Nectomys rattus

Karyotype: 2n = 52 and FN = 52. Autosomal complement: one small metacentric pair and 24 acrocentric pairs large to small decreasing in size (Gardner & Patton 1976, pp. 13, Fig. 6B; Maia et al. 1984; Yonenaga-Yassuda et al. 1988; Barros et al. 1992; Bonvicino et al. 1996; Silva & Yonenaga-Yassuda 1998a; Andrades-Miranda et al. 2001b; Oliveira & Langguth 2004; Bonvicino et al. 2005). Sex chromosomes: X chromosome presented three different morphologies, a large submetacentric, a large subtelocentric, and a large to medium acrocentric; Y chromosome presented two different morphologies, a medium submetacentric, and a medium to small subtelocentric (Gardner & Patton 1976; Maia et al. 1984; Yonenaga-Yassuda et al. 1988; Barros et al. 1992; Silva & Yonenaga-Yassuda 1998a; Andrades-Miranda et al. 2001b; Bonvicino et al. 2005). A different diploid number of 53 to 55 were reported due to the presence of 1 to 3 supernumerary chromosomes. Three different types of supernumerary chromosomes were reported: a large subtelocentric, a medium submetacentric, and a small acrocentric (Maia et al. 1984; Yonenaga-Yassuda et al. 1988; Barros et al. 1992; Silva & Yonenaga-Yassuda 1998a; Andrades-Miranda et al. 2001b). C-banding metaphases exhibited blocks of constitutive heterochromatin on the pericentromeric region of all autosomes. The X chromosome presented a large variability on the amount and distribution of heterochromatin that encompass the pericentromeric region, the entirely short arm, and sometimes a lightly band on its long arm. The Y chromosome presented a large variability on the amount and distribution of heterochromatin that encompass the pericentromeric region, the distal portion of the long arm, the whole long arm and a large part of the short arm. C-banding of B chromosomes presented different patterns on the distribution of heterochromatin: entirely heterochromatic, the long arm entirely heterochromatic, or a heterochromatic block on the end of the long arm. G- and R-banding were also performed (Maia et al. 1984; Yonenaga-Yassuda et al. 1988; Silva & Yonenaga-Yassuda 1998a). Multiple NORs, varying from two to fourteen were localized at the telomeric regions of the short arms of acrocentric pairs (Yonenaga-Yassuda et al. 1988). FISH with telomeric sequences revealed signals exclusively at the ends of all chromosome arms and no interstitial signals were observed (Silva & Yonenaga-Yassuda 1998a).