Marsupials and monotremes sort genome treasures from junk

Sequencing of a variety of mammalian and other vertebrate genomes is now proceeding apace, and one major goal of this work is to interpret the massive amounts of data from the Human Genome Project by aligning sequence and distinguishing conserved elements from the background of variable sequence ('phylogenetic footprinting'). Sequence data from mammals that are more or less closely related to humans (chimpanzee, mouse, dog) and more distantly related vertebrates (birds, fish) span 450 million years of evolution. But there is still an awkward gap, precisely in the region of the tree from which genomic data are most needed: species that are not so close that sequence comparison gives false-positive signals and not so far that the sequences are unalignable. Marsupials and monotremes, the earliest groups of mammals to diverge, fill this gap (Figure 1). All mammals produce milk and suckle their young, but marsupials and monotremes are distinguished from eutherian ('placental') mammals by differences in reproduction. Marsupials such as kangaroos and wallabies give birth to highly underdeveloped young and much of their development occurs while suckling in the pouch (including of the hindlimbs, eyes, gonads and a significant portion of the brain). Monotremes such as platypus lay eggs that are incubated in a burrow, where the young hatch and suckle from milk patches until they mature.In a recent landmark paper in Proceedings of the National Academy of Sciences USA, Margulies et al. [1] present the sequencing and comparative analysis of a 1.9 megabase (Mb) region from three marsupials (the North American opossum Didelphis virginiana, the Brazilian opossum Monodelphis domestica and the tammar wallaby Macropus eugenii) and a monotreme (the platypus Ornythorhynchus anatinus). Although previous studies [2] have clearly demonstrated the utility of marsupial sequences in comparative analysis, Margulies et al. [1] have analyzed a significantly larger region, looked at multiple marsu