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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Open reading frame expressed sequences tags (ORESTES) differ from conventional ESTs by providing sequence data from the central protein coding portion of transcripts. We generated a total of 696,745 ORESTES sequences from 24 human tissues and used a subset of the data that correspond to a set of 15,095 full-length mRNAs as a means of assessing the efficiency of the strategy and its potential contribution to the definition of the human transcriptome. We estimate that ORESTES sampled over 80% of all highly and moderately expressed, and between 40% and 50% of rarely expressed, human genes. In our most thoroughly sequenced tissue, the breast, the 130,000 ORESTES generated are derived from transcripts from an estimated 70% of all genes expressed in that tissue, with an equally efficient representation of both highly and poorly expressed genes. In this respect, we find that the capacity of the ORESTES strategy both for gene discovery and shotgun transcript sequence generation significantly exceeds that of conventional ESTs. The distribution of ORESTES is such that many human transcripts are now represented by a scaffold of partial sequences distributed along the length of each gene product. The experimental joining of the scaffold components, by reverse transcription–PCR, represents a direct route to transcript finishing that may represent a useful alternative to full-length cDNA cloning.