Combining cytogenetic and molecular data in taxonomically debated fish species: two case studies.
Combined molecular and cytogenetic approaches were adopted to investigate the debated-taxonomic status of fish species, belonging to unrelated families, characterized by different ecology, biology and distribution range. In details, our aim was to verify the morphological assignment of samples, to identify cytotaxonomic markers and to reconstruct phylogenetic relationships.
The first case regards species of the genus Pseudoplatystoma (Pimelodidae), distributed in the major river basins of South America, where they represent important fisheries resources and play an important ecological role due to their piscivirous and migratory habits. Traditional taxonomy recognized three species, but recent morphological analyses raised their number to eight, including two new species, P. orinocoense and P. metaense, distributed exclusively in the Orinoco basin. These two species were found to have very conservative karyotypes (2n=56) and similar cytogenetic features in terms of both constitutive heterochromatin distribution and number and location of minor and major ribosomal genes. The analysis of cytochrome b (1110 bp) and a fragment of COI gene (432 bp) confirmed the assignment of P. metaenseand P. orinocoensespecimens to the two distinctive Pseudoplatystoma molecular clades identified in the Orinoco basin. The gene genealogies obtained with these two mitochondrial markersconfirms that the identified molecular clades provide support for the recognition of only some of the eight described morphospecies.
The second case regards some Italian endemic Leuciscinae species (Cyprinidae), the Toscana stream chub, Squalius lucumonis,an endangered species restricted to the Tyrrhenian drainage basins of Central Italy (Tiber, Arno, Ombrone, and Serchio) along with three sympatric species, S. squalus, Telestes muticellus and Rutilus rubilio. The taxonomic status of S. lucumonis was repeatedly questioned by several authors, who argued that it could represent a hybrid between two of the previously mentioned sympatric species. Theanalysis of cytochrome b (1110 bp) and of a non-coding nuclear region, Cyfun P (Cyprinid formerly unknown nuclear Polymorphism), confirmed the validity of the four different taxa and proved to be useful to exclude the presence of hybrids. Comparative chromosomal localization of minor ribosomal genes through FISH with 5S rDNA probe showed that these chromosomal regions are differentially distributed among the four species and represent useful cytotaxonomic markers in leuciscine fishes.