Platypus genome unravels mysteries of mammalian evolution

Genome Research is publishing several papers related to analyses of the duck-billed platypus (Ornithorhynchus anatinus) genome sequence. The place of (egg-laying) monotremes, such as the platypus, in mammalian evolutionary history has remained controversial. Now, researchers are finding that the distinctive anatomical and physiological properties of the platypus are reflected in the newly sequenced platypus genome. Through comparative genomics, the platypus genome is providing remarkable insights into the evolution of venom components, the sex-determination system, testicular descent, and small RNA pathways. Primary research reports describing these novel insights will appear online May 8, concurrent with publication of the platypus genome sequence report in the journal Nature.

1. Evolution of platypus venom peptides

Platypus is the only mammal delivering venom through a "spur" situated on the inside of each hind limb. While searching the platypus genome for the molecular signatures of cysteine-rich antimicrobial peptides, called defensins, in an investigation into the role of defensins in monotreme immunity, researchers uncovered genes coding for components of platypus venom. Further analysis revealed that these venom genes evolved by gene duplication of antimicrobial beta-defensins. Interestingly, this finding mirrors an independent evolutionary pathway in reptiles. "Snake venom crotamines have also evolved from beta-defensins in separate gene duplication events, making this a compelling example of convergent evolution," says study leader Dr. Katherine Belov of the University of Sydney.

Reference: Whittington, C.M., Papenfuss, A.T., Bansal, P., Torres, A.M., Wong, E.S.W., Deakin, J.E., Graves, T., Alsop, A., Schatzkamer, K., Kremitzki, C., Ponting, C.P., Temple-Smith, P., Warren, W.C., Kuchel, P.W., and Belov, K. Defensins and the convergent evolution of platypus and reptile venom genes. Genome Res. doi:10.1101/gr.7149808.

Contact: Katherine Belov, Ph.D., University of Sydney, Sydney, Australia. ( , +61-293513454) or Camilla Whittington, University of Sydney, Sydney, Australia (+61-421851793)

2. Platypus genome reveals small RNA evolution and novel biology

Small non-coding RNAs have garnered significant interest for their ability to regulate gene expression. Two papers published by Genome Research have utilized the platypus genome sequence to investigate the conservation of small RNAs and associated functional pathways in the mammalian lineage. In the first study, researchers led by Dr. Gregory Hannon of Cold Spring Harbor Laboratory took a deep-sequencing approach to analyze small RNAs isolated from tissues of platypus and the echidna, another monotreme mammal. The isolated sequences were then mapped and compared to known small RNAs to identify conserved and novel RNA species. “Remarkably, we found that the platypus shares microRNA families uniquely with other mammals, but also uniquely with a representative of birds and reptiles,” explains Hannon. “Thus, the unusual morphology of these animals is also reflected at the genomic level and at the level of its small RNAs.”