The opossum genome: Insights and opportunities from an alternative mammal

  1. Paul B. Samollow1
  1. Department of Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences,Texas A&M University, College Station, Texas 77843-4458, USA; Faculty of Genetics, Texas A&M University, College Station, Texas 77843-4458, USA

Abstract

The strategic importance of the genome sequence of the gray, short-tailed opossum, Monodelphis domestica, accrues from both the unique phylogenetic position of metatherian (marsupial) mammals and the fundamental biologic characteristics of metatherians that distinguish them from other mammalian species. Metatherian and eutherian (placental) mammals are more closely related to one another than to other vertebrate groups, and owing to this close relationship they share fundamentally similar genetic structures and molecular processes. However, during their long evolutionary separation these alternative mammals have developed distinctive anatomical, physiologic, and genetic features that hold tremendous potential for examining relationships between the molecular structures of mammalian genomes and the functional attributes of their components. Comparative analyses using the opossum genome have already provided a wealth of new evidence regarding the importance of noncoding elements in the evolution of mammalian genomes, the role of transposable elements in driving genomic innovation, and the relationships between recombination rate, nucleotide composition, and the genomic distributions of repetitive elements. The genome sequence is also beginning to enlarge our understanding of the evolution and function of the vertebrate immune system, and it provides an alternative model for investigating mechanisms of genomic imprinting. Equally important, availability of the genome sequence is fostering the development of new research tools for physical and functional genomic analyses of M. domestica that are expanding its versatility as an experimental system for a broad range of research applications in basic biology and biomedically oriented research.

Footnotes

  • 1 Corresponding author.

    1 E-mail psamollow{at}cvm.tamu.edu; fax (979) 845-9972.

  • Article is online at http://www.genome.org/cgi/doi/10.1101/gr.065326.107.

  • 2 The dating of the earliest mammalian divergence events is somewhat controversial. For example, a recent, widely cited supertree analysis by Bininda-Emonds et al. (2007), which incorporated 99% of all extant mammalian species, places the prototherian–therian divergence at only 166.2 Mya and, based partially on this fixed point, sets the origin of Metatheria and Eutheria at ∼147.7 Mya. Although that study involved extensive molecular data and was calibrated by 30 fossil-based time points, the date suggested for the base of the Mammalia (166.2 Myr) was constrained by an extinct vertebrate described from a single partial jawbone fragment posited by Bininda-Emonds et al. (2007) to represent the maximum age of ancestral mammals that gave rise to the prototherian and therian lineages. However, because a single fossil can only fix the minimum age for the existence of a lineage (e.g, Woodburne et al. 2003; Glazko et al. 2005; Donoghue and Benton 2007), the actual divergence of Prototheria and Theria could be considerably older. Moreover, the fossil itself, Ambondro mahabo, was placed with uncertain taxonomic affinity in its original description (Flynn et al. 1999), and later analysis suggests it is likely to be a eutherian (Woodburne et al. 2003) rather than ancestral to prototherians (sensu Luo et al. 2002). Considering the disputed and dubious phylogenetic position of A. mahabo, it seems prudent to abide by the older dates for the earliest mammalian divergences (see main text), which are supported by considerable molecular data from several different studies (cited in main text).

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