Abstract
Corydoras Lacepède, 1803 is the most specious genus of Corydoradinae subfamily and many of its species are still unknown in relation to molecular cytogenetic markers. However, the diploid number and karyotypic formula were recorded for many species of this group. In current study, we provided the first cytogenetic information of Corydoras carlae Nijssen & Isbrücker, 1983, an endemic fish species from Iguassu River basin, Paraná State, Brazil. The individuals were collected in Florido River, a tributary of Iguassu River and analysed with respect to diploid number, heterochromatin distribution pattern, Ag-NORs and mapping of 5S and 18S ribosomal genes. The karyotype of this species comprises 46 chromosomes arranged in 22m+22sm+2st. The heterochromatin is distributed in centromeric and pericentromeric positions in most of the chromosomes, and also associated with NORs. The Ag-NORs were detected in the terminal position on the long arm of the metacentric pair 6. The double-FISH technique showed that 5S rDNA and 18S rDNA were co-localized in the terminal portion on the long arm of the metacentric pair 6. This condition of co-localization of ribosomal genes in Corydoras carlae seems to represent a marker for this species.
Highlights
In higher eukaryotes, rDNA is organized into two distinct gene classes: major class (45S rDNA), which contains the genes that code for the 18S, 5.8S and 28S rRNAs, and the minor class (5S rDNA), which contains the genes that code for 5S rRNA
The major rDNA sequences detected by fluorescent in situ hybridization (FISH) always coincided with silver-stained NORs (Ag-NORs) location, in species with multiple Ag-NORs the number of markings was usually smaller than the regions detected by the DNA probes
The double-FISH technique showed 5S rDNA cluster appears interspersed with 18S cistrons in the terminal portion of the long arm of pair 6 (Fig. 2c)
Summary
RDNA is organized into two distinct gene classes: major class (45S rDNA), which contains the genes that code for the 18S, 5.8S and 28S rRNAs, and the minor class (5S rDNA), which contains the genes that code for 5S rRNA. The major rDNA sequences detected by FISH always coincided with silver-stained NORs (Ag-NORs) location, in species with multiple Ag-NORs the number of markings was usually smaller than the regions detected by the DNA probes. The most common condition in the karyotype of different fish groups is the positioning of ribosomal genes in different chromosome pairs (Galetti Jr. and Martins 2004). Syntenic localization of the major rDNA clusters and the 5S sites were observed for the first time in the genus Corydoras Lacepède, 1803 (present study) and Callichthys callichthys (Linnaeus, 1758) (Konerat et al 2014), the other integrant of the family Callichthyidae. (Ziemniczak et al 2012) presented syntenic localization of ribosomal genes. The mapping of ribosomal genes has added important information about the chromosomal diversification in Corydoras, as in other groups of Siluriformes
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