X] heterozygous for an X
with a large inversion, In(X)lH (Evans & Phillips, 1975),
were used as a source of eggs lacking an X chromosome.
Karyotypic analyses have shown that they produce eggs of
which about 22 % are O, 72 % carry an X and 6 % carry an
X dicentric (Phillips & Kaufman, 1974; P. S. Burgoyne,
personal communication; Mann & Lovell-Badge, 1987).
Unless otherwise stated, In(X)/X females were produced
by mating In(X)/Y males (Murdoch Institute colony;
original breeding pairs kindly provided by Dr Paul Burgoyne, MRC Mammalian Development Unit, UK) to
C57BL/6J females.
Estimation of the proportion of O and X
parthenogenetically activated haploid eggs of
ln(X)/X females
The following experiment was carried out in order to assess
whether the proportion of O and X eggs produced by
In(X)/X females remained similar following parthenogenetic activation. In(X)/X females were superovulated and
their eggs parthenogenetically activated. Fertilized eggs
were obtained from (C57BL/6J X CBA/CaH)F, (hereafter
termed B6CBF|) females superovulated 4—5 h later than
In(X)/X females and mated to males carrying the X-linked
gene Pgk-la (coding for the A electrophoretic form of
PGK-1). In(X)/X females were homozygous Pgk-lh. Diploid eggs were then constructed by transplanting the paternal pronucleus of fertilized eggs to haploid parthenogenetic eggs of In(X)/X mice. Following micromanipulation,
the eggs were cultured overnight to cleave to 2-cells, then
transferred to B6CBF, or ARC Swiss recipients (Animal
Resources Centre, Murdoch, Western Australia; foundation stock of ARC Swiss mice were CD-I mice obtained
from Charles River Breeding Labs, USA). These were
autopsied at days 13|-15£ of gestation. Fetuses were sexed
by gonad morphology and the PGK-1 allozyme type of
female fetuses determined. Female PGK-1 A fetuses would
be derived from O haploid parthenogenetic eggs, whereas
all other types of fetuses would be derived from X eggs.
Construction and transfer of diploid XO and XX
parthenogenetic eggs
Two series of experiments were carried out.
Series I
This series of experiments were conducted at the MRC
Mammalian Development Unit, UK (all other experiments
described in this paper were carried out at the Murdoch
Institute). In(X)/X females were produced by mating
In(X)/Y males (MRC Mammalian Development Unit
colony) to MF1 (OLAC, UK) females. In(X)/X and
(CBA/CaH X C57BL/6J)F, (hereafter termed CBB6F,)
females were superovulated, killed 16 h after hCG injection, then cumulus-intact eggs parthenogenetically activated by ethanol treatment (Kaufman, 1982; Cuthbertson,
1983). Diploid eggs were then constructed by transplanting
the pronucleus of a haploid egg of an In(X)/X mouse to a
haploid egg of a CBB6F| mouse. The transplantation was
done in this fashion as eggs of CBB6F, mice were in
apparently better condition than those of In(X)/X (MF1
hybrid) mice following ethanol activation. If In(X)/X
females produce about 22% O, 72% X and 6% X
Two XM chromosomes and parthenogenetic inviability
131
Table 1. Development of haploid parthenogenetic eggs of In(X)/X mice into which a paternal pronucleus was
transplanted
Number of eggs
Transferred'
Implanted
Developed
postimplantation
101
76 (75 %)
36 (36 %)
Male
Female
4
10
2t
20
PGK-1
type
Classification
of egg/paternal
pronucleus
A
AB
B
n.d.
O/X
X/X
X/O
X/Y
n.d. = not done.
* Into recipients that became pregnant.
t These females presumably resulted from the loss or absence in the paternal pronucleus of a sex chromosome and have been
observed in previous experiments (Mann & Lovell-Badge, 1987; Mann, 1987).
dicentric eggs, then these diploids would be in the expected
proportions of 22% XO (viability?), 72% XX (lethal
before 12i days of gestation) and 6 % X dicentrics [periimplantation lethal (P. S. Burgoyne, personal communication)]. Eggs were cultured overnight to cleave to 2-cells,
then transferred to CBB6F| recipients which were autopsied at 9 i - l l i days of gestation. Postimplantation-stage
parthenogenetic embryos were dissected from the uterus
and XOs and XXs identified by chromosome counts.
