Comprehensive cytogenetic analysis of the most chromosomally variable mammalian genus from South America: Ctenomys (Rodentia: Caviomorpha: Ctenomyidae)

South American subterranean rodents of the genus Ctenomys are among the most diverse mammals at the species level, integrating a monotypic family of recent origin. Contrasting with their general morphological conservatism, chromosome variability is very high. Also, molecular data showed a shallow genetic divergence within several species groups with mitochondrial markers, leaving doubts about the taxonomic status of some lineages and the evolutionary significance of their chromosomal diversity. Here, we reviewed and reanalysed published chromosome data of Ctenomys and contrasted it with molecular data, including ancestral chromosomal state reconstructions in a phylogenetic hypothesis based on mitochondrial cytochrome b gene sequences. One hundred seventy-eight different chromosome complements were recorded for 48 Ctenomys species. Some species are polymorphic and/or polytypic, with differentiated populations. Both 2 n and FNa showed a wide variation, but FNa had a greater dispersion. With the exceptions of C. lessai and C. maulinus , in all other species, the X chromosome is biarmed. The Y chromosome is quite variable in morphology and size. In general, biarmed and uniarmed autosomes within karyotypes are nearly equally represented, departing from the pattern of other mammals. This may be due to fast-evolving heterochromatin. The chromosomal reconstructions suggest that the ancestral karyotype of the genus could have had an intermediate 2 n and high FNa. Most of the species groups recovered in the phylogeny show wide chromosomal variability, the most extreme being the torquatus group. This diversity was apparently generated in a short time, indicating an accelerated rate of chromosome evolution in this clade and the whole family.