Chromosomal variation in the tropical armoured catfish Callichthys callichthys (Siluriformes, Callichthyidae): implications for conservation and taxonomy in a species complex from a Brazilian hotspot.

The Neotropical catfish family Callichthyidae is characterized by wide karyotype variation, although reports are restricted to a few species/populations. This work provides new chromosomal information in two populations of armoured catfish (Callichthys callichthys) within the Atlantic forest boundaries in northeastern Brazil. Both populations of C. callichthys presented 2n=54 (16 metacentric, 24 submetacentric, 6 subtelocentric, and 8 acrocentric chromosomes) with occurrence of a metacentric B macrochromosome, reported for the first time in "2n=54" karyomorph. The lack of heterochromatin in the supernumerary chromosome suggests a recent origin of Bs with a significant difference in their frequency (p<0.05) between localities. Contrasting to the predominance of single nucleolus organizer regions (NORs) in other populations from South America, a remarkable polymorphism of 18S rDNA and GC-rich sites was detected, inasmuch as NOR phenotypes were unique for each individual, comprising up to 13 NOR-bearing chromosomes. The 5S rDNA was also mapped onto multiple chromosomes nonsyntenic to NORs. Such divergence in a number of NORs and occurrence of an euchromatic B chromosome in relation to other karyomorphs reinforces the divergent evolution of C. callichthys populations. Furthermore, these results also support central and drier Atlantic forest areas in northeastern Brazil as a biodiversity hotspot.