Metaphase spreads were made from the egg cylinder alone,
or together with the yolk sac (Burgoyne et al. 1983). XO
embryos possess 39 chromosomes. No other monosomy in
mice is known to develop after implantation (Epstein,
1985).
they were cultured for 3h in 5-0^gml ' cytochalasin B to
suppress 2nd polar body formation, washed in 2 ml of
medium for 5min, then passed through five drops of
medium under oil. They were cultured for a further 5h,
after which diploid eggs (possessing two pronuclei and no
2nd polar body) were selected (between 50 % and 70 % of
the total) and cultured overnight to cleave to 2-cells.
Usually, five 2-cell eggs per oviduct were transferred to
(ARC Swiss x BALB/c)F, recipients using medium PB1
(Whittingham & Wales, 1969). These were autopsied at day
9i of gestation.
Results
Series II
Diploid parthenogenetic 1-cell eggs were constructed by
transplanting the pronucleus of a haploid egg of an In(X)/X
mouse to another of the same. The In(X)/X mice used were
C57BL/6J hybrids, from which haploid pronuclear eggs of
good quality were obtained following parthenogenetic activation. The constructed diploids would be in the expected
proportions of 32 % XO, 52 % XX, 11 % X dicentrics and
5 % OO (preimplantation lethal). Eggs were cultured
overnight to cleave to 2-cells, then transferred to B6CBF,
or ARC Swiss recipients. These were autopsied at day 9i of
gestation. XO and XX parthenogenetic embryos were
identified as described above. As only these are able to
develop after implantation, the probability of an embryo
being XO or XX, given that XOs and XXs are equally
viable, would be 0-32x1/0-84 = 0-38 and 0-52x1/0-84 =
0-62, respectively.
Production and transfer of diploid XX
parthenogenetic eggs produced by suppressing 2nd
polar body extrusion
This was carried out in order to assess the frequency of
postimplantation development that could be obtained in
diploid parthenogenetic eggs produced by suppressing 2nd
polar body extrusion and not involving pronuclear transplantation. Immediately following exposure of eggs of
B6CBF| females to ethanol for parthenogenetic activation,
Estimation of the proportion of O and X
parthenogenetically activated haploid eggs of
In(X)/X females
In transplanting a paternal pronucleus into haploid
parthenogenetic eggs of In(X)/X mice, fusion of the
egg fragment occurred in 119 out of 143 (83 %)
micromanipulated eggs. All but one cleaved to
2-cells. They were transferred to 12 recipients. One
did not possess implantations at autopsy. The frequency of implantation and postimplantation development of these eggs, and the number of fetuses of
each type obtained is shown in Table 1. As four
female PGK-1 A fetuses (derived from O eggs) were
obtained, at least four OY eggs would also have been
expected, these being preimplantation lethal. 32
fetuses that must have been derived from X eggs were
also obtained. Thus, the ratio of O to X haploid
parthenogenetic eggs produced that were able to
participate in normal development was 8:32 = 1:4.
This ratio is similar to that expected from previous
estimates of the proportion of O and X eggs produced
by In(X)/X females, i.e. 22% O and 72% X eggs;
1:3-3. Hence, it is apparent that ethanol activation of
eggs of In(X)/X females does not result in markedly
132
J. R. Mann and R. H. Lovell-Badge
Table 2. Development of diploid XO and XX parthenogenetic eggs
Postimplantation-stage embryos obtained
Number of eggs
Transferred*
Implanted
Genotype
116(86%)
XO
XX
Developmental
stage
Number
obtained
(Recipient No.t)
Series I
135
(30 XO, 97 XX)t
9
25 somites
8 somites
16 somites
1(0
1(1)
1(2)
Series II
108
(35 XO, 56 XX)t
72 (67 %)
XO
XO
XO
XO
XO
XO
egg cylinder-like
head fold
14 somites
16 somites
20 somites
25 somites
XX
20 somites
egg cylinder-like
5 ( 3 , 3 , 5 , 8 , 14)
1(5)
1(12)
1(11)
3 (6, 9. 13)
6 ( 5 . 7 . 8 , 9 . 10. 10)
17
1(4)
4 (3. 4, 4. 6)
* Into recipients that possessed implantations at autopsy.
t Expected numbers of XO and XX eggs transferred, calculated from the expected proportions of O and X eggs present (see
Materials and methods section).
