J. evol.
Biol.
2: 373-378
101&061X/89/053734b6
0 1989 Birkhluser
(1989)
Chromosomal rearrangements, speciation and
reproductive isolation: The example of two karyotypic
of the genus Sorex
Cornelis
$ 1.50 +0.20/O
Verlag, Base1
species
R. Neet and Jacques Hausser
Institut de Zoologie et d’Ecologie Animale, Universitk de Lausanne,
Bfitiment de Biologie, CH-1015 Lausanne, Switzerland
Key words: Speciation;
cytogenetics; Sorex.
chromosomal
evolution;
reproductive
isolation;
Introduction
During the last twenty years, growing evidence appeared in favour of the role
of chromosome rearrangements
as a factor that may promote speciation (White,
1978). Several theoretical models have been proposed (Sites and Moritz, 1987)
many of which are based on empirical data from various taxa, including, among
others, the genus Drosophila (Wallace, 1953), grasshoppers
(Shaw, 1981), lizards
(King, 1981) and mice (Capanna, 1982). However, Sites and Moritz ( 1987) still find
this evidence for a chromosomal
involvement
in speciation too fragmentary.
Moreover, chromosomal
speciation models are often taken as sympatric speciation
models and thus assume that when pre-mating barriers have evolved to reduce the
production of unfit hybrids, this should have occurred by reinforcement of characters increasing assortative mating, a process that still lacks good theoretical and
empirical support (Butlin, 1987; Coyne and Barton, 1988). One further problem is
that, in many natural cases, hybrid zones are found between karyotypic
races,
sometimes with appreciable stability and persistence that does not necessarily
suggest current speciation (Barton and Hewitt,
1985). For example, a study on
British karyotypic races of the shrews of the Sorex araneus complex has shown that
between some races, each characterized by different sets of metacentric chromosomes, natural hybridization
may occur without any evidence for strong pre- or
post-mating barriers. On the contrary, in the hybrid zone, a karyotype with a higher
number of acrocentric chromosomes and that may hybridize with other karyotypes
appears to be favourably selected (Searle, 1984a, 1986).
In this paper, we present results obtained by biochemical and karyological
analyses of two other taxa of the Sorex araneus complex, Sorex coronatus and the
Vaud race of Sorex araneus (Hausser et al., 1986). These taxa are considered to be
373
374
Neet and Hausser
two karyotypic
species, and are more differentiated
than the British karyotypic
races. They are distinguishable
by subtle morphological
characters and a few
biochemical characters (Hausser et al., 1985), and their distributional
ranges are
parapatric, with a few narrow contact zones where syntopic populations are found
(Hausser 1978; Hausser et al., 1985). On the basis of karyotype,
hybrids between
these species would have a high probability
of being sterile (Olert and Schmid,
1978) but almost nothing is known of their reproductive isolation in nature. The
individuals we studied were all taken from syntopic populations that have coexisted
over periods of at least two years and which both reproduced (within taxa) in the
contact zone (Neet and Hausser, 1989). The populations also exhibit interspecific
territoriality,
and, during the mating period, interspecific contacts between males
and females were frequent (Neet, 1989). Thus, in these contact zones, we may
expect to find evidence either for hybridization
or for complete reproductive
isolation. Since it has previously been hypothesized that S. coronatus and S. araneus
have been separated in the recent past by a chromosomal
speciation process
(Hausser et al., 1985), it is of great interest to evaluate whether reproductive
isolation without hybridization,
i. e. pre-mating isolation, has evolved between them
since. If such was the case, this example would provide corroborative
evidence for
the hypothesis that chromosomal
rearrangements
were involved in the process of
speciation.
Materials
and methods
Shrews were trapped in two contact zones, one in the heights of Bassins, in the
Swiss Jura mountains, the other on the coast of the Lake of Neuchltel, between
Yverdon and Yvonand (Switzerland).
For karyological
analyses the shrews were
taken to the laboratory where direct, air-dried, mitotic chromosome preparations
were made from bone marrow.
The. preparations
were either Gimsa stained or
G-banded. For electrophoretic analyses, a blood sample of about 2 ~1 was taken, in
the field, from the tail. The blood samples were diluted in a buffer solution and run
on a disc-polyacrylamide
gel system (Hausser and Zuber, 1983) to compare serum
albumin patterns of both species. Each species has a characteristic
pattern that
permits precise identifications (see below). The technique has, so far, given 100%
correct identifications when tested with about 50 individuals of S. coronatus and 70
individuals of the Vaud race of S. araneus by karyology.
Results
No hybrid karyotypes were found in the sample (Table 1). The electrophoretic
results are best understood after considering the serum albumin patterns (Fig. 1).
