ISSN 0012-4966, Doklady Biological Sciences, 2007, Vol. 417, pp. 487–489. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © N.A. Shchipanov, S.V. Pavlova, 2007, published in Doklady Akademii Nauk, 2007, Vol. 417, No. 6, pp. 847–849.
GENERAL
BIOLOGY
Hybridization of the Common Shrew (Sorex araneus L.)
Chromosomal Races Moscow and Seliger: The Probability
of Crossing and Survival of Hybrids
N. A. Shchipanov and S. V. Pavlova
Presented by Academician D.S. Pavlov June 18, 2007
Received June 18, 2007
DOI: 10.1134/S0012496607060221
The common shrew (Sorex araneus L.) is a species
that forms as many as 70 chromosomal races parapatrically distributed over the species area [1, 2]. In zones of
contact between the chromosomal races, hybrid zones
appear, which differ in width and configuration. Their
interaction may serve as an important forming factor
[3]. In Western Europe, 13 hybrid zones have been
described, whereas in Russia only a hybrid zone
between the Tomsk and Novosibirsk races in Western
Siberia was known until recently [4]. In the European
part of Russia, a new zone of contact between two chromosomal races have been found, which is promising for
studying hybridization between the Moscow and
Seliger races [5–7]. Karyotypic distinctions of these
two races reach the maximum level among common
shrew chromosomal races. In addition to invariant autosome pairs (af, bc, jl, tu) and sex chromosomes (XX
and XY1Y2 in females and males, respectively), which
are typical of all S. araneus chromosomal races, karyotypes of the above two races contain diagnostic metacentrics (they differ in arm composition) and acrocentrics with two arms and one arm, respectively. In the
race Moscow, all diagnostic chromosomes are metacentrics (gm, hi, kr, no, pq) according to the standard
nomenclature for the chromosomal set of S. araneus
[8]. In the race Seliger, the same chromosomal arms
form four metacentrics (hn, ik, mq, pr) and two unfused
acrocentrics (g and o). The varying number of metacentrics in the given pair of races is reflected in the diploid
number of autosomes, which is smaller in the race Moscow and higher in the race Seliger (2na = 18 and 2na =
20, respectively). In these races, heterozygotes for the
diagnostic metacentrics gm/g,m (Moscow) and
mgq/m,q (Seliger) are rare [5].
The karyotypes of F1 hybrids of pure homozygous
races combine 11 monobrachial homologues—gm, hi,
kr, no, pq/g, hn, ik, mq, pr, o (2na = 19)—forming, in
meiosis I, a chain of 11 elements, which is the longest
among those observed in S. araneus chromosomal
races [6].
Does the probability of hybridization between different races and viability of hybrids depend on the level
of distinctions? Simulation may provide answers to
these questions [9]. We have collected empirical field
data in the hybrid zone between the Moscow and
Seliger races [5, 7]. Our data may be helpful for
detailed analysis of this issue.
The natural frequencies of the common shrew chromosomal races Moscow and Seliger and F1 hybrids
were calculated using the data on karyotyping of 240
S. araneus individuals from 25 localities (Table 1); no
other variants of hybrid karyotypes were analyzed.
Karyotype descriptions were reported in [6]. The coor-
Severtsov Institute of Ecology and Evolution,
Russian Academy of Sciences, Leninskii pr. 33,
Moscow, 117071 Russia
Table 1. The observed and expected frequencies of the
karyotypic variants in the contact zone
Observed frequencies
Subzone
no.
Seliger
Moscow
1
2
3
4
5
6
7
8
9
8
38
69
16
1
0
0
0
0
2
12
34
31
7
10
Expected frequencies
MosF1 hySeliger
brids
cow
0
0
4
16
16
1
0
0
–
–
36.36
61.00
8.73
0.03
–
–
–
–
0.36
4.00
26.73
31.00
–
–
F1 hybrids
–
–
7.27
32.00
30.55
1.70
–
–
Note: The data excluded from calculations are italicized (subzones
1−2, 7–8). Dash, no data.
