Here we report, for the first time, the complete mitochondrial genome of Astyanax giton which, together with other species, are popularly known as tetras. The mitogenome’s length is 16,643 bp, containing 13 protein-encoding genes (CDS), two ribosomal RNAs (rRNA), 22 RNA transfer (tRNA) and one control region (D-loop). As for other vertebrates, all genes are encoded on the heavy strand except for ND6 and eight tRNA genes. In the phylogenetic analyses, this species and other Astyanax were paraphyletic.

This work constitutes the first cytogenetic characterization of a trans-Andean species of Heptapteridae. The catfish Pimelodella cf. chagresi from the Upper Rio Magdalena was studied, applying standard cytogenetic techniques (Giemsa, C-banding, and argyrophilic nucleolar organizer region [Ag-NOR]) and fluorescence in situ hybridization techniques using repetitive DNA probes: microsatellites (CA and GA) and ribosomal RNA (rRNA) multigene families (18S and 5S recombinant DNA [rDNA] probes). The species showed a unique diploid chromosome number 2n = 50 (32m [metacentrics] +14sm [submetacentrics] +4st [subtelocentrics]) and a XX/XY sex chromosomal system, where the heteromorphic Y-chromosome revealed a conspicuous accumulation of all the assayed domains of repetitive DNA. P. cf. chagresi karyotype shares common features with other Heptapteridae, such as the predominance of metacentric and submetacentric chromosomes, and one pair of subtelomeric nucleolar organizer regions (NORs). These results reflect an independent karyological identity of a trans-Andean species and the relevance of repetitive DNA sequences in the process of sex chromosome differentiation in fish; it is the first case of syntenic accumulation of rRNA multigene families (18S and 5S rDNA) and microsatellite sequences (CA and GA) in a differentiated sex chromosome in Neotropical fish.

The tree frog <i>Aplastodiscus</i> is a Neotropical taxon that encompasses 15 species in the Atlantic forest biome, with one isolated species in the Central Brazilian Cerrado. To date, only 8 species have been karyotyped, showing high levels of diploid number variation, which allowed clustering species in chromosome number groups: 2n = 24 (<i>Aplastodiscus perviridis</i> group), 2n = 22 (<i>Aplastodiscus albofrenatus</i> group), 2n = 20, and 2n = 18 (both within <i>Aplastodiscus albosignatus</i> group). This study aims to report karyotypic information on 4 species from the last 2 groups using classical and molecular cytogenetic techniques and hypothesize chromosomal evolutionary trends within the species groups. <i>Aplastodiscus weygoldti</i> showed 2n = 22; Ag-NOR and FISH 18S rDNA signals were located in the interstitial region of the short arms of chromosome pair 6. <i>Aplastodiscus cavicola, Aplastodiscus</i> sp. 4, and <i>Aplastodiscus</i> sp. 6 showed 2n = 18; Ag-NOR and FISH 18S rDNA bands were located in the terminal region of the long arm of chromosome pair 9. Our results support multiple and independent chromosome fusion events within <i>Aplastodiscus</i>, including a new chromosome fission event<i>.</i> Ag-NOR and FISH 18S rDNA patterns were restricted to the small chromosome pairs, similar to the other species within this genus, and confirm overall chromosome morphology conservation among the genera of Cophomantinae.

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The genus Enyalius is composed of 10 described species inhabiting forest areas in Amozônia, Cerrado and Atlantic forest biomes. Currently, eight species with high levels of chromosome variation have been karyotyped. The study aims to characterize the karyotype of Enyalius boulengeri, with classical and molecular techniques, and improve knowledge about the karyotype evolution of the lizard genus Enyalius. The species has 2n = 36 chromosomes (8m + 4sm + 24mc), FN = 24; NORs and 18S rDNA were subtelomeric and located on chromosome pair 2. Repetitive DNA probes (CAT)10 accumulated on centromeric and terminal regions of some macrochromosomes. (GA)15 probe showed conspicuous accumulation on the pericentromeric region of chromosome pairs 1 and 6. Repetitive FISH patterns obtained with (GC)15 probe marked the pericentromeric region of the first chromosome pair. All probes showed accumulation in the microchromosomes. The chromosomal formula found on E. boulengeri has been considered the ancestral karyotype for pleurodont Iguania. The genus Enyalius is characterized by two distinctive chromosomal groups; one with highly conserved karyotypes, whereas the other is karyotypically diverse. Our molecular cytogenetics data are promising and will increase knowledge about the genus Enyalius chromosome evolution.