Caracterização Cromossômica de três espécies de Serrasalmini (Serrasalmidae: Characiformes)

Abstract

The Serrasalmini tribe (Bleeker 1859) comprises six genera, with several species popularly known as “piranhas”. It is a diverse group, distributed throughout South America, which presents taxonomic uncertainties, especially in paraphyletic genera, due to their great morphological similarity. Several studies have been carried out to solve and/or understand this problem, through ecological, morphological, molecular or cytogenetic studies. Classical and molecular cytogenetic approaches have already been carried out in some species, representing the various clades of the Serrasalmidae family, but not all. Thus, this study aims to characterize, cytogenetically, a clade that is found in the Serrasalmini tribe and that had no published results, for a better understanding of the chromosomal evolution of the family. Three species, from three genera, were analyzed: Pristobrycon striolatus, Catoprion absconditus and Pygopristis denticulata. All of them have a diploid number (2n) equal to 62 chromosomes, with one and two arms, with species-specific karyotypic formulas and fundamental number. The heterochromatin pattern was evidenced in the centromeric and terminal (bitelomeric) regions in most chromosomes, in addition to a conspicuous interstitial block in the 1(m) pair in the species. The Nucleolus Organizer Regions (Ag-NOR) were multiple and positive for the C band, and their location was confirmed by 18S ribosomal DNA mapping. However, additional 18S rDNA sites were evidenced. The 5S rDNA was located in a chromosomal pair, interstitial in the long arm, corresponding to the 1(m) pair in the three species. Furthermore, we evidenced synteny between 18S and 5S in the species C. absconditus and P. denticulata which, according to fiber-FISH, are intercalated. Telomeric sequences were detected in terminal regions of the chromosomes of the three species, without the presence of interstitial telomeric sequences (ITS). Thus, the maintenance of 2n (62) in the three species corroborates their phylogenetic relationship, based on molecular characters, and the diversification of chromosomal formulas within the clade shows the presence of non-Robertsonian rearrangements. The 1(m) pair can be considered a cytotaxonomic marker for the clade, as the three species have an interstitial heterochromatin block associated with 5S rDNA, a chromosomal pair that has been considered homeologous within the family. Synteny of 18S and 5S rDNA in C. absconditus and P. denticulata corroborate their phylogenetic proximity and also the paraphyletism of the genus Pristobrycon.