The unusual high number of chromosomes signals rare multiple fission events in the Polynemidae (Carangaria, Teleostei)
Polynemidae, commonly known as threadfins, are important fishing and aquaculture species. This is a peculiar fish group that is morphologically characterized by long tactile filaments of the pectoral fins. Although inserted in the Percomorpha, its relationships with other groups of this clade are uncertain. Cytogenetic investigations have helped clarify various evolutionary aspects of marine fish groups, including stock delimitation, speciation, taxonomy, hybridization, phylogenetic relationship, sex chromosomes, and genome changes. However, the chromosomal characteristics of threadfins are still unknown. We investigated the chromosomal features of two cryptic species, Polydactylus virginicus and P. oligodon found in Northeast Brazilian coastal waters (Tropical Southwestern Atlantic region). Cytogenetic data were obtained by conventional staining (Giemsa and fluorochrome staining, C-banding and Ag-NORs techniques) and molecular cytogenetic analyses through fluorescent in situ hybridization using rDNA probes (18S and 5S rDNA). The two species have similar karyotypes at the macro and microstructural levels. An unexpected high diploid number was found, with 2n=58 acrocentric chromosomes, including four microchromosome pairs. The expanded karyotypes are a synapomorphy for these species, likely resulting from sharing at least five chromosome fissions from a basal karyotype with 2n=48. Besides the macrostructural karyotype similarities, the Ag-NOR/GC-rich/18S rDNA (chromosome 19) and 5S rDNA (chromosome 20) loci have an identical organization in apparently homeolog chromosomes of these species, suggesting the conservation of large syntenic chromosomal regions. We conclude that although chromosome fissions are very rare events among other Carangaria fish, they played an important disruptive role in the evolution of some species of Polynemidae.