FIRST DESCRIPTION OF THE KARYOTYPE AND Ag-NORs LOCALIZATION OF STEPHANOLEPIS SETIFER (BENNETT, 1831) (TETRAODONTIFORMES: MONACANTHIDAE) WITH REMARKS ON SEX CHROMOSOME DIFFERENTIATION

Resumen

ABSTRACT: A cytogenetic study of Stephanolepis setifer by conventional Giemsa stain and localization of NucleolusOrganizer Regions (NORs) by silver staining is reported. Females possess a diploid chromosome number of 2n=34, all of whichare acrocentric, Males exhibit a karyotype with 2n=33 chromosomes, consisting of 32 acrocentric chromosomes includingtwo unpaired acrocentric chromosomes, and one medium-size metacentric chromosome. The results suggest the presence ofa multiple sex chromosome system of X1X1X2X2/X1X2Y type. The X1 and X2 chromosome sizes suggest that themetacentric chromosome is derived from Robertsonian fusion between the original Y chromosome and an autosome whichprobably corresponded to one element of the pair 16 and another of the pair 17, resulting in a neo-Y sex chromosome. Silverstaining of metaphase spreads revealed a single pair of Ag-NORs interstitially located on the chromosome pair numbered 7.The comparison of our results with previous reports in Tetraodontiformes species suggest that its evolutionary divergence hasbeen accompanied by macrostructural karyotype changes as far as microstructural alteration in NORs cluster.

Key words: Cytogenetic, multiple sex chromosomes, Ag-NORs

RESUMEN: A cytogenetic study of Stephanolepis setifer by conventional Giemsa stain and localization of the NucleolusOrganizer Regions (NORs) by silver staining is reported. Females feature a diploid chromosome number of 2n=34 chromosomes,all of which are acrocentric. Males exhibit a karyotype with 2n=33 chromosomes, consisting of 32 acrocentric chromosomes,including two unpaired ones and one medium-size metacentric chromosome. The results suggest the presence of a multiple sexchromosome system of the X1X1X2X2/X1X2Y type. The size of chromosomes X1 and X2 suggests that the metacentricchromosome is derived from a Robertsonian fusion between the original Y chromosome and an autosome, probablycorresponding to one element of pair 16 and to another of pair 17, resulting in a neo-Y sex chromosome. Silver staining ofmetaphase spreads revealed a single pair of Ag-NORs interstitially located on the seventh pair. The comparison of our resultswith previous reports in other Tetraodontiforme species suggests that their evolutionary divergence has been accompanied byboth macrostructural karyotype changes and microstructural NOR alterations.

Key words: cytogenetics, multiple sex chromosomes, Ag-NORs