Sympatric occurrence of four cytotypes and one extra chromosome in Bryconamericus ecai (Characidae): 18S rDNA polymorphism and heterochromatin composition

Abstract

In the present study, specimens of Bryconamericus ecai collected from the Forquetinha River/RS, were cytogenetically analyzed, disclosing a wide karyotypic diversity in this species. All individuals had 2n = 50, with different karyotypic formulae, resulting in four cytotypes and one B macrochromosome observed in cytotype III. Heterochromatin was distributed in the pericentromeric region of most chromosomes on the four cytotypes and also on a chromosome pair with interstitial markings in cytotype IV. Staining with CMA(3) and DAPI fluorochromes revealed a C-band region rich in AT base pairs in cytotypes I, II and III, and a pair with GC-rich heterochromatin in cytotypes II and III. Cytotype IV presented CMA(3) and DAPI positive heterochromatin. Silver nitrate impregnation, in situ hybridization, and fluorochrome staining showed a multiple system of AgNORs, 18S rDNA and CMA(3) sites in cytotypes I, III and IV, with both inter-and intraindividual variability in the number and location of these sites. Cytotype II had only one pair of NORs coincident with the 18S rDNA and CMA(3) sites, indicating a simple system. The chromosomal polymorphism observed among the specimens of B. ecai added to the literature data show that chromosomal rearrangements, especially pericentric inversions, play an important role in the karyotypic evolution of this group of fish. It can also be implied that more than one species of Bryconamericus is probably occurring, living in sympatry in the Forquetinha River/RS.

Occurrence of Cytotypes and Variability of AgNORs in a Population of <i>Bryconamericus</i> aff. <i>iheringii</i> (Characidae)

“…iheringii analyzed so far, including those of the present study (Table 1), 9 cytotypes were found and none were repeated among the populations. From an evolutionary viewpoint, some authors have suggested that the changes in the fundamental numbers are caused by chromosomal rearrangements, translocations, and mainly pericentric inversions and that these are the mechanisms of diversification among the species of Bryconamericus (Wasko et al 1996;Wasko and Galetti Jr. 1998;Santos et al 2012), and this may be occurring among these populations of Bryconamericus aff. iheringii.…”

“…However, one cannot rule out the possibility that, in some cases, this may be occurring between different species. Santos et al (2012) analyzed a population of Bryconamericus ecai from the Forquetinha River/RS and found 2n=52 and different karyotypic formulae resulting in 4 cytotypes. The authors also suggested that pericentric inversions might be occurring between individuals of B. ecai or that more than 1 species was present at that location.…”

“…E, analyzed by Wasko and Galetti Jr (1998), presents a different diploid number, 2n=54 and the variations in the karyotypic structure of this group of fish are significant, as listed in Table 1. The nucleolus organizer regions (NORs) are also quite variable in this genus, and may present intra-and intervariations in the number and location of ribosomal DNA cistrons, thus featuring Bryconamericus as a group with multiple NORs (Wasko et al 1996, Wasko and Galetti Jr. 1999, Paintner-Marques et al 2002a, Capistano et al 2008, Santos et al 2012. However, simple NORs have already been found in 2 populations of B. aff.…”

“…However, simple NORs have already been found in 2 populations of B. aff. iheringii Boulenger, 1887 by Paintner-Marques et al (2003) and Capistano et al (2008), and recently, Santos et al (2012) have observed single and multiple NORs in the same population of Bryconamericus ecai.…”

“…The species of the genus Bryconamericus present specific C-banding patterns, showing variability in the heterochromatin location, are commonly found in pericentromeric heterochromatic regions and terminal (Paintner-Marques et al 2003 and some additional interstitial bands (Wasko et al 1996, Santos et al 2012.…”