Holochilus sciureus (Wagner, 1842)

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 : 19-20

Holochilus sciureus

Karyotype: 2n = 56 and FN = 56. Autosomal complement: one small metacentric pair, and 26 acrocentric pairs large to small decreasing in size. Sex chromosomes: X, a medium acrocentric; Y, a small acrocentric ( Gardner & Patton 1976; Vidal et al. 1976; Patton et al. 2000).A fundamental number of 55 was reported due to a heteromorphic centric fusion ( Freitas et al. 1983, pp. 17, Fig. 6 ). A different morphology of sex chromosomes was reported by Yonenaga-Yassuda et al. (1987), a subtelocentric X and a submetacentric Y. These authors also reported the presence of one or two large submetacentric B chromosomes. C-banding metaphases exhibited blocks of constitutive heterochromatin on the pericentromeric region of all autosomes. The X chromosome presented constitutive heterochromatin on the pericentromeric region and on the short arm, while the Y chromosome was completely heterochromatic. The B chromosomes presented heterochromatic blocks on the pericentromeric region and on both arms ( Freitas et al. 1983; Yonenaga-Yassuda et al. 1987). G- and R-banding were also performed ( Baker et al. 1983; Freitas et al. 1983; Yonenaga-Yassuda et al. 1987). Multiple NORs, varying from six to fourteen were localized at the telomeric regions of the short arms of small autosomes ( Yonenaga-Yassuda et al. 1987).

Supernumerary chromosomes of H. sciureus were isolated by flow-sorted and used as probe for understand the origin of B chromosomes in tribe Oryzomyini . In situ hybridization of Bs and sex chromosomes of H. sciureus were performed on metaphases of 15 species of Oryzomyini , five of which had Bs. The results revealed that 12 of the species sampled, all belonging to a monophyletic Oryzomyini subclade (clade C and D, recovered by Weksler 2006), were positive for an anonymous ‘Oryzomyini’ shared heterochromatic region (OSHR) on the X and Y chromosomes. The OSHR was also present on Bs of H. sciureus , Nectomys rattus , and N. squamipes , but not on Bs of Oligoryzomys flavescens , and Sooretamys angouya . Two distinct additional OSHR/autosome associations were observed on S. angouya . According to the authors, the OSHR possibly arose 3.0 to 7.8 million years ago on the sex chromosomes of an ancestral species and spread through the action of transposable elements. These results suggest that the diversity of Oryzomyini Bs in number, size, morphology, and genetic content may be explained by their independent origin in different subgroups of species (Ventura et al. 2015).

Figures

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.