Comparison of evolutionarily conserved mammalian chromosomes homologous to human chromosome 17, performed with microdissected painting probes, revealed rearrangements inside these chromosomes in mink and pig and a disruption of this conserved region in the fox. Detection of a homologous region on an Iberian shrew chromosome showed the efficiency of microdissected painting probes for delineation of homologous chromosome regions in species belonging to orders that diverged at least 100 million years ago.

The telomere structure in the Iberian shrew Sorex granarius is characterized by unique, striking features, with short arms of acrocentric chromosomes carrying extremely long telomeres (up to 300 kb) with interspersed ribosomal DNA (rDNA) repeat blocks. In this work, we investigated the telomere physiology of S. granarius fibroblast cells and found that telomere repeats are transcribed on both strands and that there is no telomere-dependent senescence mechanism. Although telomerase activity is detectable throughout cell culture and appears to act on both short and long telomeres, we also discovered that signatures of a recombinogenic activity are omnipresent, including telomere-sister chromatid exchanges, formation of alternative lengthening of telomeres (ALT)-associated PML-like bodies, production of telomere circles, and a high frequency of telomeres carrying marks of a DNA damage response. Our results suggest that recombination participates in the maintenance of the very long telomeres in normal S. granarius fibroblasts. We discuss the possible interplay between the interspersed telomere and rDNA repeats in the stabilization of the very long telomeres in this organism.T elomeres are nucleoprotein structures that protect chromosome ends from degradation and end-to-end fusions (1). Telomeres also play a major role in tumorigenesis, cell senescence, and apoptosis (2). Telomere length, which is an important indicator of telomere function, is the result of the equilibrium between shortening events and lengthening activities (3). To neutralize telomere erosion, which occurs with every cell division, the majority of immortal cells, as well as germ line cells, use telomerase, a dedicated ribonucleoprotein enzyme that is able to add telomere repeats to the 3= ends of chromosomes (1). However, the majority of human somatic cells express telomerase at very low levels, and as a result, telomeres progressively shorten (2). The sustained loss of telomeric DNA eventually triggers cell senescence, thus limiting the proliferation capacity of somatic cells. This proliferation barrier in humans is deemed to play a fundamental role in tumor suppression (2). It was shown recently that in the case of telomerase-mediated telomere lengthening in the human male germ line and in phytohemagglutinin (PHA)-stimulated leukocytes, an additional mechanism of telomere length control operates, leading to telomere shortening through rapid deletions (telomere trimming) (4). This is a homologous recombination-based excision of the T loop, a structure formed by the folding back of the telomere and invasion of the double-stranded portion by the 3= telomere overhang (5). The resolution of these T loops results in the production of extrachromosomal T circles (6). The production of T loops is also a characteristic feature of alternative lengthening of telomeres (ALT), a telomere maintenance mechanism based on homologous recombination, found in some telomerase-negative immortal cell lines and primary cancers (7).Despite the fact that both telomere stru...

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A major problem in studies of synaptonemal complexes (SC) is the difficulty in distinguishing individual chromosomes. This problem can be solved combining SC immunostaining with FISH of chromosome-specific sequences. However, this procedure is expensive, time-consuming and applicable only to a very limited number of species. In this paper we show how a combination of SC immunostaining and DAPI staining can allow identification of all chromosome arms in surface-spreads of the SC of the common shrew (Sorex araneus L.). Enhancement of brightness and contrast of the images with photo editing software allowed us to reveal clear DAPI-positive and negative bands with relative sizes and positions similar to DAPI landmarks on mitotic metaphase chromosomes. Using FISH with DNA probes prepared from chromosome arms m and n we demonstrated correct recognition of the chromosomes mp and hn on the basis of their DAPI pattern. We show that the approach we describe here may be applied to other species and can provide an important tool for identification of individual bivalents in pachytene surface-spreads.