Heterogeneity and meiotic behaviour of B and sex chromosomes, banding patterns and localization of (TTAGGG)(n) sequences by fluorescence in situ hybridization in the neotropical water rat Nectomys (Rodentia, Cricetidae)

A cytogenetic study using fluorescence in situ hybridization (FISH) of telo mere probes, CBG, GTG and RBG banding patterns and synaptonemal complex dat a was carried out in 41 specimens of Nectomys from three Brazilian states: Pernambuco, Mate Grosso and Sao Paulo. The specimens presented 2n = 52, 53, 56 and 57, and the differences in chromosome number were due to the presen ce of three different supernumeraries and also to the occurrence of tandem fusions. The tandem fusions involved chromosome pairs 3 + 11 and 5 + 24 fro m karyotype with 2n = 56 that originated pairs 1 and 4 in specimens with 2n = 52. Sex chromosome polymorphism was also detected, and the X presented t hree different morphologies, which could be explained by heterochromatin ad dition/deletion. FISH results revealed interstitial telomeric bands (ITBs) In a submetacentric B, but no ITB was detected in the chromosomes originate d by tandem fusion. The supernumeraries presented a remarkable heterogeneit y of size and morphology, constitutive heterochromatin pattern and localiza tion of telomeric sequences. Synaptonemal complex by light and electron mic roscopy showed the supernumerary as an autopaired univalent. The sex chromo some pairing in meiotic cells involved the heterochromatic short arm of the X chromosome and the short arm of the Y chromosome.