Multiple interstitial ribosomal sites (NORs) in the Brazilian squirrel Sciurus aestuans ingrami (Rodentia, Sciuridae) with 2n = 40: an overview of Sciurus cytogenetics

This is the first time the karyotype of the Brazilian squirrel Sciurusaestuans ingrami, with 2n = 40, is described. The karyotype of this species comprises 18 pairs of biarmed and one minute pair of acrocentric autosomes, a medium-sized submetacentric X and a medium-sized acrocentric Y. Four pairs have an interstitial secondary constriction, co-located with nucleolar organizer regions (NORs), identified by silver-staining technique and fluorescent in situ hybridization (FISH) with ribosomal 18S/28S probes. The occurrence of multiple interstitial NORs is rare in rodents, and this is one of the few examples, identified by a molecular cytogenetics approach