Marsupial (infraclass Metatheria) and eutherian mammals combined make up the subclass Theria. The egg-laying monotremes (subclass Prototheria) are the earliest offshoot of the class Mammalia and are represented by 1 extant species of platypus and 4 species of echidna. Mammals are divided into 3 major lineages that diverged from a common ancestor between 161 and 217 million years ago (mya) (fig. Here, we discuss the significant insight into karyotype evolution afforded by the combination of recently sequenced marsupial and monotreme genomes with cytogenetic analysis, which has provided a greater understanding of the events that have shaped not only marsupial and monotreme genomes, but the genomes of all mammals.ĭeep divisions in class Mammalia provide an opportunity to look back in time and reconstruct the events shaping their genomes. Chromosome painting and gene mapping have played a vital role in piecing together the puzzle of monotreme karyotypes, particularly their complicated sex chromosome system. The application of molecular cytogenetic techniques has greatly advanced our understanding of the evolution of marsupial chromosomes and allowed the reconstruction of the ancestral marsupial karyotype. In contrast, monotreme genomes are divided into several large chromosomes and many smaller chromosomes, with a complicated sex chromosome system that forms a translocation chain in male meiosis. They possess many unique features including their distinctive chromosomes, which in marsupials are typically very large and well conserved between species. Marsupial and monotreme mammals fill an important gap in vertebrate phylogeny between reptile-mammal divergence 310 million years ago (mya) and the eutherian (placental) mammal radiation 105 mya.