Karyotypic differentiation through chromosome fusion and number reduction in Imparfinis hollandi (Ostariophysi, Heptapteridae)

The Neotropical Heptapteridae fish Imparfinis hollandi, endemic to the Iguaçu River Basin (Brazil), was cytogenetically analyzed and the diploid chromosome number of 2n = 42 chromosomes was determined (22m + 10sm + 10st), the lowest diploid number in this genus and family. Like other Heptapteridae species, only one NOR-bearing chromosome pair was detected by silver nitrate staining. Dark heterochromatic blocks were visualized in only three chromosome pairs, and chromomycin A3 + bands were coincident with Ag-NORs. Although no intercalary (TTAGGG)n sequence was observed through FISH with a telomere probe, an asymmetric karyotype showing four large chromosome pairs with diploid chromosome number reduction suggests that tandem chromosome fusions probably occurred during the karyotypic differentiation of Imparfinis hollandi.