$ Recipients 1 and 2 were CBBF,, 3-7 ARC Swiss and 8-14 B6CBF,.
Series I. Fusion of the egg fragment occurred in 177 out of 208 (85 %) micromanipulated eggs. One did not cleave to 2-cells. These
were transferred to twenty-eight recipients. Seven did not possess implantations of autopsy. XO embryo; chromosomes were counted
using the yolk sac (of twenty-seven spreads, twenty-three had 39, and the remainder had 34-38 chromosomes each). XX embryo;
chromosomes were counted using the whole embryo (of twenty-two spreads, eighteen had 40, and the remainder had 39, 38 or 36
chromosomes each). The unclassified 16-somite embryo was necrotic; chromosome counts were not attempted.
Series II. Fusion of the egg fragment occurred in 122 out of 137 (89%) micromanipulated eggs. Two did not cleave to 2-cells. 108
were transferred into 17 recipients. One did not possess implantations at autopsy and twelve eggs were not transferred due to a lack of
recipients on one occasion. The nine egg-cylinder-like embryos were very small and disorganized in appearance. Four of these could
not be identified as the spreads were of insufficient quality. Fifteen embryos were classified as XO according to the presence 39
chromosomes in at least ten spreads, with no counts of 40; the mean number of additional spreads counted (all with 36-3S
chromosomes) was 1-9. Two more were classified as XO. One was egg-cylinder-like, in which only five spreads could be counted; all
had 39 chromosomes. The other was a 20-somite embryo in which 40 spreads had 39 chromosomes, with one of four non-modal
spreads having 40. One embryo was classified as XX; of eight spreads counted, seven had 40 chromosomes and one had 39.
different proportions of O and X parthenogenetic
eggs compared to fertilized eggs.
Development of diploid XO and XX
parthenogenetic eggs
Series I
Results are shown in Table 2. Only three postimplantation-stage embryos were obtained. Two of these,
the 25-somite and 8-somite embryos, were present in
the same uterine horn of a recipient autopsied at 91
days.
Series II
22 postimplantation-stage embryos were obtained.
Chromosome counts were made in 18 of these; 17
were XO and one was XX. Given that the probability
of an embryo being XO or XX is 0-38 and 0-62,
respectively, given that they are equally viable (see
Materials and methods section), then the probability
of obtaining 17 or more XOs out of 18 embryos
is (0-38)17(0-62) x 18!/17!l! + (0-38)'H = 8-3 x 1CT7.
Where more than one embryo was obtained in a
recipient, there was no obvious tendency for these to
be similar in developmental stage. In all embryos, the
development of the trophoblast was very limited,
often represented by only a small piece of necrotic
tissue. A distinctive Reichert's membrane was often
present, as has been observed previously (Surani et
al. 1984), that could be easily separated from the
decidua. This could be attributed to the sparse
development of trophoblast. The extent of development of the yolk sac appeared consistent with that of
the embryo proper. Three of the somite-stage parthenogenetic embryos obtained in the Series II experiments are shown in Fig. 1.
Two XM chromosomes and parthenogenetic inviability
133
t,Rm
Fig. I. Somite-stage parthenogenetic embryos obtained in the series II experiments. (A,B) 25-somite XO embryos.
(C) 20-somite XX embryo. Trophoblast (t), Reichert's membrane (Rm), yolk sac (ys) and embryo-proper (e). Gross
abnormal features were their small size (about half the dimensions of normal embryos) and a poor development of
trophoblast tissue. A curious finding was the large structure observed in the most advanced XO embryos, probably the
allantois (indicated by arrows). Bar, 10mm.
Table 3. Development of diploid XX
parthenogenetic eggs produced by suppressing 2nd
polar body extrusion
Number of eggs
Transferred*
Implanted
16t§
581-H
12 (75 %)
27 (47 %)
53 (90 %)
Implantation sites
with embryonic
derivates
* Into recipients that possessed implantations at autopsy.
t Activation of cumulus-intact eggs.
t Activation of cumulus-free eggs.