487
488
SHCHIPANOV, PAVLOVA
Table 2. The ratios between karyotypic variants in the hybrid zone. N, the number of individuals
The frequency distribution
of chromosomal forms
Karyotype
Seliger
Moscow
F1 hybrids
autumn
spring
N
%
N
%
79
32
28
57
23
20
12
5
4
57
24
19
dinates of capturing sites were determined in the hybrid
zone using GPS and were then put onto an electronic
map that served as the basis for data treatment. To evaluate a success of overwintering in the same plot, the
samples were taken in the autumn of the previous year
and in the spring of the current one. The data studied
were obtained on 160 animals, 139 of which were captured in that year (autumn 2005) and 21 ones had wintered and were captured in spring 2006 (Table 2).
The central line of the previously determined [6]
hybrid zone was arbitrarily drawn as a straight line
equidistant from the boundary points of the areas
inhabited by the pure races Moscow and Seliger. A 10km-wide zone (5 km northwest and 5 km southeast
from the central line) was subdivided into 1.25-km
strips (subzones), which were numbered in the direction from the north to the south (1–8). The central line
was between subzones 4 and 5. The number of purerace individuals and hybrids found in a subzone were
summarized to obtain the natural frequencies. F1
hybrids were encountered only in subzones 3 to 6. As
Frequency
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
3
4
5
Subzone no.
6
The distribution of the observed (bars) and simulated (lines)
frequencies in subzones 3–6 of the hybrid zone of two chromosomal races of S. araneus. White color, the Seliger race;
black color, the Moscow race; gray color, F1 hybrids. The
lines with squares show the expected distribution after free
crosses; the lines with triangles, the frequencies of hybrids
provided Moscow males were excluded; the lines with circles, the frequencies of hybrids provided Seliger males were
excluded.
determined from Hardy–Weinberg equation, the
expected number of hybrids was twice as high as the
actual number (Table 1). This deviation from the frequency expected in the case of free crosses could be
explained by a low survival rate of hybrids; therefore,
in the autumn of the previous year and spring of the current year, we compared the ratio of the number of individuals of each pure race to that of hybrids in zones 4
and 5, i.e., in the center of the contact zone. After overwintering, this ratio remained almost unchanged
(Table 2); i.e., no significant differences were found
(χ2 = 0.0947, P < 0.953). Since the largest decrease in
the shrew population density occurs in winter, when
competitive relationships are the most acute [10], our
results suggest that the competitive abilities of both
races and their hybrids were similar.
Since the number of hybrids was two times lower
than the expected one, it could be suggested that one of
two sexes was excluded from crossing, e.g., females of
one race avoid mating with males of the other race. To
test this assumption, we used Hardy–Weinberg equation to calculate the expected number of hybrids after
exclusion of male “alleles” of either Seliger or Moscow
race from crosses. To correct our calculations, the real
sex ratio was taken into account. In the pooled sample
(n = 303) collected in the studied area with earlier data
[5, 6] added to the pool, males accounted for 60%. The
fit between the expected and actual numbers proved to
grow after excluding males of either race from crosses,
though this parameter was somewhat higher after the
exclusion of males of the Seliger race (figure). A highly
significant difference was obtained between the distributions of the actual and stimulated frequencies (χ2 =
32.2, p < 0.001); after the exclusion of the Seliger and
Moscow males, these parameters were equal to χ2 =
7.90; p = 0.43 and χ2 = 8.40 p = 0.39, respectively.
Reasoning from the general assumption that karyotype differences affect the results of hybridization, we
could expect that the numbers of both born and surviving F1 hybrids would be significantly lower than those
expected in the case of free crosses, as was the case
with the Tomsk and Novosibirsk races [4]. In general, a
low competitiveness of F1 hybrids is assumed to be the
cause of their displacement to worse environments [11,
12]. Our data on the hybrid zone between the Moscow
and Seliger races suggest other possibilities. Note that
the competitiveness of individuals with a hybrid karyotype remained unchanged after overwintering. This evidence suggests that the independence (unmixing) of the
chromosomal races is a result of not only karyotype differences, but also of a wide range of biological features
that prevent both mutual expansion of the parental individuals into strange areas and crosses between the
parental races.
DOKLADY BIOLOGICAL SCIENCES
Vol. 417
2007
HYBRIDIZATION OF THE COMMON SHREW (SOREX ARANEUS L.)
ACKNOWLEDGMENTS
This study was supported by the INTAS (project 0351-4030) and Foundation for Supporting Russian Science.
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