Nectomys squamipes (Brants, 1827)

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 : 40

Nectomys squamipes

Karyotype: 2n = 56 and FN = 56. Autosomal complement: one small metacentric pair and 26 acrocentric pairs large to small decreasing in size ( Maia et al. 1984, pp. 123, Fig. 2E ; Yonenaga-Yassuda et al. 1988; Barros et al. 1992; Bonvicino et al. 1996; Silva & Yonenaga-Yassuda 1998a; Andrades-Miranda et al. 2001b; Paresque et al. 2004; Pinheiro & Geise 2008; Di-Nizo et al. 2014; Gatto-Almeida et al. 2016). Sex chromosomes: X chromosome presented two different morphologies, a large submetacentric, and a large subtelocentric; Y chromosome presented three different morphologies, a medium metacentric, a medium submetacentric, and a medium to small subtelocentric ( Maia et al. 1984; Yonenaga-Yassuda 1988; Barros et al. 1992; Silva & Yonenaga-Yassuda 1998a; Andrades-Miranda et al. 2001b; Paresque et al. 2004; Di-Nizo et al. 2014; Gatto-Almeida et al. 2016). A different diploid number of 57 to 59 were reported due to the presence of 1 to 3 supernumerary chromosomes. Two different types of supernumerary chromosomes were reported: a medium submetacentric, and a medium acrocentric ( Maia et al. 1984; Yonenaga-Yassuda et al. 1988; Barros et al. 1992; Silva & Yonenaga-Yassuda 1998a; Andrades-Miranda et al. 2001b; Paresque et al. 2004; Di-Nizo et al. 2014; Gatto-Almeida et al. 2016). Another diploid number of 55 was reported by Maia et al. (1984), for a sample from São Paulo, state of Brazil ( Table 7), due to a Robertsonian rearrangement between two acrocentric chromosomes, these authors also reported a mosaicism of 56/57 with a B chromosome. Barros et al. (1992), reported a fundamental number of 54 for a sample from Misiones, state of Argentina ( Table 7, Fig. 11 ), however, without any further description. 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 chromosome 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 ( Maia et al. 1984; Yonenaga-Yassuda et al. 1988; Silva & Yonenaga-Yassuda 1998a; Gatto-Almeida et al. 2016). G- and R-banding were also performed ( Baker et al. 1983; Maia et al. 1984; Yonenaga-Yassuda et al. 1988; Silva & Yonenaga-Yassuda 1998a). Multiple NORs, varying from four to nine were localized at the telomeric regions of the short arms of small acrocentric pairs ( Yonenaga-Yassuda et al. 1988). FISH with telomeric sequences revealed signals at the ends of all chromosome arms, and additional telomeric sequences were found on the proximal region of the long arm of submetacentric B chromosome ( Silva & Yonenaga-Yassuda 1998a).

The species N. rattus and N. squamipes with the basic karyotype (2n = 52 and 2n = 56, respectively) were raised in captivity for crossed breeding. Cytologic and histologic analyses of the hybrid meiosis showed that the rearrangements between these two karyotypes represented a drastic reproductive barrier ( Bonvicino et al. 1996). According to Yonenaga-Yassuda et al. (1998) based on the G-banding pattern, two tandem fusions have been recognized as the mechanism involved in the differentiation of these karyotypes.

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 11. Distribution of diploid and fundamental number of karyotyped specimens of genus Nectomys. (ê) N. apicalis; (■) N. m. grandis; (l) N. palmipes; (○) N. rattus; (▲) N. squamipes.