Neacomys spinosus

Karyotype: 2n = 64 and FN = 68. Autosomal complement: three small metacentric pairs, and 28 acrocentric pairs (one very large, and the remaining large to small decreasing in size). Sex chromosomes: X, a large subtelocentric; Y, a small acrocentric (Gardner & Patton 1976, pp. 13, Fig. 6E; Patton et al. 2000, pp. 110, Fig. 75D). We also report this same karyotype for samples collected in Mato Grosso and in the Rio Içá in Amazonas, states of Brazil, plus a metacentric Y chromosome for the sample from Rio Içá (Fig. 8A). Furthermore, we performed C-banding for the karyotype with a metacentric Y chromosome, and G-banding and FISH with telomeric sequences for the karyotype with an acrocentric Y. C-banding metaphase exhibited blocks of constitutive heterochromatin on the pericentromeric region of all autosomes. The X chromosome presented pericentromeric heterochromatic blocks and an interstitial lightly stained C-band on its long arm. The Y chromosome was entirely heterochromatic (Fig. 8B). G-banding was performed to allow the correct identification of all homologous pairs (Fig. 8C). FISH with telomeric sequences revealed signals exclusively at the ends of all chromosome arms and no interstitial signals were observed (Fig. 8D). The same karyotype was also reported by Silva et al. (2019) for a sample from Mato Grosso, Brazil, with a different morphology of the X chromosome, a medium submetacentric.