Transandinomys talamacae (Allen, 1891)

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 : 80-81

Transandinomys talamacae

This species presents a remarkable chromosomal variability that range from 2n = 34 to 2n = 54, with four distinct karyotypes ( Perez-Zapata et al. 1986; Musser et al. 1998).

Karyotype 1: 2n = 34 and FN = 64. Autosomal complement: two large submetacentric pairs, one large subtelocentric pair, and 13 metacentric and submetacentric pairs medium to small decreasing in size. Sex chromosomes: X, a medium acrocentric; Y, a small subtelocentric. A secondary constriction was observed at the long arm of one large submetacentric pair, and in two small metacentric pairs ( Perez-Zapata et al. 1986, pp. 371, Fig. 2 ; Musser et al. 1998). This karyotype was reported for samples from Aragua, Delta Amacuro and Yaracuy, states of Venezuela ( Table 10, Fig. 21 ).

Karyotype 2: The diploid numbers ranges from 40 to 42, and were reported a different chromosomal set in each one of the four animals karyotyped for samples from Zulia, state of Venezuela ( Table 10, Fig. 21 ) ( Musser et al. 1998, pp. 159, Fig. 69): (i) sample (male) 2n = 40 and FN = 66 or 67 (fundamental number comprises autosomal and sex chromosomes). Chromosomal complement: ten large to small metacentric and submetacentric pairs decreasing in size, four subtelocentric pairs (two large and two medium), four acrocentric pairs (two large and two small), and four unpaired elements that include one medium metacentric, one small and another very small acrocentric, and the medium acrocentric X chromosome, which has very short arm. The Y chromosome was one of the first three unpaired elements ( Musser et al. 1998); (ii) sample (male) 2n = 41 and FN = 63 or 64 (fundamental number comprises autosomal and sex chromosomes). Chromosomal complement: nine medium to small metacentric and submetacentric pairs decreasing in size, three subtelocentric pairs (two large and one medium), seven acrocentric pairs (two large and five medium to small), and three unpaired elements that include a small metacentric, a small acrocentric (one of which probably represents the Y chromosome), and the medium-sized acrocentric X chromosome with a very short arm ( Musser et al. 1998); (iii) sample (female) 2n = 41 and FN = 66. Autosomal complement: nine medium to small metacentric and submetacentric pairs decreasing in size, three subtelocentric pairs (two large and one medium), six acrocentric pairs (two large and four medium to small), and three unpaired elements that include a medium submetacentric, a small metacentric, and a medium subtelocentric. Sex chromosomes: X, a medium acrocentric with a very short arm ( Musser et al. 1998). (iv) sample (male) 2n = 42 and FN = 66. Autosomal complement: nine large to small metacentric and submetacentric pairs decreasing in size, four subtelocentric pairs (two large and two medium), and seven acrocentric pairs (two large and five medium to small). Sex chromosomes: X chromosome was a medium subtelocentric, and the presumed Y was a small submetacentric ( Musser et al. 1998).

According to Musser et al. (1998) the major difference between the variable karyotypes (2n = 40–42) and the apparently stable karyotypes (2n = 34) was that the latter has an entirely biarmed autosomal complement. The num-ber of acrocentric pairs varies from four to seven, not counting the unpaired element present in karyotypes with 2n = 40 and in one with the 2n = 41. The karyotype with 2n = 34 was characterized by two pairs of very large submetacentric chromosomes that were conspicuous not only for their size, but also for the secondary constrictions evident in one of the pairs. This karyotype also has only one pair of large subtelocentrics. In contrast, the karyotypes with 2n = 40–42 have an additional pair of large subtelomeric, but lack the two pairs of very large submetacentric. Both Robertsonian rearrangements and pericentric inversions were required to explain that karyotypic differences. The configurations of these karyotypes do not suggest supernumerary chromosome involvement.

Karyotype 3: 2n = 36 and FN = 60. Autosomal complement: two subtelocentric pairs (one large and one medium), 11 metacentric and submetacentric pairs (four large, three medium, two small, and two minute), and four acrocentric pairs (two large and two minute). Sex chromosomes: X, a medium acrocentric; Y, a small subtelocentric ( Musser et al. 1998, pp. 236, Fig. 70). This karyotype was reported only for a sample from Esmeraldas, states of Ecuador ( Table 10, Fig. 21 ).

Karyotype 4: 2n = 54 and FN = 60. Autosomal complement: four metacentric pairs (one medium, one small and two minute), and 22 acrocentric pairs (three large and the remaining medium to minute decreasing in size). Sex chromosomes: X, a large subtelocentric; Y, a small acrocentric ( Musser et al. 1998, pp. 236, Fig. 70). This karyotype was reported only for a sample from El Oro, states of Ecuador ( Table 10, Fig. 21 ).

According to Musser et al. (1998) the major differences between karyotypes 3 and 4 can be explained by either fissions or Robertsonian rearrangements. Both explanations presuppose whole-arm homologies between the chromosome set of these karyotypes, with one having been derived from the other, however, confirmation will require the analysis of banding patterns. Karyotypes from intervening populations were unknown and evidence for or against hybridization was lacking between the forms with 2n = 36 and 2n = 54.

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 21. Distribution of diploid and fundamental number of karyotyped specimens of genus Transandinomys. (■) T. bolivaris; (l) T. talamacae.