Extraordinary chromosome diversity in the southernmost populations of the montane grass mouse Akodon montensis (Cricetidae, Sigmodontinae)

Abstract

Akodon montensis is abundant and widely distributed in the Atlantic Forest, which southern limit extends to northern Argentina, principally in Misiones province. Chromosomal data from more centrally distributed populations in Brazil showed significant chromosome diversity; however, data from its southern limit are scarce. To explore the chromosomal diversity of this species, we conducted conventional chromosome staining, C-banding, CMA3/DAPI fluorochromes, and in situ hybridization with a telomere probe in samples from Argentina. Most specimens had the standard karyotype 2n=24, although numerical variations due to aneuploidies and supernumerary (B) chromosomes were also detected. We registered novel structural polymorphisms involving pair 11 and sex chromosomes, and a rearrangement involving pairs 2 and 4, possibly due to a spontaneous chromosomal mutation. Most A. montensis females are homogametic with XX sex chromosomes, although XY and X0 females were observed. Most individuals carrying B chromosomes had 1 B and, to a lesser extent, 2 and 3 B. The chromosomal variability detected at the southern limit of A. montensis distribution is high and similar to other geographically distant populations, despite it could be a region recently colonized. Some of these variations are unique and could have originated independently in southern populations, while others shared throughout the species distribution could be older.