KARYOTYPE OF THE COMMON VOLE (MICROTUS ARVALIS) FROM THE SOWIE MOUNTAINS

Magdalena Moska, Barbara Kosowska, Katarzyna Brzezińska

ABSTRACT

A description of the karyotype of six specimens of the common vole (Microtus arvalis) from the Sowie Mountains is given. All animals had got diploid number of chromosomes 2n = 46. Among small autosomes were 13 pairs of bi-armed chromosomes (meta- and submetacentrics) and 4 pairs of telocentrics. The number and morphology of chromosomes show that all studied animals represent Microtus arvalis species - form ,,arvalis” .

Key words: Microtus arvalis, Microtus rossiaemeridionalis, sibling species, chromosomal forms, karyotype.

INTRODUCTION

The common vole Microtus arvalis Pallas, 1779 is the species, that for many years drows attention of cytogeneticists and nowadays is one of the most karyologically studied free-living mammals in Europe. In spite of a good knowledge about number and morphology of chromosomes of the group of Microtus arvalis, very complicated taxonomic and nomenclatoric problems exist within the group.

In Europe the group of common vole comprises two sympatric sibling species, which differ from each other in number and morphology of chromosomes. M. arvalis has 46 chromosomes (most of all metacentrics) while the other species which name is rather problematic has 54 chromosomes (all acrocentrics except one pair).

In Poland the M. arvalis species (2n = 46) divide into several subspecies: M. a. duplicatus Rörig i Börner, 1905 (the northern Poland), M. a. incognitus Stein, 1931 (majority our territory), M. a. cimbricus Stein, 1931 (Pomorze Zachodnie), M. a. levis Miller, 1908 (the southern part of Poland). It is possible, that in our fauna one more subspecies exists - M. a. rossiae-meridionalis Ognev, 1924, which inhabits the west part of Ukraine and reach the Polish border. All subspecies could be distinguished on the ground of morphological features, however the criteria of their subspecies division are obscure [14].

M. arvalis species is furthermore divided into two chromosomal forms: “arvalis” and “obscurus”. The “arvalis” form inhabits the western and central part of Europe while the “obscurus” form occupies the Eastern Europe. [12]. In both forms the diploid number of chromosomes is identical (2n = 46). Chromosomal forms differing in centromere positions on several pairs of small autosomes and also the Y chromosome. The “arvalis” form has got 13 pairs of small bi-armed chromosomes (metacentrics and submetacentrics) and 4 pairs of telocentrics while the “obscurus” form has got 7 and 10 pairs respectively [1,6,8,9,10,16,17,18]. Both chromosomal forms never occur sympatricly and the hybrid offspring produced by them in laboratory are completely fertile [10,11,12,13].

Our knowledge about taxonomy of M. arvalis group in Poland is insufficient. The only karyotypes of this species which are known from our country were described by [4] from Białowieża, [1] from Warsaw suburbs and Moska [unpubl.] from Wielkopolska. On the basis of cytogenetical analysis the authors found that in all localities specimens represented the M. arvalis species form “arvalis”.

The aim of our study was to analyse the karyotypes of M. arvalis from the Sowie Mountains. This paper refers to the pilot study being a small part of a big project which is planned to be realized in the whole territory of Poland. During the study it is intended to analyse karyotypes of M. arvalis in Poland and verify its systematic belongings, analyse sex chromosome aneuploidy described in this species and check whether the sibling species, with 54 chromosomes, lives in Poland.

MATERIAL AND METHODS

A total of 6 individuals (4 males and 2 females) of M. arvalis were caught at the base of the Sowie Mountains (Jugów, Dzikowiec). The studies were carried out from June to August 2003. Animals were trapped using metal cone traps. Trapping was done in biotopes which were preferred by voles i.e. in grasslands, meadows, uncultivated crops and skirt (?) of forests.

Mitotic preparations were made from the spleen after colchicine treatment in vivo [5]. The slides subjected to differential staining for G-bands were prepared using trypsin and Giemsa stain [15]. The karytotype of each specimen was described on the base of 5 metaphasal plates. The number and morphology of chromosomes were analysed.

