Cytogenetic identification of invasive fish species following connections between hydrographic basins

Chromosome analyses were carried out for several fish species in a region impacted by the connection between two hydrographic basins, where a river originally pertaining to the Parana River basin was transposed to the So Francisco River basin in the state of Minas Gerais, Brazil. The purpose was to assess the dispersal of invasive species from one hydrographic basin to the other and the consequent new geographical distributions of species that had been previously isolated from each other. Species of the families Parodontidae (Apareiodon piracicabae, A. ibitiensis, Parodon hilarii, and P. nasus), Sternopygidae (Eigenmannia virescens), and Gymnotidae (Gymnotus sylvius) were studied. The four species of Parodontidae had 2n = 54 chromosomes, but species-specific karyotype structures, including the presence of the ZZ/ZW sex chromosome system in A. ibitiensis and P. hilarii. Two distinct karyomorphs (A and B) were identified for E. virescens, although with the same diploid number (2n = 38 chromosomes). G. sylvius had 2n = 40 chromosomes. P. hilarii and E. virescens (karyomorph B) are considered to be native to the So Francisco River basin, whereas A. piracicabae, A. ibitiensis, E. virescens (karyomorph A), and G. sylvius are known for the Parana River basin, and P. nasus is known for the Parana-Paraguay basin. The presence of the last five species in the So Francisco River basin indicates that they migrated from the Parana River to the So Francisco River basin due to the transposition of the river, and can therefore be considered invasive species in this basin. Moreover, a natural migratory pathway through the former wetland may have affected this dispersal.