ISSN 1022-7954, Russian Journal of Genetics, 2009, Vol. 45, No. 4, pp. 389–396. © Pleiades Publishing, Inc., 2009.
Original Russian Text © N.B. Rubtzov, Yu.M. Borissov, T.V. Karamysheva, M.N. Bochkarev, 2009, published in Genetika, 2009, Vol. 45, No. 4, pp. 449–457.
The analysis of karyotypes of the Korean ﬁeld
Thomas, 1907) from dif-
ferent populations of its vast range revealed a remark-
able diversity of B chromosomes by their number, mor-
phology, and DNA composition against the background
of the constant system of A chromosomes [1–6]. The
number of B chromosomes in individuals of this spe-
cies varies from 0 to 24 [2, 7]. However, the mecha-
nisms of formation of different systems of B chromo-
somes in different geographical regions have not been
practically discussed so far. Main attention in this work
is given to possible mechanisms responsible for the
occurrence and further evolution of B chromosomes.
When analyzing the processes determining the current
diversity of B chromosomes in this mammalian spe-
cies, we took into consideration the data on the mecha-
nisms of chromosomal rearrangements obtained in
investigations of human chromosomal pathologies and
the results of investigations of B chromosomes in
Korean ﬁeld mice
regions of the area of this species.
The published results of cytogenetic analysis of ani-
mals from natural populations from the northeastern
shore of the Teletskoye Lake (settlement of Artybash,
Republic of Altai) [8, 9], from the southern shore of the
Baikal Lake (in the neighborhood of the town of
Baikal’sk, Irkutsk oblast, settlements of Vydrino,
Tankhoi, and Babushkin, Republic of Buryatia) ,
and from the Ussuriyskii Reserve (Primorskii krai) 
are used in the work.
As a result of the above analysis, several hypotheses
concerning the mechanisms and causes of formation of
different organizations of B chromosomes in popula-
tions of mice from geographically separated regions
have been proposed.
ORGANIZATION OF THE SYSTEM
OF B CHROMOSOMES IN KOREAN FIELD MICE
AND THE PROBLEMS
OF ITS ANALYSIS
The karyotypes of all animals involved in this study
contained from 48 to 61 chromosomes. The basic set
consisted of a descending number of acrocentric chro-
mosomes (23 autosome pairs and sex chromosomes)
and corresponded to the previously described one .
The variability of the karyotypes was determined by the
multiplicity of variants of B chromosomes whose num-
ber varied from 0 to 13. The number of large double-
armed B chromosomes varied from 0 to 3, the number
of medium-size double-armed chromosomes varied
from 0 to 5, small double-armed chromosomes varied
in number from 0 to 4, small acrocentric chromosomes
from 0 to 2, and the number of point chromosomes, or
B microchromosomes, varied from 0 to 12.
It should be noted that there are some problems in
the classiﬁcation of chromosomes whose size corre-
sponds to the borderline size for two neighboring
classes, for example, I and II, II and III. In addition, the
classiﬁcation of some B microchromosomes (class V,
point B chromosomes without clear-cut morphology)
may depend on the quality of cytological preparations.
For instance, in the course of analysis of prometaphases
and prophases it was possible to detect small arms in B
microchromosomes in some animals, whereas in other
animals the arms of B microchromosomes were unde-
tectable even in very weakly condensed chromosomes.
The Mechanisms of Formation and Evolution of B Chromosomes
in Korean Field Mice
N. B. Rubtzov
, Yu. M. Borissov
, T. V. Karamysheva
, and M. N. Bochkarev
Institute of Cytology and Genetics, Russian Academy of Sciences, Novosibirsk, 630090 Russia;
Severtsev Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, 119071 Russia
Received June 5, 2007
—Several hypotheses concerning variations in the frequency of some elementary events determining
the formation and reorganization of mammalian B chromosomes are proposed on the basis of the data on their
number, morphology, and DNA composition in Korean ﬁeld mice
from natural populations of Altai, Buryatia, Irkutsk oblast, and Primorye. The mechanisms and causes respon-
sible for the formation of B chromosomes and differences in their organization in populations of mice from geo-
graphically separated regions are discussed.
REVIEWS AND THEORETICAL ARTICLES