Surgeonfishes are a species-rich group and a major biomass on coral reefs. Three species are commonly found throughout South Atlantic, Acanthurus bahianus, A. chirurgus, and A. coeruleus. In this paper, we present the first cytogenetic data of these species, revealing a sequential chromosomal diversification. A. coeruleus was characterized by a relatively conserved karyotype evolved by pericentric inversions of some pairs (2n = 48, 2sm + 4st + 42a). In contrast, the karyotypes of A. bahianus (2n = 36) and A. chirurgus (2n = 34) were highly differentiated by the presence of six large metacentric pairs in A. bahianus (12m + 2sm + 4st + 18a) and A. chirurgus (12m + 2sm + 4st +1 6a) probably derived by chromosomal fusions that corroborate their closer relationship. A discernible in tandem fusion represents an autapomorphic character to A. chirurgus. In spite of macrostructure variation, single nucleolar organizer regions (NORs) on short arms of a subtelocentric pair and similar distribution of C-bands were observed in the three species. Overlapping of chromosomal data with molecular phylogeny indicated pericentric inversions which took place nearly at 19 Ma while centric fusions are as recent as 5 Ma. A physical mapping of coding and noncoding sequences in Acanthurus could clarify the role of additional rearrangements during their chromosomal evolution.

Cytogenetic studies in Neotropical electric knifefish of genus Gymnotus have shown a remarkable interspecific variability, including distinct sex chromosome systems. In this study, we present the first chromosomal data in Gymnotus bahianus from Contas River basin, northeastern South America. Based on extensive analyses, the modal diploid values were 2n = 36 (30m/sm + 6st) for females and 2n = 37 (32m/sm + 5st) for males. Therefore, a novel XX/XY1Y2 sex chromosome system is described for the genus. Single nucleolar organizer regions (NORs) interspersed to GC-rich sites were detected on a subtelocentric pair (7th) for both sexes and confirmed by fluorescent in situ hybridization with 18S rDNA probes. Heterochromatin was detected at pericentromeric regions of all chromosomes and interspersed to NORs on pair 7 and 5S rDNA cistrons on pair 9. The highly differentiated karyotype of Gymnoytus bahianus, with low diploid numbers and a unique XX/XY1Y2 system, reinforces the independent origin of sex chromosomes in Gymnotiformes and seems to reflect the particular evolutionary history of this species in a small and isolated drainage system. Moreover, in spite of morphological similarities, the present results indicate a remarkable chromosomal divergence in relation to closely related species such as G. sylvius and G. carapo.

The first chromosomal data in Hoplosternum littorale from an isolated South American drainage in north-eastern Brazil are presented. All specimens were characterized by a diploid number (2n) of 60 chromosomes divided into three metacentric, one submetacentric and 26 acrocentric pairs; single nucleolar organizer regions (NOR) on the sixth pair; centromeric and interstitial heterochromatin; GC-rich sites on four large acrocentric chromosomes, including the NOR-bearing pair, and 5S ribosomal genes at terminal region on short arms of two acrocentric pairs. These data are invariably similar to previous reports in H. littorale from distant localities throughout South America, which contrasts with the chromosomal diversity of Callichthyidae and reinforces the role of human activities on the dispersal and colonization of this fish.

The Neotropical catfish family Callichthyidae is characterized by wide karyotype variation, although reports are restricted to a few species/populations. This work provides new chromosomal information in two populations of armoured catfish (Callichthys callichthys) within the Atlantic forest boundaries in northeastern Brazil. Both populations of C. callichthys presented 2n=54 (16 metacentric, 24 submetacentric, 6 subtelocentric, and 8 acrocentric chromosomes) with occurrence of a metacentric B macrochromosome, reported for the first time in "2n=54" karyomorph. The lack of heterochromatin in the supernumerary chromosome suggests a recent origin of Bs with a significant difference in their frequency (p<0.05) between localities. Contrasting to the predominance of single nucleolus organizer regions (NORs) in other populations from South America, a remarkable polymorphism of 18S rDNA and GC-rich sites was detected, inasmuch as NOR phenotypes were unique for each individual, comprising up to 13 NOR-bearing chromosomes. The 5S rDNA was also mapped onto multiple chromosomes nonsyntenic to NORs. Such divergence in a number of NORs and occurrence of an euchromatic B chromosome in relation to other karyomorphs reinforces the divergent evolution of C. callichthys populations. Furthermore, these results also support central and drier Atlantic forest areas in northeastern Brazil as a biodiversity hotspot.