Astyanax scabripinnis possesses a widespread polymorphism for metacentric B chromosomes as large as the largest chromosome pair in the A complement. On the basis of C-banding pattern, it was hypothesized that these B chromosomes are isochromosomes that have arisen by means of centromere misdivision and chromatid nondisjunction. In the present paper we test this hypothesis by analysing (i) the localization of a repetitive DNA sequence on both B chromosome arms, and (ii) synaptonemal complex formation, in order to test the functional homology of both arms. Genomic DNA digested with KpnI and analysed by gel electrophoresis showed fragments in a ladder-like pattern typical of tandemly repetitive DNA. These fragments were cloned and their tandem organization in the genome was con®rmed. A 51-bp long consensus sequence, which was AT-rich (59%) and contained a variable region and two imperfect reverse sequences, was obtained. Fluorescence in situ hybridization (FISH) localized this repetitive DNA into noncentromeric constitutive heterochromatin which encompasses the terminal region of some acrocentric chromosomes, the NOR region, and interstitial polymorphic heterochromatin in chromosome 24. Most remarkably, tandem repeats were almost symmetrically placed in the two arms of the B chromosome, with the exception of two additional small clusters proximally located on the slightly longer arm. Synaptonemal complex (SC) analysis showed 26 completely paired SCs in males with 1B. The ring con®guration of the B univalent persisting until metaphase I suggests that the two arms formed chiasmata. All these data provided strong support for the hypothesis that the B chromosome is an isochromosome.Keywords: Astyanax scabripinnis, B chromosomes, FISH, heterochromatin, isochromosome, satellite DNA. IntroductionB chromosomes, also referred to as supernumerary or accessory chromosomes, are`additional dispensable chromosomes that are present in some individuals from some populations in some species, which have probably arisen from the A chromosomes but that follow their own evolutionary pathway ' (Camacho & Parker, 1993). The molecular analysis of B chromosomes has revealed that they are mostly composed of satellite DNA, which is consistent with their heterochromatic nature. Some of these satellite DNAs are speci®c to Bs whereas others are shared with the A chromosomes (reviewed in Beukeboom, 1994; Hackstein et al., 1996; Camacho et al., in press). In some cases, satellite DNA has provided a useful tool to ascertain the intraspeci®c (Lo pez-Leo n et al., 1994 3,4 ) or interspeci®c (Mcallister & Werren, 1997 3,4 ) origin of B chromosomes (see also Camacho et al., 2000).A metacentric macrochromosome B, with dierent amounts of C-heterochromatin, has been described in some populations of the characid ®sh Astyanax scabripinnis (for review, see Vicente et al., 1996). On the basis of the roughly symmetrical pattern of C-banding response of the two arms of this B, Vicente et al. (1996) suggested that this B is an isochromosome that arose b...

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Chromosome banding patterns obtained by ammoniacal silver staining (Ag-AS) and alkaline Giemsa (CBG) have been analysed in several amphibian species of the genus Odontophrynus from South America. Ag-AS bands were found at secondary constrictions, mainly of chromosomes 4 and 11. The CBG technique revealed centromeric and telomeric constitutive heterochromatin on almost all chromosomes of these species. Moreover, intercalary bands were found at particular sites of several chromosomes. Some inter- and intra-population polymorphisms were found for the Ag-AS and C-banding patterns. The species variability in the number and position of the Ag-AS bands, known to be regions of active ribosomal cistrons, as well as the specific sites of intercalary heterochromatin, are used to discuss the possible evolutionary relationships among these species.

Non-melanoma skin cancers are the most common malignant neoplasms in humans. About 95% of all non-melanoma skin cancers are represented by basal cell carcinoma and squamous cell carcinoma. Their prevalences are still increasing worldwide, representing an important public health problem. The genetic alterations underlying basal cell carcinoma and squamous cell carcinoma development are only partly understood. Much interest lies in determining the genetic basis of non-melanoma skin cancers, to explain their distinctive phenotypes, biological behaviors and metastatic potential. We present here a molecular genetic update, focusing on the most frequent genes and genomic instability involved in the development and progression of non-melanoma skin cancers. Keywords: Carcinoma, basal cell; Carcinoma, squamous cell; Chromosomal instability; Loss of heterozygosity; Microsatellite repeats; Skin neoplasms; Skin neoplasms/genetics Resumo: Os cânceres cutâneos não melanoma são as neoplasias malignas mais comuns em humanos. O carcinoma basocelular e o carcinoma espinocelular representam cerca de