Hylaeamys megacephalus (Fischer, 1814)

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 : 24-25

Hylaeamys megacephalus

Karyotype: 2n = 54 and FN = 62. Autosomal complement: five metacentric and submetacentric pairs medium to small decreasing in size, and 21 acrocentric pairs large to small decreasing in size. Sex chromosomes: X, a large acrocentric; Y, a small acrocentric ( Perez-Zapata et al. 1986; Svartman & Almeida 1992b, pp. 965, Fig. 1 ; Musser et al. 1998; Andrades-Miranda et al. 2000; Bonvicino et al. 2005; Nagamachi et al. 2013). C-banding metaphases exhibited blocks of constitutive heterochromatin on the pericentromeric region of all autosomes. The X chromosome presented pericentromeric heterochromatic blocks and two interstitial blocks on its long arm. The Y chromosome presented strongly heterochromatic bands on its long arm. G- and R-banding were also performed ( Baker et al. 1983; Svartman & Almeida 1992b; Nagamachi et al. 2013). Multiple NORs, varying from five to ten were localized at the telomeric regions of the short arms of acrocentric autosomes ( Svartman & Almeida 1992b). FISH with telomeric sequences revealed signals exclusively at the ends of all chromosome arms and no interstitial signals were observe. Additionally, whole chromosome probes were made for H. megacephalus by flow cytometry from sorted chromosomes of fibroblast cell cultures ( Nagamachi et al. 2013). These probes were used to established maps of chromosomal homology between H. megacephalus and Cerradomys langguthi ( Nagamachi et al. 2013) ; Neacomys sp. A, Neacomys sp. B ( Silva et al. 2017); Neacomys sp. (cytotype A, and cytotype B), Neacomys sp. E, N. paracou , and N. spinosus ( Silva et al. 2019) ; Oecomys catherinae ( Malcher et al. 2017) ; Akodon montensis and Thaptomys nigrita ( Suárez et al. 2015) ; and Necromys lasiurus and Akodon sp. ( Pereira et al. 2016). The results showed a huge genomic reorganization between the karyotype of these species.

A different diploid number of 50 was reported by Nagamachi et al. (2013), for a fetus from a specimen from Pará. The fluorescent in situ hybridization with whole chromosome probes of H. megacephalus (2n = 54 and FN = 62) revealed that chromosome rearrangements in the fetal karyotype was due to X chromosome monosomy, an isochromosome, and heterozygosis for two centric fusions. The authors concluded that the fetal karyotype was not a new cytotype and suggests that the fetus probably would be non-viable or infertile, as widely reported in humans.

The same diploid number (2n = 54) with a different number of arms was reported for samples from Suriname, with the fundamental number of 58–59 ( Table 5, Fig. 6 ) ( Koop et al. 1983; Kerridge & Baker 1990), and for samples from French Guiana with the fundamental number of 64 ( Table 5, Fig. 6 ) ( Volobouev & Aniskin 2000).

Figures

FIGURE 1. Collection localities of Oryzomyini species analyzed in this work. (1) Claúdia, Mato Grosso state; (2) and (3) Rio Japurá, Amazonas state; (4) and (5) Rio Içá, Amazonas state.

FIGURE 6. Distribution of diploid and fundamental number of karyotyped specimens of genus Hylaeamys. (□) H. megacephalus; (■) H. oniscus; (○) H. perenensis; (l) H. seuanezi; (ê) H. yunganus.