RESULTS

In all specimens that were examined we found a diploid number of 46 chromosomes and the number of chromosomal arms was NF = 83/84. Autosomes of M. arvalis are divided in two groups: large and small chromosomes. In all researched specimens 5 pairs of the large chromosomes including one pair of metacentrics, 3 pairs of submetacentrics and one pair of subtelocentrics were found. In the group of small autosomes, 17 pairs in total: 13 pairs of bi-armed chromosomes (metacentrics and submetacentrics) and 4 pairs of telocentrics were found. The X chromosome was a medium size metacentric, while the Y chromosome was a small acrocentric, similar in size to the small acrocentric autosomes (Fig. 1 a, b). Based on karyotypes we found that all analysed specimens represent M. arvalis species, form arvalis.

The Microtus arvalis group is the group within which very important taksonomic and nomenclatoric problems exist. At present we know, that two sibling species exist sympatricly in Europe: with 46 chromosomes (most of all metacentrics) – M. arvalis Pallas, 1779 and with 54 chromosomes (all acrocentrics except one pair). Both sibling species are very similar morphologically. Except the number and morphology of chromosomes they differ from each other shape and size of spermatozoid head, and baculum shape [14].

The question of nomenclature of voles with 54 chromosomes has been a problematic issue for many years. Big freedom in onomastics of the species observed in the literature, makes this problem especially difficult. The species with 54 chromosomes was named M. subarvalis [11]. The name turned out to be invalid because it was a homonime – it was used for the fosil form of M. subarvalis Heller, 1933. Therefore, as proper and obligatory name for this species name M. a. epiroticus Ondrias, 1966 was accepted. Other species name, for voles with 54 chromosomes was introduced by [8] – M. rossiaemeridionalis. As this name is a synononim to: M. a. epiroticus Ondrias, 1966 and M. subarvalis [11], it should not have the priority before M. a. epiroticus Ondrias, 1966. What is more, the name M. a. rossiae-meridionalis Ognev, 1924 has functioned earlier for the subspecies of M. arvalis which inhabited the west part of Ukraine [14]. Then, i f only name M. a. rossiae-meridionalis Ognev, 1924 is the authoritative name for the species with 54 chromosomes, this species name should has a priority.

Our analyses of karyotype of voles trapped in the Sowie Mountains show that all researched specimens represent M. arvalis species - form ,"arvalis". The presence of sibling species with 54 chromosomes was not proved in spite of a fact that trapping of animals was performed in biotopes prefered by both species such as open area (meadows, pastures), shrubby area and unharvested fields. That fact however, does not mean that it is absent in our theriofauna. The range of the species with 54 chromosomes spreads from Balkan Penisula in the south-west as far as Petersburg region and Finland in the north and western Kazakstan in the east [3,7,11]. Moreover, the species inhabits territory situated directly behind the east border of Poland. What is more, the border of the species is moving to the west [3], and it is possible that the species started to colonize our country. For verification of this hypothesis karyologically study of M. arvalis on the whole Polish territory seems to be indispensable. We have only the data about karyotypes of M. arvalis from north-east Poland, Mazowsze and Wielkopolska [1,4], [Moska (unpubl.)]. A total number of the karyologically researched specimens is 64, what in a scale of the whole Polish territory gives rather unsatisfactory results. Also the results of our study are unsatisfactory because of too low number of cytogenetically researched specimens. It is a result of exceptionally unfavourable atmospherical conditions (drought and very high temperatures of air) which were in the summer 2003 and influenced very poor trapping.

During our study, similarly to [1,4], [Moska (unpubl.)], we did not observe abnormality in the number of chromosomes of the researched animals. There is some information about sex chromosome aneuploidy of this species. [18] analysed karyotypes of 264 specimens from Central and South-Eastern Europe and found 4 animals with sex chromosome aneuploidy (i.e. 1.5% of all individuals karyotyped). Two females had got 45 chromosomes (X0) and two males had got 47 chromosomes (XYY, XXY). Also [2] searched 222 prophase nuclei from spermatocytes of M. arvalis and observed a triple synaptonemal complex between 3 small chromosomes in the 4 spermatocytes.

SUMMARY

The karyotypes of all researched specimens from the Sowie Mountains suit (?) M. arvalis species - form arvalis (2n = 46, NF = 83/84). We did not confirm the occurance the sibling species with 2n = 54 in the studied area.

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