Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com Original Article Cytogenet Genome Res DOI: 10.1159/000331584 A New Allopatric Lineage of the Rodent Deltamys (Rodentia: Sigmodontinae) and the Chromosomal Evolution in Deltamys kempi and Deltamys sp. K. Ventura   a V. Fagundes   b G. D’Elía   c A.U. Christoff   d Y. Yonenaga-Yassuda   a   a  Departamento de Genética e Biologia Evolutiva, Instituto de Biociências, Universidade de São Paulo, São Paulo, b  Departamento de Ciências Biológicas, Centro de Ciências Humanas e Naturais, Universidade Federal do Espírito Santo, Vitória, Brazil; c  Instituto de Ecología y Evolución, Universidad Austral de Chile, Valdivia, Chile; d  Departamento de Biologia, Museu de Ciências Naturais, Universidade Luterana do Brasil, Canoas, Brazil Chromosome painting with Akodon paranaensis chromo- some 21 (a small metacentric akodont marker) paint revealed total homology with the smallest acrocentric Deltamys sp. chromosome, DSP19. This suggests the occurrence of a peri- centric inversion or centromeric shift when compared to other akodontines, with a posterior tandem rearrangement giving rise to DKE4. In DKE, large blocks of pericentromeric constitutive heterochromatin are present on the autosomes and the X, and the Y/autosome has an entirely heterochro- matic short arm. In DSP, small heterochromatic blocks are observed on autosomes and X, and the Y is a very small, mostly heterochromatic acrocentric. The cytogenetic analy- ses suggest that the Deltamys sp. karyotype is ancestral, with the derived condition resulting from a tandem fusion (DSP7 + DSP19) and the Y/autosome translocation giving rise to the multiple sex chromosome system. The autosomal rearrange- ments, the differences in CBG-banding patterns and Ag- NOR localization, as well as the presence of X 1 X 1 X 2 X 2 /X 1 X 2 Y and XX/XY sex determination mechanisms, possibly acting as a reproductive barrier, and the phylogenetic position within the Deltamys genus, with high genetic divergence, call for a taxonomic review of the genus. Copyright © 2011 S. Karger AG, Basel Key Words Akodontini  Chromosome painting  Cytochrome b  Phylogeny  Sex chromosomes  Tandem rearrangement Abstract Deltamys Thomas 1917 is a poorly studied and rarely collect- ed taxon of Akodontini (Sigmodontinae). The single de- scribed species, Deltamys kempi (DKE), has a basic karyotype with a diploid number of 2n = 37 in males and 2n = 38 in fe- males, a fundamental number FN = 38 for both sexes, and an X 1 X 1 X 2 X 2 /X 1 X 2 Y sex determination system. Herein, a new al- lopatric form, Deltamys sp. (DSP), is reported, based on spec- imens from southern Brazil, with 2n = 40, FN = 40 and XX/XY sex chromosomes. We describe the karyotype and mecha- nism of chromosomal differentiation between both Delta- mys complements. Phylogenetic analyses, based on the complete sequence (1,140 bp) of the mitochondrial cyto- chrome b gene, grouped Deltamys sp. as sister species to D. kempi, with up to 12% genetic divergence between them. The GTG-banding patterns show complete autosomal cor- respondence between D. kempi and Deltamys sp. and iden- tify a tandem rearrangement involving DSP7, DSP19 and DKE4 that is responsible for the differences in 2n and FN. Accepted: July 13, 2011 by M. Schmid Published online: September 20, 2011 Karen Ventura Departamento de Genética e Biologia Evolutiva Instituto de Biociências, Universidade de São Paulo, Rua do Matão, 277 São Paulo, SP 05508-090 (Brazil) Tel. +55 11 3091 7574, E-Mail kabiousp  @  yahoo.com.br © 2011 S. Karger AG, Basel 1424–8581/11/0000–0000$38.00/0 Accessible online at: www.karger.com/cgr