Holochilus chacarius (Thomas, 1906)

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 : 18-19

Holochilus chacarius

Karyotype: 2n = 48 and FN = 56. Autosomal complement: five metacentric pairs (four large and one small), and 18 acrocentric pairs medium to small decreasing in size. Sex chromosomes: X, a medium acrocentric; Y, a small acrocentric. C-banding metaphases exhibited blocks of constitutive heterochromatin on the pericentromeric region of all autosomes, with the exception of one large metacentric pair. The X chromosome presented pericentromeric heterochromatic blocks, the Y chromosome was almost entirely heterochromatic. G-banding was also performed ( Nachman & Myers 1989, pp 6668, Fig. 1A ). The karyotype of H. chacarius was highly variable, with diploid number ranging from 48 to 55, and fundamental number from 56 to 60. This variation occurs within and between populations from Paraguay and Argentina ( Table 4, Fig. 5 ), and was resulted from multiple Robertsonian translocations ( Nachman & Myers 1989; Nachman 1992a). Furthermore, two different types of supernumerary chromosome were reported: B1 was a small to large metacentric or submetacentric chromosome that contains only centromeric heterochromatin, and B2 was a small acrocentric chromosome that was largely heterochromatic. The authors reported a pericentromeric inversion of a single X chromosome in one female, terminal euchromatic addition to the short arm of the X chromosome in nine specimens, euchromatic addition adjacent to the pericentromeric region in one autosomal pair of two specimens, and one female with only one X chromosome ( Nachman & Myers 1989; Nachman 1992a).

In the original report, Nachman & Myers (1989), and latter Nachman (1992a), considered the fundamental number as the number of all chromosome arms, including sex and B chromosomes. Here the fundamental number was given as the number of autosomal arms, excluding sex and supernumerary chromosomes. A karyotype with 2n = 48 and FN = 60, was reported by Gardner & Patton (1976) for a sample from Paraguay, however this specimen presented a submetacentric X chromosome, that accounts for the difference from the former karyotype. Vidal et al. (1976) reported a karyotype variation for genus Holochilus , 2n = 50–54 and FN = 56–60, for samples from Argentina that seems to belong to H. chacarius . We assumed this correlation, as this variability of diploid and fundamental number was described only for H. chacarius , and due to the proximity of collection localities.

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 5. Distribution of diploid and fundamental number of karyotyped specimens of genus Holochilus. (■) H. brasiliensis; (□) H. chacarius; (l) H. sciureus; (○) H. venezuelae; (▲) H. vulpinus.