Karyotype structure of hypostomus cf. Plecostomus (Linnaeus, 1758) from tapajós river basin, southern amazon: Occurrence of sex chromosomes (ZZ/ZW) and their evolutionary implications

Abstract

Hypostomus is a group of fish with numerical and struc­tural karyotypic variability. Among them, only six species, three of which belong to the Amazon basin, show a sex chromosome. In this study, we present the karyotype structure of Hypostomus cf. plecos­tomus from the Teles Pires river basin in the municipality of Alta Flo­resta, MT. The species has 2n = 68 and the karyotype formula 14m+ 24sm+ 14st+ 16a [fundamental number (FN) = 120] in males and 15m+ 24sm+14st+15a (FN = 121) in females and sex chromosomes ZZ/ZW. Argyrophilic nucleolar organizer regions (AgNORs) were identified in two pairs of chromosomes at different positions: short arm of the pair 21and long arm of the pair 27, matching the signals displayed by 18S FISH and indicating multiple NORs. Analysis of band C detected few blocks of constitutive heterochromatin in the pericentromeric regions of most chromosomes and the telomeric regions of some pairs, includ­ing the nucleolar pair 21. However, large blocks on the long arm of the nucleolar pair 27 still stood out. GC-rich heterochromatin (CMA3) was visualized only coincidently with nucleolar sites. Mapping of 5S rDNA sites with FISH revealed markings in eight chromosomes, demonstrat­ing synteny between the 18S and 5S sites. The data obtained for H. cf. plecostomus are important for taxonomic studies of this Amazon com­plex “H. plecostomus group”. The occurrence of sex chromosomes in Amazon species of Hypostomus suggests an evolutionary event that is independent of other species in the group.