PHYLOGENETIC ANALYSIS OF G-BANDED KARYOTYPES AMONG THE SOUTH-AMERICANSUBTERRANEAN RODENTS OF THE GENUS CTENOMYS (CAVIOMORPHA, OCTODONTIDAE), WITH SPECIAL REFERENCE TO CHROMOSOMAL EVOLUTION AND SPECIATION

The chromosomes of subterranean rodents of the South American genus Ct enomys are highly variable with diploid numbers ranging from 10 to 70. The phylogenetic relationships of this group have been analysed cladi stically using G-banded karyotypes as have the chromosomal rearrangeme nts involved in its karyotypic differentiation. One group, called the 'Corrientes group', has very variable chromosomes but low allozymic an d morphological differentiation among its members. This group has been analysed with respect to chromosomal speciation. Using a member of an other subfamily (Octodontomys gliroides) as an outgroup, the results i ndicate that karyotypes with low diploid and fundamental numbers are p lesiomorphic. The range of diploid numbers studied here is between 22 and 70, while the fundamental numbers are between 40 and 86. It was fo und that the main chromosomal rearrangement that transforms karyotypes towards higher diploid and fundamental numbers is the acquisition of new chromosomal material via unknown mechanisms, followed by pericentr ic inversions that generate new chromosomal arms, centric fusions and centric fissions. In spite of their low differentiation regarding allo zymic and morphological features, it was found that the karyomorphs of the Corrientes group have enough chromosomal differentiation to consi der them as distinct species. Beside the range of diploid and fundamen tal numbers of this group (42-70 and and 80-84 respectively), their pa irwise chromosomal differences are high. The most closely related of t hem differ in one nonhomologous arm, one Robertsonian change and a who le chromosome duplication. The most differentiated taxa differ in 20 a rms with lack of homology, 12 Robertsonian changes (one with monobrach ial homology), six pericentric inversions and the above mentioned prob able arm duplication. For these reasons, it is probable that some kind of chromosomal speciation has occurred in the Corrientes group.