Comparative chromosome mapping of the rRNA genes and telomeric repeats in three Italian pine voles of the Microtus savii s.l. complex (Rodentia, Cricetidae)

Ekaterina Gornung,Riccardo Castiglia,Alexandra M R Bezerra

doi:10.3897/compcytogen.v5i3.1429

Ekaterina Gornung, Riccardo Castiglia + Show 1 more

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https://doi.org/10.3897/compcytogen.v5i3.1429

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Abstract

The Microtus (Terricola) savii s. l. complex is a group of five species/subspecies of the Italian pine voles, which diverged at different times either with or without chromosomal differentiation. The evidence of chromosomal diversification has so far concerned the shape of the sex chromosomes, especially the X chromosome. Three taxa of the group, Microtus savii savii, Microtus savii nebrodensis, and Microtus savii tolfetanus have identical karyotypes with metacentric X chromosomes. The X chromosomes of Microtus brachycercus and Microtus brachycercus niethammericus are, respectively, subtelocentric and acrocentric in shape. The Microtus savii complex has been long an object of conventional karyological studies, but comparative molecular cytogenetic data were completely missing. Therefore, we conducted a comparative chromosomal mapping of rRNA genes (rDNA) and telomeric repeats in three of the five taxa of the group: Microtus savii savii, Microtus savii nebrodensis, and Microtus brachycercus niethammericus, each of which belongs to a distinct mitochondrial clade.The survey revealed that differentiation of the clades was accompanied by remarkable changes with regard to the number and locations of the rDNA sites. Thus, Microtus savii savii and Microtus savii nebrodensis have especially high numbers of rDNA sites, which are located in the centromeric regions of, correspondingly, 18 and 13 chromosome pairs, whereas Microtus brachycercus niethammericus shows variable (8–10) and heteromorphic rDNA sites on both centromeric and telomeric regions. Interstitial telomeric sites (ITS), which are believed to indicate possible breakpoints of recurring chromosomal rearrangements, are present on the largest biarmed chromosomes and on the metacentric X chromosomes in Microtus savii savii and Microtus savii nebrodensis. These preliminary results are discussed in the context of recent advances in phylogeny of the group, as well as the rDNA genomic organization and X chromosome rearrangements in the genus Microtus.

Highlights

The Italian endemic pine voles are distributed throughout the Apennine peninsula from the Alps to Sicily (Contoli et al 2008)
M. s. savii and M. s. nebrodensis have especially high numbers of rDNA sites, which are located in the centromeric regions of, correspondingly, 18 and 13 chromosome pairs, whereas M. b. niethammericus shows variable (8–10) and heteromorphic rDNA sites on both centromeric and telomeric regions
The number of signal-bearing chromosomes in metaphase plates of M. s. savii was as large as 36 (18 chromosome pairs) (Fig. 2a), while 28 signals were distributed on 13 chromosome pairs in M. s. nebrodensis (Fig. 2b)

Summary

Introduction

The Italian endemic pine voles are distributed throughout the Apennine peninsula from the Alps to Sicily (Contoli et al 2008). The chromosomal and morphological polytypism of this group led to the identification of five forms: “savii”, “brachycercus”, “nebrodensis”, “niethammericus”, and “tolfetanus”, ascribed to the Microtus (Terricola) savii complex de Sélys-Longchamps, 1838 (Galleni et al 1994, 1998, Contoli 2003). The systematic ranks and relationships of these taxa have been recently reconsidered (Contoli 2008, Contoli and Nappi 2008) in the light of new insight into the phylogeny of the group (Castiglia et al 2008) and chromosomal morphology (Table 1). Karyological studies in the M. savii sensu lato complex revealed the same diploid number (2n=54) and invariable set of autosomes (NFa=58) in all these taxa, but only three of the five taxa showed sex chromosomes similar in size and shape. Albeit the karyological similarity with M. s. savii, the Sicilian form presently ascribed to M. s. nebrodensis showed unexpectedly high genetic divergence, which suggested its possible specific status (Castiglia et al 2008)

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