Tandem fusion, centric fusion, and chromosomal evolution in the cotton rats, genus Sigmodon

G-banded and C-banded karyotypes of three closely related and morphologically similar species of cotton rats, Sigmodon hispidus (2n=52; 52 autosomal arms), S. mascotensis (2n=28; 28 autosomal arms), and S. arizonae (2n=22,24; 38 autosomal arms) are presented and compared. Despite the enormous difference between the higher diploid number of hispidus and the lower diploid numbers of mascotensis and arizonae, a great deal of G-band homology is retained. If the karyotype of hispidus is considered ancestral in the group, then chromosome evolution toward lower diploid numbers has proceeded through the fixation of a large number of both tandem and centric fusions. Centromeric heterochromatin has not, however, persisted as intetstitial C-band material on the tandem fusion products. The karyotypes of mascotensis and arizonae share several tandem fusion products of hispidus chromosomes indicating a common origin of the former two species from an ancestor of intermediate diploid number. Chromosome evolution from the ancestral hispidus-like karyotype to the karyotype of the common ancestor of arizonae and mascotensis proceeded almost exclusively by tandem fusion. This orthokaryotypic trend continued in the evolution of the karyotype of modern mascotensis. In the line leading from the arizonae-mascotensis ancestor to modern arizonae a trend of centric fusion predominated.