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...

Concerted evolution leading to homogenization of tandemly repeated DNA arrays is widespread and important for genome evolution. We investigated the range and nature of the process at chromosomal and array levels using the 1.688 tandem repeats of Drosophila melanogaster where large arrays are present in the heterochromatin of chromosomes 2, 3, and X, and short arrays are found in the euchromatin of the same chromosomes. Analysis of 326 euchromatic and heterochromatic repeats from 52 arrays showed that the homogenization of 1.688 repeats occurred differentially for distinct genomic regions, from euchromatin to heterochromatin and from local arrays to chromosomes. We further found that most euchromatic arrays are either close to, or are within introns of, genes. The short size of euchromatic arrays (one to five repeats) could be selectively constrained by their role as gene regulators, a situation similar to the so-called "tuning knobs."

Fluorescence in situ hybridisation (FISH) and double FISH experiments were carried out to ascertain the chromosomal distribution pattern of the 45S and 5S ribosomal (r) DNAs in four populations of the characid fish Astyanax scabripinnis--a group considered to be a species complex for its wide karyotypical and morphological diversity. The results regarding the 45S rDNA agreed with this hypothesis, since these sites showed intra- and inter-populational, numerical and positional variations. However, the data obtained with the 5S rDNA probe revealed a highly conserved chromosomal distribution pattern of these sequences among individuals of each population, as well as among the populations analysed. We consider this contrasting situation as a functional divergence between 45S and 5S ribosomal DNAs, which may reflect the localisation of these sequences in distinct nuclear compartments, leading them to undergo differentiated evolutionary processes.

Karyotype and other chromosomal markers of Characidium cf. gomesi were analyzed using conventional (Giemsa-staining, Ag-NOR and C-banding) and molecular (Fluorescent in situ hybridization (FISH) with 18S and 5S rDNA biotinylated probes) techniques. Both sexes had invariably diploid chromosome number 2n = 50 while karyotypes of males and females differed. That of male consisted of 32 metacentric + 18 submetacentric chromosomes and that of female consisted 31 metacentric + 18 submetacentric + 1 subtelocentric chromosomes. The Z chromosome was medium-sized metacentric, while W was highly heterochromatinized subtelocentric element. NORs as revealed by Ag-staining were situated at 2-7 telomeric regions while FISH with 18S probes showed consistently 10 signals at telomeric regions. FISH with 5S rDNA probe showed constantly signals at one metacentric pair. Distribution of centromeric heterochromatin was mostly in all chromosome pairs, besides some telomeric sites. The common origin of the sex chromosome system of ZZ/ZW type in the karyotypes of other representatives of the genus analyzed so far might be hypothesized based on biogeography and partial phylogeny of the group.

We present cytogenetic analyses of four fish species, belonging to four Loricariidae subfamilies: Neoplecostomus microps (Neoplecostominae) with 2n ¼ 54 chromosomes, Harttia loricariformis (Loricariinae) with 2n ¼ 56 chromosomes, Hypostomus affinis (Hypostominae) with 2n ¼ 66 chromosomes and Upsilodus sp. (Upsilodinae), with 2n ¼ 96 chromosomes. In addition to karyotypes, data on the location of 18s rDNA sites are presented, derived from indirect (silver nitrate impregnation) and direct (FISH) methods. There is only one pair of nucleolar organizing regions (NORs) per species, except in H. affinis. Diversity and NOR macrokaryotypic evolution in the species analyzed are discussed in relation to the evolution of the Loricariidae as a whole. In addition, a revision of the cytogenetic data available for this family is presented.

Cytogenetic studies were conducted on 154 specimens of AstyΑnΑx scabripinnis collected at three localities in the Campos do JordÃo region (State of SÃo Paulo, Brazil). The C-banding pattern suggested that the metacentric B chromosome found in most of the specimens is an isochromosome derived from chromosome 24, the only chromosome in the standard complement that carries interstitial C-bands similar to those present in each arm of the B chromosome. The sex ratio was biased toward females in the CÓrrego das Pedras and RibeirÃo do Casquilho streams and toward males in the RibeirÃo das Perdizes stream. In all three populations analyzed, the B chromosome was more frequent in females than in males. In the most exhaustively sampled population (those from CÓrrego das Pedras), there was a highly significant association between B-chromosome frequency and sex-ratio distortion, with a disproportionately high number of males without B chromosomes and females with one B chromosome.

A chromosome analysis was carried out in two sympatric fish species of the genus Parodon, Parodon sp. and P. tortuosus, from the Paraná basin, São Paulo State, Brazil. Although both species showed the same diploid number (2n=54), an interspecific diversity was detected concerning their karyotypic formulas and banding patterns, besides a ZZ/ZW sex chromosome system detected in Parodon sp., which was caracterized as a new species for this genus. No heteromorphic sex chromosomes were found in P. tortuosus. These data are discussed concerning the characterization of the regional ictiofauna and its evolutionary aspects.

Hoplias malabaricus, a widely distributed neotropical freshwater fish, shows a conspicuous karyotypic diversification. An overview of this diversity is presented here comprising several Brazilian populations, and some others from Argentina, Uruguay and Surinam. Seven general cytotypes are clearly identified on the basis of their diploid number (2n = 39 to 2n = 42), chromosomal morphology and sex chromosome systems, which can be clustered into two major karyotypic groups. This clustering suggests that karyotype structure would be more informative than the diploid number regarding cytotype relationships in this fish group. While some cytotypes show a wide geographical distribution, some others appear to be endemic to specific hydrographic basins. Sympatric cytotypes can occur without detection of hybrid forms; this situation points to a lack of gene flow, a fact that is also reinforced by studies with genomic markers. The karyotypic data support the view that the nominal taxon H. malabaricus corresponds to a species complex comprising distinct evolutionary units, each with well-established chromosomal differences.