ISSN 1022-7954, Russian Journal of Genetics, 2008, Vol. 44, No. 5, pp. 501–512. © Pleiades Publishing, Inc., 2008.
Original Russian Text © V.N. Orlov, A.I. Kozlovskii, A.E. Balakirev, Yu.M. Borisov, 2008, published in Genetika, 2008, Vol. 44, No. 5, pp. 581–593.
The early cytogenetic studies of plants and
in the 1920s–1930s revealed not only variability,
but also genetic monomorphism in the species.
Yu.P. Altukhov  was among the ﬁrst researchers who
turned attention to the phenomenon of protein mono-
morphism of species. Species monomorphism of chro-
mosome sets permitted wide use of genetic methods for
studying the taxonomy of mammals [2, 3] and other
groups of animals. In recent years, monomorphism of
some mtDNA regions (DNA barcoding) has been used
for species diagnostics .
The causes and conditions of ﬁxation of chromo-
some and gene mutations are central to the speciation,
because the formation of isolating mechanisms is
directly connected with mutation ﬁxation (appearance
of a monomorphic character) rather than the appear-
ance of that mutation in the population.
Although the objectivity of genetic monomorphism
of the species is beyond doubt, investigation of its
appearance is somewhat problematic. Monomorphic
proteins, karyotypes, and DNA regions, differing
between closely related species, are typically devoid of
any within-species variation, making equiprobable
polymorphism loss during the evolution and salutatory
speciation without intermediate stages.
The classical views on the ways of formation of spe-
cies monomorphic characters imply a prolonged transi-
tory stage of the polymorphism. However, some
authors believe that large chromosome or gene muta-
tions are ﬁxed, i.e., attain 100% frequency, after a short
stage of polymorphism or even without it, like genomic
mutations [5–7]. Note that so far, salutatory formation
of monomorphic characters of a species, according to
which “two individuals meet in a marginal deme and
produce normally viable homozygotes—a novel spe-
cies” [7, p. 279] has not been recorded in any model
In rare cases, not only species, but also populations
can be monomorphic for some characters. Investigation
of the transition from the polymorphic population to the
monomorphic one will shed light on the appearance of
monomorphic species characters in evolution. The
L. is a helpful model for
studying the formation of populations monomorphic
for chromosome rearrangements. Seven international
symposia (1987–2005) were devoted to the molecular
aspects of the evolution of this species.
The polymorphism of Robertsonian translocations
(fusion of two acrocentric chromosomes into one meta-
centric) of the common shrew was ﬁrst found in
England , while its wide occurrence in European
populations of this species was discovered in the 1960s
[9, 10]. To date, 70 chromosome races of the common
shrew have been described throughout of the Palearctic
range of the species, from England and the Pyrenees to
Baikal Lake [11–13]. The obsolete taxonomic term
race is rarely used in systematics and genetics. Tradi-
tionally, chromosome races of the common shrew and
some other species refer to territorial groups, i.e., pop-
ulations on a part of the species range, which are char-
acterized by various metacentric chromosomes or their
absence. In some populations, metacentric chromo-
somes are in heterozygous state, whereas in other pop-
ulations they are ﬁxed.
Here, based on a survey of chromosome races of the
common shrew from the eastern part of Europe (east of
the Dnestr and Visla rivers and Botnicheskii Bay) and
Fixation of Metacentric Chromosomes in Eastern Europe
Populations of the Common Shrew
V. N. Orlov, A. I. Kozlovskii, A. E. Balakirev, and Yu. M. Borisov
Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, 119071 Russia
Received February 16, 2007
—In this review, we discuss the processes of ﬁxation of Robertsonian chromosome fusions in popu-
lations of the common shrew
L. Various Robertsonian fusions, accumulating in populations, cre-
ate an illusion of large chromosomal rearrangements, reciprocal translocations of complete chromosome arms.
The use of these rearrangements for phylogenetic reconstructions results in false conclusions. Robertsonian
fusions accumulate in populations at such stages of the species evolution, when large open or subdivided pop-
ulations prevail (populations of warm periods of Pleistocene and many present-day populations) and are ﬁxed
in small isolated populations and glacial refugia. The formation of monomorphic chromosome races requires a
long time, several glaciation epochs during the whole Pleistocene.