The RAPD-PCR patterns indicate strong genetic differentiation within and among populations in the Paraná and the Tibagi Rivers. In the Paraná River at Porto Rico, the high levels of allelic polymorphism may represent the pristine composition of different species or populations within the H. malabaricus species complex, or they may be the outcome of faunal mixing caused by the closing of the Itaipu Dam in 1978 and the consequent flooding of Sete Quedas, another Oligocenic geographic barrier (Sampaio, 1988). Sete Quedas was the natural boundary for the Upper and Middle Paraná basins. The presence of an outlier in the Paraná River similar to the Iguaçu population suggests that the Paraná may indeed be a recipient of trahiras of diverse origins.
The Londrina sample from the Tibagi River is most similar to the populations of the Paraná (Figures 1, 2, and 3). On the other hand, while the Londrina sample is genetically homogeneous, the sample from Sertanópolis included one specimen with a molecular profile similar to Londrina, and the remaining four specimens different from all other samples along the Tibagi River (Figure 3). These well-defined clusters suggest that at least three yet undescribed species, or highly differentiated populations of H. malabaricus, co-exist in the Tibagi River. A similar situation was reported for H. malabaricus in the Aguapey River (Province of Corrientes, Argentina) (Dergam, 1996). To test these hypotheses, sequencing and morphological analyses of voucher material from the Tibagi River are currently being done by J.A.D. The patterns of genomic variation obtained with RAPDs suggest a close kinship between the newly established population of H. malabaricus in the Iguaçu and the sample from the Tibagi at Ponta Grossa. Lower genetic variation of RAPDs in the Tibagi sample than that observed in the Iguaçu/Segredo sample may be either an artifact of small sample size, the outcome of random genetic drift, or a combination of both factors: the Tibagi/Ponta Grossa sample came from a lagoon close to the headwaters of the Tibagi.
Although Haseman (1911) described the division between the Tibagi and the Iguaçu as "short and low", molecular similarity is not sufficient to support or reject the possibility that founder animals came from the Tibagi. The possibility of the existence of other source populations is supported by the fact that small samples (N = 3 each) from coastal populations of this species (Perequê River in Paranaguá and Ribeira River at Registro) display patterns of variation of RAPD alleles similar to the sample from Tibagi/Ponta Grossa (Dergam, 1996). Phylogenetic sequence analyses of the mitochondrial DNA of the Ponta Grossa, Segredo, and coastal populations corroborate that they are more closely related to each other than to any of the populations from the Tibagi or Paraná, and that all those populations are members of a more inclusive coastal clade (Dergam, 1996).
High levels of genetic diversity (genomic and mtDNA data) in the Tibagi and the Paraná Rivers suggest the existence of differentiated populations or species within the H. malabaricus species complex. Moreover, the introduced population of H. malabaricus in the Iguaçu was most likely derived from populations with a genetic composition similar to the sample from the headwaters of the Tibagi.
Alberto Fenocchio kindly provided samples from Argentina. J.A.D. was supported by a travel and study from CAPES (Process No. 2732/92).