Karyotype evolution of marsupials: from higher to lower diploid numbers.

A basic 2n = 14 ancestral marsupial karyotype giving rise to higher diploid numbers through chromosome fissions has been widely accepted for the last three decades. Our finding of interstitial telomeres in two South American species, one with the 2n = 14 "ancestral karyotype" and the other with 2n = 18, indicates that these complements evolved from a karyotype with a higher diploid number. A new scenario for the karyotype evolution in the group is put forward. In this scenario an ancestral karyotype with at least 22 chromosomes would have originated the basic karyotype with 2n = 14 before the radiation of marsupials.