Cryptic Diversity in the Terminal Portion of the Chromosomes of the Dogtooth Characins, Family Cynodontidae (Ostariophysi: Characiformes)

Karyotypic variation of two populations of the small freshwater stingray Potamotrygon wallacei Carvalho, Rosa & Araújo 2016: A classical and molecular approach

Potamotrygoninae comprises a group of Neotropical fishes with an ancient relationship with marine environments. In the last few years, 11 new Potamotrygon species were described, including Potamotrygon wallacei Carvalho, Araújo e Rosa 2016. Cytogenetic data about this species are limited to classical markers (Giemsa, C-Banding and Ag-NOR techniques), these studies highlighted a rare sexual chromosome system XX/X0 with males presenting 67 chromosomes and females 68 chromosomes. The classical analyses performed here reveled populational variation in the karyotype formula, as well as, in the heterochromatin regions. Besides the classical markers, our molecular experiments showed multiple sites for 18S rDNA sequence (including in the X chromosomes) and single sites for 5S rDNA sequence, we did not find interstitial telomeric sequences. In addition, (AC)15, (AG)15, and (CAC)15 microsatellites showed association with the several autosome pair, and the (GT)15 clutters were found in only one population. On the other hand, (GATA)4 sequence showed association with the sexual chromosomes X in all males and females analyzed. Our results showed that pericentric inversions, in addition to fusions, shaped the karyotype of P. wallacei once we found two populations with distinct karyotype formula and this could be a result of the past events recovered by our modeling experiments. Besides, here we described the association of 18S and (GATA)4 motifs with sexual chromosomes, which indicated that these sequences had a novel in the differentiation of sexual chromosomes in P. wallacei.

Evolutionary Dynamics of Two Classes of Repetitive DNA in the Genomes of Two Species of Elopiformes (Teleostei, Elopomorpha)

The order Elopiformes includes fish species of medium to large size with a circumglobal distribution, in both the open sea, coastal, and estuarine waters. The Elopiformes are considered an excellent model for evolutionary studies due to their ample adaptive capacity, which allow them to exploit a range of different ecological niches. In this study, we analyzed the karyotype structure and distribution of two classes of repetitive DNA (microsatellites and transposable elements) in two Elopiformes species (Elops smithi and Megalops atlanticus). The results showed that the microsatellite sequences had a very similar distribution in these species, primarily associated to heterochromatin (centromeres and telomeres), suggesting these sequences contribute to the chromosome structure. In contrast, specific signals detected throughout the euchromatic regions indicate that some of these sequences may play a role in the regulation of gene expression. By contrast, the transposable elements presented a distinct distribution in the two species, pointing to a possible interspecific difference in the function of these sequences in the genomes of the two species. Therefore, the comparative genome mapping provides new insights into the structure and organization of these repetitive sequences in the Elopiformes genome.