Cytogenetic Analysis in the Toad Species Bufo spinosus, Bufotes viridis and Epidalea calamita (Anura, Bufonidae) from the Mediterranean Area

Taxonomy in Bufonidae witnessed notable transformations. Bufotes viridis and Epidalea calamita, previously included in genus Bufo, were relocated in other genera, while the genus Bufo was restricted to members of the earlier Bufo bufo group. On the other hand, Bufo bufo sensu lato now includes four species: Bufo bufo, Bufo spinosus, Bufo verrucosissimus and Bufo eichwaldi. In this study, we examined three species of three Bufonidae genera (B. spinosus, B. viridis and E. calamita) by conventional (C-banding and Ag-NOR staining) and molecular (in situ hybridization with probes for telomeric repeats and rDNA loci, and genomic in situ hybridization (GISH)) cytogenetic methods. C-banding patterns are reported for the first time for B. spinosus and E. calamita populations from Iberian Peninsula and for B. viridis from Greece, and reveal several differences with the reported C-banded karyotypes described for other European populations of these species. Silver staining shows size heteromorphisms of the signals at the Nucleolar Organizing Region (NOR). By contrast, FISH with ribosomal probes only reveal size heteromorphism of rDNA sequences in E. calamita, suggesting that the differences observed after silver staining in B. spinosus and B. viridis should be attributed to differences in chromosomal condensation and/or gene activity rather than to differences in the copy number for ribosomal genes. Regarding telomeric repeats, E. calamita is the only species with interstitial telomeric sequences (ITS) located on centromeric regions, probably originated by accumulation of telomeric sequences in the centromeric heterochromatin. Finally, we analyzed the composition and distribution of repetitive sequences by genome in situ hybridization. These experiments reveal the accumulation of repetitive sequences in centromeric regions of the three species, although these sequences are not conserved when species from different genera are compared.

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

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.