ISSN 1022-7954, Russian Journal of Genetics, 2007, Vol. 43, No. 6, pp. 658–664. © Pleiades Publishing, Inc., 2007.
Original Russian Text © L.D. Safronova, E.V. Cherepanova, 2007, published in Genetika, 2007, Vol. 43, No. 6, pp. 796–803.
Electron microscopic studies of meiosis have been
performed for many mammalians, including rodents.
Electron microscopic patterns of synaptonemal com-
plexes (SCs), along with differential staining of mitotic
chromosomes, is broadly used for karyotype descrip-
tion . The synapsis of sex chromosomes is known to
be more diverse than that of autosomes. In most
rodents, synapsis of sex chromosome axes is conﬁned
to the short pseudoautosomal region, where recombina-
tion occurs (for review, see ). The lengths of this
region are different in different species [3, 4]. In some
species, sex chromosomes do not synapse [5, 6]. Anal-
ysis of the synapsis of sex chromosomes and banding of
mitotic chromosomes within a single family sometimes
showed that not only major but even minor changes in
the structure of the X and Y chromosomes resulted in
loss of synapsis [5, 7].
The difference in the behavior of sex chromosomes
in early meiosis can also be detected by light micros-
copy. In most species, the X and Y chromosomes at
diakinesis–metaphase I associate in a end-to-end con-
ﬁguration. At metaphase I of species with asynaptic sex
chromosomes, they can associate end-to-end or disso-
Here we describe early meiotic stages and SC kary-
otyping in three ﬁeld mouse species:
. Although numerous karyological studies have been
performed in ﬁeld mice [10–12], information on meio-
sis is available only for two species: the yellow-necked
 and the Korean ﬁeld
[14, 15]. Only in the
latter species meiosis has been studied by EM .
MATERIALS AND METHODS
Experiments were performed with the following
wood mouse species: yellow-necked wood mouse
Melchior 1834 (Tver
region, two males), Caucasian wood mouse
Sviridenko 1936 (Krasnodar region, one male),
and pygmy wood mouse
(Saratov region, two males).
Mitotic chromosome prepara-
tions were made from bone marrow according to the
conventional method . Meiotic chromosome prepa-
rations for light microscopy were prepared from testis
cell suspension . Chromosomes were visualized by
Preparations of total spread
spermatocyte I nuclei for analysis of synaptonemal
complexes (SCs) were made according to Dresser and
Moses  and stained with 50% silver nitrate solution.
Cells were examined under a JEM 100C electron
microscope. Substages of the ﬁrst meiotic division
were analyzed according to Moses’ classiﬁcation
[19, 20]. We examined 10
cells, and 8
cells. For SC kary-
otyping, we examined cells at early–middle pachytene
(6, 14, and 5 cells, respectively) and late pachytene (two,
four, and one cells). Chromosomes were identiﬁed by
measuring axial elements of SCs in autosomes and sex
Behavior of Sex Chromosomes at Early Meiosis Stages
in Three Wood Mice Species
of the Genus
L. D. Safronova and E. V. Cherepanova
Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, 117071 Russia;
Received August 2, 2006
—The prophase of the ﬁrst meiotic division was studied in ﬁeld mice of the species
by light and electron microscopy. The karyotypes of
three species were described on the base of electron microscopy of synaptonemal complexes in spermatocytes I.
The axial elements of the sex chromosomes at early–middle pachytene synapse along the major portion of the
Y axis; at late pachytene–early diplotene, the synapsis region shrinks; and at diakinesis–metaphase I, X and
Y chromosomes associate end-to-end in all species studied. The behavior of sex chromosomes in the synapsis
in the species studied was quite uniform. The results are discussed in the context of earlier data on the behavior
of sex chromosomes in various rodent species in meiosis prophase I and their banding.