Abstract
Karyotypic features of Rhoadsia altipinna Fowler, 1911 from Ecuador were investigated by examining metaphase chromosomes through Giemsa staining, C-banding, Ag-NOR, and two-color-fluorescence in situ hybridization (FISH) for mapping of 18S and 5S ribosomal genes. The species exhibit a karyotype with 2n = 50, composed of 10 metacentric, 26 submetacentric and 14 subtelocentric elements, with a fundamental number FN=86 and is characterized by the presence of a larger metacentric pair (number 1), which is about 2/3 longer than the average length of the rest of the metacentric series. Sex chromosomes were not observed. Heterochromatin is identifiable on 44 chromosomes, distributed in paracentromeric position near the centromere. The first metacentric pair presents two well-defined heterochromatic blocks in paracentromeric position, near the centromere. Impregnation with silver nitrate showed a single pair of Ag-positive NORs localized at terminal regions of the short arms of the subtelocentric chromosome pair number 12. FISH assay confirmed these localization of NORs and revealed that minor rDNA clusters occur interstitially on the larger metacentric pair number 1. Comparison of results here reported with those available on other Characidae permit to hypothesize that the presence of a very large metacentric pair might represent a unique and derived condition that characterize one of four major lineages molecularly identified in this family.
Highlights
The study of fish chromosomes has become an active area of research in recent decades providing basic information on the number, size and morphology of chromosomes, nucleolus organizers regions (NORs), distribution of constitutive heterochromatin and other more specific markers, detected through the application of molecular techniques (Nirchio and Oliveira 2006a)
Impregnation with silver nitrate (Fig. 2c) showed a single pair of Ag-positive NORs located at terminal regions of the short arms of the subtelocentric chromosome pair number twelve
Cytogenetic studies in Characidae disclose great karyotype diversity related to the high variability of chromosome morphology among species and populations (Arai 2011), and the description of the karyotype of R. altipinna adds new data to this picture
Summary
The study of fish chromosomes has become an active area of research in recent decades providing basic information on the number, size and morphology of chromosomes, nucleolus organizers regions (NORs), distribution of constitutive heterochromatin and other more specific markers, detected through the application of molecular techniques (Nirchio and Oliveira 2006a). Characiformes comprises 2,081 valid species grouped in 23 families: Characidae is the largest with 15 subfamilies and 1,086 valid species (Eschmeyer and Fong 2015) These fish have the larger geographic distribution within this order occupying almost all environments of freshwater, with distribution in the Americas, from southwestern United States to South of Argentina (Lucena 1993). In Ecuador, among the freshwater fishes, the Characiformes is the second largest order for number of species (345), after Siluriformes (365) (Barriga 2012) and chromosome studies in the Neotropical area have been performed for 475 species of Characiformes (Oliveira et al 2009) until now there is an absolute absence of data from Ecuador
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