Zygodontomys brevicauda (Z. b. brevicauda (Allen & Chapman, 1893)

Moreira, Camila Do Nascimento, Ventura, Karen, Percequillo, Alexandre Reis & Yonenaga-Yassuda, Yatiyo, 2020, A review on the cytogenetics of the tribe Oryzomyini (Rodentia: Cricetidae: Sigmodontinae), with the description of new karyotypes, Zootaxa 4876 (1), pp. 1-111 : 82

Zygodontomys brevicauda (Z. b. brevicauda )

Two different karyotypes were reported for Z. b. brevicauda . Karyotype 1: 2n = 84 and FN = 116 or 118. Autosomal complement: one medium submetacentric pair, one medium subtelocentric pair, 18 small autosomal pairs, and 21 minute autosomal pairs. According to authors, due to the small size of most autosomes, it was hard to recognize the morphology of many of the chromosomal pairs, however based on the best-defined karyotypes the estimate of autosomal fundamental number as made of 116 or 118 arms. Sex chromosomes: X, a small subtelocentric; Y, a minute subtelocentric ( Kiblisky et al. 1970; Reig et al. 1990, pp. 363, Fig. 2 ). A secondary constriction was observed at one small autosomal pair. C-banding metaphases exhibited blocks of constitutive heterochromatin on the pericentromeric region of almost all chromosomes, the medium metacentric and submetacentric pairs lack a striking heterochromatic region, five small autosomal pairs exhibit both pericentromeric and terminal heterochromatic block, three small to minute autosomal pairs presented the small arm entirely heterochromatic, five minute autosomal pairs were entirely heterochromatic, all the remaining autosomal pairs showed different degrees of Cband on the pericentromeric regions. The X chromosome presented a pericentromeric heterochromatic block, and the banding pattern of the Y chromosome was not clearly defined. According to authors, the heterochromatic nature of some short arms, may be responsible for polymorphic variants in the morphology of several autosomes among different individuals ( Reig et al. 1990). Karyotype 2: 2n = 88 and FN = 116–118. Autosomal complement: one medium submetacentric pair, one medium subtelocentric pair, 18 small autosomal pairs, and 23 minute autosomal pairs. Sex chromosomes: X, a small subtelocentric; Y, a minute subtelocentric ( Gardner & Patton 1976; Perez-Zapata et al. 1984; Reig et al. 1990, pp. 365, Fig. 4 ). According to Reig et al. (1990), the difference in diploid numbers of 2n = 84 and 2n = 88, that occurs in different localities of Venezuela ( Table 11, Fig. 22 ), were due to addition of two pairs of minute chromosomes. Besides, the metacentric and the submetacentric small pairs found in the 2n = 84 karyotype was lacking in the 2n = 88 karyotype. These differences can be easily explained by a Robertsonian rearrangement affecting two small pairs of the 2n = 84 karyotype. C-banding was performed and a great deal of the genome were C-band positive, although the small size and high number of chromosomes hampered efforts to obtain a clear metaphase. It was evident, however, that eleven very minute autosome pairs were entirely heterochromatic, and five small autosome pairs appear to be entirely heterochromatic ( Reig et al. 1990).

Figures

FIGURE 2. Distribution of diploid and fundamental number of karyotyped specimens of genus Cerradomys. (□) C. akroai; (■) C. goytaca; (ê) C. langguthi; (r) C. maracajuensis; (▲) C. marinhus; (○) C. scotti; (☆) C. subflavus; (l) C. vivoi.

FIGURE 4. Karyotype of Euryoryzomys sp., male with 2n = 58 and FN = 92. (A) Conventional staining (JAP 228); (B) C-banding (JAP 228); (C) G-banding (JAP 228); (D) Metaphase plate after fluorescent in situ hybridization with telomeric probe (left) and DAPI staining (right) (JAP 228), arrows indicate the hybridization of interstitial telomeric sequence.

FIGURE 22. Distribution of diploid and fundamental number of karyotyped specimens of genus Zygodontomys. (l) Z. b. brevicauda; (■) Z. b. cherriei; (○) Z. b. microtinus; (ê) Zygodontomys sp. 1; (□) Zygodontomys sp. 2.