§ Unilateral and || bilateral transfer into recipients 15 weeks of
age that had reared one litter.
H Bilateral transfer into 8- to 12-week-old nulliparous
recipients.
Development of diploid XX parthenogenetic eggs
produced by suppressing 2nd polar body extrusion
The frequency of implantation and postimplantation
development of these eggs is shown in Table 3. Of 133
eggs transferred to recipients, only three implantation sites were obtained in which postimplantation
development was evident. However, only yolk sac
vesicles had developed.
Discussion
Two series of experiments were conducted in order to
determine whether a difference exists in the developmental potential of diploid XO and XX parthenogenetic eggs. In the experiments of series I, the majority
of XO and XX parthenogenetic eggs did not develop
to postimplantation stages. This result was similar to
that obtained with XO and XX gynogenetic eggs
(Mann & Lovell-Badge, 1987), and demonstrates that
in these experiments the lack of A p chromosomes was
sufficient to cause the inviability of parthenogenones
at this stage. However, in the experiments of series II,
the frequency of postimplantation development of
XO eggs (1 out of an expected 56 transferred) was
much greater than in XX eggs (17 out of an expected
134
J. R. Mann and R. H. Lovell-Badge
35 transferred). The probability of obtaining this
result by chance is extremely low (see Results section). Therefore, it can be safely concluded that in the
experiments of series II, XO blastocysts were better
able to continue development after implantation than
XX blastocysts. Considering that ethanol activation
can cause aneuploidy in up to 19 % of haploid
parthenogenetic eggs (Kaufman, 1982) and that aneuploidy often results in death at implantation (Epstein,
1985), the frequency of postimplantation development of euploid XO eggs was likely to be as high as
that obtained following transplantation of pronuclei
between synchronous fertilized eggs, i.e. 67 % (Mann
& Lovell-Badge, 1987).
In the experiments of series II then, the increased
frequency of postimplantation development of XO
compared to XX parthenogenones must have
depended on the absence of a second X M , this being
the only difference between them. It is evident
therefore that two XM chromosomes can contribute
significantly to parthenogenetic inviability, the effect
being manifest at implantation. This result is consistent with XX parthenogenones having difficulties in
achieving normal X-dosage compensation in the
trophectoderm and primitive endoderm, presumably
because of a failure of X inactivation in these tissues
where in normal conceptuses Xp is preferentially
inactivated. If this explanation is correct, then the
survival of some XX parthenogenones beyond implantation could be due to just enough trophectoderm and primitive endoderm cells managing to
inactivate an X to enable further development. Also,
the higher frequency of postimplantation development of XX parthenogenetic blastocysts subjected to
implantational delay (Kaufman et al. 1977), may
result from the longer time available to inactivate an
X, or from an increased number of cells with an
inactive X resulting from an overall increase in
delayed blastocyst cell number (Copp, 1982). These
explanations are in keeping with observations of a
high proportion of cells with an inactive X in the yolksac endoderm (derived from the primitive endoderm)
in postimplantation-stage XX parthenogenones (Rastan et al. 1980; Endo & Takagi, 1981).
From the above, it could be predicted that the few
androgenones that develop to postimplantation
stages (Barton et al. 1984) may be of an XPY
constitution, whereas XPXP androgenones may be
less viable due to difficulties in achieving normal Xdosage compensation at the time of trophectoderm
and primitive endoderm formation.
The abnormal development observed in the postimplantation-stage XO parthenogenones was essentially equivalent to that reported previously in their
XX counterparts. However, some retardation of XOs
relative to XXs would have been expected, as XO
embryos derived from fertilized eggs are slightly
retarded in comparison to their XX counterparts at 7£
days of gestation (Burgoyneef al. 1983). Significantly,
the trophoblast was very poorly developed (Fig. 1) as
has been described previously in X M X M A M A M eggs
(Surani et al. 1984) (where A refers to autosomes).
