The evolution of eutherian chromosomes.

The gross organization of the genome of Eutheria (placental mammals) into chromosomes follows a simple architecture that, with some minor changes, is almost completely conserved for more than 100 million years in various species of almost all extant mammalian orders. Recent molecular cytogenetic results--especially those from the assumed oldest clade, the Afrotheria--suggest an ancestral karyotype that would calculate the "default" frequency of gross rearrangements to less than two changes within 10 million years of mammalian evolution. The main changes are the fission, movement and subsequent fusion of large chromosome segments or of chromosome arms. Reciprocal translocations are the exception. Chromosome numbers may have increased or decreased significantly in this fusion/fission process but, in most instances, the main architecture still remains evident. There are, however, some exceptions in mammals with extremely derived karyotypes.