S. coronatus is characterised by a slow double banded albumin pattern (C2) and the
S. armeus Vaud race by a faster double band (A2). In some samples, the upper one
of the two bands is less visible and may even be unnoticed on gel photographs (Cl
Chromosomal
speciation
375
in Sorex
Table 1. Numbers
of individuals
identified
by karyology
and electrophoresis
of serum albumins.
individuals
were taken from contact zones between the karyotypic
species S. cor~na~u~ and the Vaud
of s. araneus.
S. coronalus
Karyology:
Serum albumin
electrophoresis:
a Only
one of these individuals
s. araneus
Vaud race
Presumable
hybrids
Total
22
50
0
72
103
224
4”
331
was karyotyped.
It had a typical
S. araneus
The
race
karyotype.
and Al). However, a careful inspection of the gels shows that the double banded
pattern is always present. Although the intensity of both bands varies with sample
concentration (the blood sample is taken in the field and cannot always be exactly
dosed at 2 ~1) we have noticed that the upper band is more variable in intensity
than the lower band. Attempts were made to find the reason for this variability, e. g.
by controlling for sample storage time or the age of the individuals from which
samples were taken, but the source of this variability remains unclear. Nevertheless,
the interspecific difference is always clearcut, the lower band being always very
distinct.
As shown on Table 1, on four occasions a triple band pattern was observed (H3
in Fig. 1). Such a pattern may be interpreted as a hybrid since it combines the two
double band patterns into a triple band pattern. A similar pattern, but with the
Fig. 1. The serum albumin patterns obtained
by disc-electrophoresis
coronatus (Cl and C2) and for the Vaud race of S. araneu.7 (Al
pattern.
on polyacrylamide
gels for Sorex
and A2). H3 is a presumable
hybrid
376
Neet and Hausser
middle band being more intense, was actually obtained by mixing blood samples of
S. araneus (A2) and S. coronatus (C2). However, in the case of the H3 individuals,
the triple band pattern was not an artifact as it was obtained in different electrophoretic migrations and with various neighboring samples. Actually, the only H3
individual for which G-banded karyological
preparations were obtained shows a
typical S. araneus Vaud race karyotype,
with no indication of hybridization.
Therefore, we should not consider this triple band pattern to be a clear indication
of hybridization.
Having, on the other hand, no reason to doubt that a possible
hybrid would have this pattern, we must infer that hybrids between S. corona&s and
S. araneus Vaud race, if any, are rare and do not represent more than 1% of the
syntopic individuals.
Discussion
The very low frequency of hybridization, if any, in the contact zones between these
two karyotypic species suggests reproductive isolation. Most current definitions of
reproductive isolation (Hauser, 1987) only require extensive hybrid sterility, while in
this example hybrids are almost absent. As shown by Searle (1988), fitness reduction
in Robertsonian heterozygotes is, as far as we know, almost only due to losses of
fertility rather than reduced viability. It is therefore reasonable to assume that if Fl
hybrids were, in fact, regularly produced, we would have found some. Thus, our
results strongly suggest the existence of a pre-mating isolation mechanism between
the species. According to Templeton (1981), such mechanisms are far more likely to
evolve before secondary contact than after. In other words, it may seem likely that
S. coronatus and the Vaud race of S. araneus have been separated by a geographic
barrier in the past, and that speciation occurred by geographical isolation. The actual
distribution of these species suggests that this barrier should have been the Pyrenees,
during the last glacial periods (Searle 1984b; Hausser et al., 1985).
This view has the drawback that, during the Pleistocene, the iberic peninsula has
never been completely isolated from the rest of the continent (Nilsson,
1983).
Moreover,
the genetic distances within the Sorex araneus complex are rather
homogeneous, i. e., there is no clearcut difference between S. coronatus and the
numerous karyotypic
races of S. araneus (Catzeflis,
1984). Thus, with respect to
genetical differentiation,
there is no clear support for a long temporal separation
between S. coronatus and the different races of S. araneus. For these two reasons,
other speciation models that may produce pre-mating barriers without geographical
isolation (e. g. Endler, 1977; Rosenzweig, 1978; Templeton, 1981) may apply equally
well to this case.
Our results show that these two karyotypic species of the Sorex araneus complex
are not only separated by a very low probability of hybrid fertility (Olert and Schmid,
1978), but also by a pre-mating barrier. Since the evidence for a long geographical
isolation is at least weak, our results support the notion that the pre-mating barrier
could well have appeared after Robertsonian chromosomal rearrangements had cut
the gene flow between S. araneus and S. coronatus.
Chromosomal
speciation
in Sorex
377
Acknowledgements
We thank A.-M.
Mehmeti
for technical assistance, N. Zuber for material,
Dr. Mark A. Burgman and
an anonymous
referee for their comments
on an earlier draft of the manuscript.
We also thank Prof. P.
Vogel and the University
of Lausanne
for support.
References
Barton,
N. H., and G. M. Hewitt,
1985. Analysis
of hybrid zones. A. Rev. ecol. Syst. 16: 113-148.