Thus, it is apparent that the presence of two XM
chromosomes does not contribute significantly to the
characteristically poor development of trophoblast,
or any other developmental abnormality in postimplantation-stage XX parthenogenones. Rather, as
M
0 A M A M eggs
X M O A M A P eggs a r e vjable? a n d X
are not, these abnormalities can be attributed to a
lack of A p chromosomes.
Why was the frequency of postimplantation development of X M OA M A M eggs high in the series II
experiments, yet low in two previous sets of experiments (Mann & Lovell-Badge, 1987 and series I)? It
should first be noted that X M X M A M A M eggs are also
variable in terms of the frequency that they will
develop to postimplantation stages, as well as in the
degree of developmental normality attained. Low
frequencies of postimplantation development have
been consistently obtained by the authors (Mann &
Lovell-Badge, 1984, 1987 and in the present experiments) following transfer of 2-cell eggs to oviducts,
and have been also been reported by others following
transfer of blastocysts to the uterus (Tarkowski et al.
1970; Witkowska, 1973; Kaufman & Gardner, 1974).
In contrast, a frequency of 25% development to
somite stages has been reported by Kaufman et al.
(1977) after inducing delayed implantation in blastocysts and up to 22 % developing to various postimplantation stages by Surani & Barton (1983) and
Surani et al. (1984) following transfer of 2-cell eggs to
oviducts. Concerning the degree of developmental
normality obtained, at midgestation this can vary
from disorganized egg-cylinder-like structures to
reasonably well-formed embryos that are not retarded at least in terms of somite number.
Therefore, the success or failure of parthenogenetic eggs developing after implantation appears to
depend on the penetrance and expressivity of certain
loci at which maternal duplication affects development at the immediate postimplantation stage. These
loci would appear to be associated with both the X
chromosome and autosomes, as the results of the
present and previous experiments (Mann & LovellBadge, 1987) have demonstrated that the presence of
(i) two sets of AM and (ii) two XM chromosomes
both have the potential to affect the viability of
X M X M A M A M eggs at this stage T h e genetic back.
ground may be involved, as in the experiments of
series II, a different strain combination had been used
compared to previous experiments (series I; Mann &
Lovell-Badge, 1984, 1987). However, we have only
Two XM chromosomes and parthenogenetic inviability
P. S., TAM, P. P. L. & EVANS, E. P. (1983).
Retarded development of XO conceptuses during early
pregnancy in the mouse. J. Reprod. Fert. 68, 387-393.
ever obtained very low frequencies of postimplantation development of X M X M A M A M eggs derived
from B6CBF, mice, whereas others have reported
much higher frequencies using the same eggs (Surani
& Barton, 1983; Surani et al. 1984). Conditions of the
experimental or maternal (recipient) environment,
and possibly certain qualities of the egg cytoplasm
may also influence the success of parthenogenetic
postimplantation development. In the experiments of
series I, two out of the three postimplantation-stage
parthenogenones were present in one recipient in the
same uterine horn. Also, Kaufman has described four
very advanced XX parthenogenones obtained in a
single recipient (Kaufman, 1981). However, in the
experiments of series II, no such pattern was evident.
Given the above, X M OA M A M eggs would have a
greater potential to continue development after implantation than X M X M A M A M eggs as they have only
to contend with a lack of A p chromosomes. Presumably therefore, in the experiments of series II, the
environment and/or genetic background usually
resulted in a low expressivity of relevant autosomal
loci (in XO parthenogenones), but not of X-linked
loci (in XX parthenogenones).
Further definition of the causes for the variable
development of parthenogenones beyond implantation awaits future study. Nevertheless, in the experiments of series II, there was essentially no variability
in the experimental and maternal conditions and in
the genetic background of XO and XX parthenogenetic eggs. Furthermore, the estimated frequency of
postimplantation development obtained in XOs is at
least double that which has been obtained in XXs in
any other study, by ourselves or others, following
transfer of 2-cell eggs to oviducts. This demonstrates
that, at least under some circumstances, the presence
of two XM chromosomes does have a deleterious
effect on the viability of parthenogenones at the
immediate postimplantation stage.
BURGOYNE,
We thank Anne McLaren and Paul Burgoyne for helpful
discussions.
KAUFMAN, M. H., BARTON, S. C. & SURANI, M. A. H.
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{Accepted 11 May 1988)