ISSN 1022-7954, Russian Journal of Genetics, 2007, Vol. 43, No. 2, pp. 213–215. © Pleiades Publishing, Inc., 2007.
Original Russian Text © G.K. Isakova, 2007, published in Genetika, 2007, Vol. 43, No. 2, pp. 280–283.
Two twin types are known to occur in mammals:
monoval (monozygotic) and polyoval (polyzygotic).
Monozygotic twins are produced by fertilization of a
single egg followed by division of the resulting zygote
into two or more parts, each of which forms an individ-
ual embryo. By the beginning of implantation, all
embryos share one chorion and develop in one fetal
chamber. They should be genotypically identical.
Polyzygotic twins are formed by fertilization of two or
more simultaneously ovulating eggs. Each embryo has
a chorion of its own and develops in a separate fetal
chamber. Such embryos differ from one another no less
than from other progeny of the same couple of parents
The American mink (
) is a multipa-
rous animal. It produces on average six to eight
embryos, which develop in separate fetal chambers and
are polyzygotic twins. It has been noted, though, that
sometimes a single fetal chamber contains two
embryos sharing one chorion (, our unpublished
results). This brings up the question: what is the origin
of such monochorionic pairs and are they genetically
As shown in our cytogenetic studies, a characteristic
feature of the American mink is high frequency of
mosaic embryos, consisting of two cell lines: diploid
and triploid (2
) or two diploid but possessing dif-
ferent chromosome sets (2
, XY) [4, 5]. An
adult mink with the chromosome constitution 2
, XXY has been reported . Comparison of
expected and observed frequencies of occurrence of
individuals with different sex chromosome com-
binations in two cell lines in different fertilization
abnormalities has shown that mosaics of this type are
most likely to originate from binuclear cells. Indepen-
dent fertilization of either of the nuclei followed by
zygote fusion can give rise to a 2
embryo [7, 8].
The same way can bring about mosaics of type 2
with XX/XX, XX/XY, or XY/XY sex chromosome sets
[4, 5, 7]. Mosaics emerging during fertilization are
referred to as chimeras . We considered an alterna-
tive putative pathway of the development of a binuclear
egg, in which the zygotes produced by fertilization of
both nuclei separately develop within one egg instead
of fusion. The embryos have a common chorion; there-
fore, they should be considered monozygotic twins.
However, they are genetically different, because they
are formed by fertilization of two maternal pronuclei by
two or more spermatozoa. The chromosome constitu-
tions of such embryo pairs can be 2
with identical or different sex chromosome sets .
The action of such mechanism can be conﬁrmed by
detection of two embryos with one chorion but different
karyotypes in one fetal chamber. Both members of a
formerly found monochorionic pair had the karyotype
, XY [11, 12].
Here, the study of the frequency of monochorionic
twin pairs and karyotypes of their embryos in the
American mink is reported.
Experiments were performed with two-year old
minks of standard brown color (wild type) and sapphire
minks (mutant for fur color) bred at the SD RAS exper-
imental fur farm, Novosibirsk. Formerly, we found that
sapphire minks had signiﬁcantly higher preimplanta-
tional embryo mortality than standard ones. The main
cause of this mortality is oogenesis abnormalities [13, 14].
In addition, 2
, XY chimeras also
occur at the preimplantation stage in sapphire minks
more often than in standard ones .
Three Twinning Types
in Mink (
G. K. Isakova
Institute of Cytology and Genetics, Russian Academy of Sciences, Novosibirsk, 630090 Russia;
fax (383) 333-12-78; e-mail: firstname.lastname@example.org
Received May 5, 2006
—The frequency of occurrence of monochorionic twins in the postimplantational embryogenesis of
the American mink and their karyotypes were studied. Monochorionic twin pairs were found in which embryos
had different chromosome sets: 2
, XX and 2
, XY or 2
. This fact contradicts the idea that monochori-
onic twins should be monozygotic and genetically identical but conﬁrms our earlier hypothesis that a third twin-
ning type exists in mink: monozygotic but genetically different. The mechanism of the emergence of this twin-
ning type in mammals is discussed. It is suggested that the high (up to 4.5%) frequency of its emergence in the
American mink is related to obligate embryonic diapause, causing abnormal fertilization.