Butlin, R. 1987. Speciation
by reinforcement.
Trends in Ecology and Evolution
2: 8-13.
Capanna,
E. 1982. Robertsonian
numerical
variations
in animal speciations:
Mus musculus, an emblematic model, pp. 155-177. In C. Bariogozzi
(ed.), Mechanisms
of speciation.
A. R. Liss. New York.
Catzeflis,
F. 1984. Systtmatique
biochimique,
taxonomie
et phyloginie
des musaraignes
d’Europe
(Soricidae,
Mammaha).
These, Universite
de Lausanne.
Coyne, J. A., and N. H. Barton,
1988. What do we know about speciation?
Nature 331: 485486.
Endler, J. A. 1977. Geographic
Variation,
Speciation,
and Clines. Princeton
University
Press, Princeton.
Hauser, C. L. 1987. The debate about the biological
species concept -a review. Z. zoo]. syst. Evolutforsch. 25: 241-257.
Hausser, J. 1978. Repartition
en Suisse et en France de Sorex araneus L., 1758 et de Sorex coronarus
Millet,
1828 (Mammalia,
Insectivora).
Mammaha
42: 3299341.
Hausser, J., F. Catzeflis,
A. Meylan,
and P. Vogel, 1985. Speciation
in the Sorex araneus complex
(Mammalia:
Insectivora).
Acta Zool. Fennica
170: 1255130.
Hausser, J., E. Dannelid,
and F. Catzeflis,
1986. Distribution
of two karyotypic
races of Sorex araneus
(Insectivora,
Soricidae)
in Switzerland
and the post-glacial
recolonization
of the Valais: First
results. Z. zool. Syst. Evolut.-forsch.
24: 3077314.
Hausser, J., and N. Zuber, 1983. Determination
specifique d’individus
vivants des deux esp&es jumelles
Sorex araneus et S. coronatus,
par deux techniques
biochimiques
(Insectivora,
Soricidae).
Rev.
suisse Zool. 90: 8577862.
King, M. 1981. Chromosome
change and speciation
in lizards, pp. 2622285. In W. R. Atchley and D.
Woodruff
(eds.), Evolution
and Speciation.
Essays in honor
of M. J. D. White. Cambridge
University
Press, London.
Neet, C. R. 1989. Evaluation
de la territorialite
interspecifique
entre Sorex araneus et S. coronatus dans
une zone de syntopie (Insectivora,
Soricidae).
Mammaha
53 (in press).
Neet, C. R., and J. Hausser. 1989. Habitat
selection in zones of parapatric
contact between the common
shrew Sorex nraneus and Millet’s
shrew S. coronatus.
J. Anim. Ecol. (in press).
Nilsson, T. 1983. The Pleistocene.
Geology
and Life in the Quaternary
Ice Age. F. Enke. Stuttgart.
Olert, J. and M. Schmid.
1978. Comparative
analysis of karyotypes
in European
shrew species.
Cytogenet.
Cell Genet. 20: 3088322.
Rosenzweig,
M. L. 1978. Competitive
speciation.
Biol. J. Linn. Sot. IO: 2755289.
Searle, J. B. 1984a. Hybridization
between Robertsonian
karyotypic
races of the common
shrew Sorex
areaneus. Experientia
40: 876878.
Searle, J. B. 1984b. Three new karyotypic
races of the common
shrew Sorex araneus (Mammalia:
Insectivora)
and a phylogeny.
Syst. Zool. 33: 184194.
Searle, J. B. 1986. Factors
responsible
for a karyotypic
polymorphism
in the common
shrew, Sorex
araneus. Proc. R. Sot. Lond. B 229: 277-298.
Searle, J. B. 1988. Selection and Robertsonian
variation
in Nature,
pp. 5077531. In A. Daniel (ed.), The
Cytogenetics
of Mammalian
Autosomal
Rearrangements.
A. R. Liss, New York.
Shaw, D. 1981. Chromosomal
hybrid zones in orthopteroid
insects, pp. 146-170. In W. R. Atchley and
D. Woodruff
(eds.), Evolution
and Speciation.
Essays in honor of M. J. D. White. Cambridge
University
Press, London.
378
Neet
and Hausser
Sites, J. W., and C. Moritz.
1987. Chromosomal
evolution
and speciation
revisited.
Syst. Zool. 36:
1533174.
Templeton,
A. R. 1981. Mechanisms
of speciation -A population
genetic approach.
A. Rev. ecol. Syst.
12: 2348.
Wallace, B. 1953. On coadaptation
in Drosophila.
Am. Nat. 87: 343-358.
White, M. J. D. 1978. Modes of Speciation.
W. H. Freeman,
San Francisco.
Received 30 June 1988;
accepted 20 Dec. 1988.
Corresponding
Editor: M. Bulmer