An undescribed karyotype for Thaptomys (2n = 50) and the mechanism of differentiation from Thaptomys nigrita (2n = 52) evidenced by FISH and Ag-NORs

Abstract

Thaptomys nigrita is a monotypic species with 2n = 52 from Akodontini tribe. The karyotype is composed by 25 pairs of autosome being 24 acrocentric decreasing in size and a small metacentric pair. X and Y are respectively a medium size acrocentric and a small submetacentric.

In this paper we report for the first time a karyotype with 2n = 50 for an undescribed species of genus Thaptomys. This new karyotype is encompasses by 24 pairs of acrocentric autosomes decreasing in size; X and Y chromosomes are respectively a large acrocentric and a small submetacentric; heterochromatic blocks are observed in the pericentromeric regions of all autosomes and of the X, whereas the long arm of the Y is entirely heterochromatic. Multiple Ag-NORs are located at the telomeric regions of the long arm of the autosomes, and a single chromosome pair (24) presents Ag-NORs in both telomeric regions, which is similar to the pattern observed in the metacentric autosome pair 25 of Thaptomys nigrita with 2n = 52. It can be suggested that this pair 24 has undergone a pericentric inversion and originated the acrocentric pair in Thaptomys sp. with 2n = 50.

G-banding pattern and interstitial telomeric signal (ITS) by FISH suggest that the karyotype differentiation between both karyomorphs with 2n = 52 in Thaptomys nigrita and 2n = 50 of Thaptomys sp. was due to a tandem fusion involving respectively pairs 2 and 24 from the former resulting in pair 2 of the latter. We propose that this new karyotype with 2n = 50 belongs to a new and cryptic species for the genus Thaptomys, since these two entities seem to be morphologically indistinguishable and the geographic localization plus the chromosome rearrangements can represent a reproductive barrier between these two forms.

Key words:

• Rodentia
• Thaptomys
• karyotype
• cryptic species
• tandem fusion