Cytogenetic and Molecular Characterization of aff. (Gymnotiformes: Sternopygidae): A First Report of System of Sex Chromosomes ZWW/ZZ in Gymnotiformes.

Gymnotiformes a monophyletic group of fish endemic to the Neotropics, represent an important component of the freshwater ichthyofauna that presents relevant taxonomic problems. Thus, in view of the morphological complexity involving (Gymnotiformes) fish species, this study aimed to characterize aff. of the upper Paraguay River basin through cytogenetic and molecular analyses, to help in the correct identification and delimitation of species. This study reports a multiple sex system of the type ZWW/ZZ, with 2 = 31 for females and 2 = 30 for males. A single pair of chromosomes carrying the nucleolar organizing regions (NORs) was detected. The heterochromatin was colocated in NOR sites and mainly located in the centromeric regions of chromosomes. Besides that, individual sequences from the specimens of . aff. were obtained, totalizing three haplotypes. The distance between the haplotypes in . aff. , ranged from 0.2% to 7.1%. Species delimitation tests indicated the existence of two possible operational taxonomic units of . aff. . Thus, this study reports a new multiple sex system in Gymnotiformes and these specimens previously identified as . aff. may belong to two distinct species.