Chromosomes and zoogeography of fossorial rodents of the genus Ctenomys (Rodentia, Caviomorpha) from the South American lowlands east of the Andes

The polytypic fossorial rodents of the genus Ctenomys are autochthonous fro m South America, ranging between 17 degrees S and 54 degrees S with a pecul iar evolutionary history closely related to geomorphological events in the past and present distribution range. Basically what must be considered are: (1) The origin of the genus in the Bolivian plateau, ca. 1.9 MY BP; (2) A progressive expansion from north to south along the Andean highlands; (3) T he constant development of derived forms which have undergone a very active process of radiation resulting in extension of their ranges to the orienta l lowlands, the steppes of Patagonia and Tierra del Fuego and to the west T rans Andean regions; (4) At present, particular processes are going on, sim ilar to those producing chromosome divergence between populations; (5) Exte nsive chromosomal diversity has been accompanied with few morphological cha nges. The following lineages are distinguished: Bolivian Matogrossan, North ern Chacoan or Bolivian Paraguayan, Uruguayan Pampas (a relict group, almos t superimposed by the preceeding one which colonised the Pampas and the cen tral area), and an apomorphic lineage comprising all the Patagonian species of Ctenomys.