Platypus genome unravels mysteries of mammalian evolution

The second paper describes an investigation of small nucleolar RNAs (snoRNAs) that sheds light on the evolution of this class of mammalian non-protein-coding RNAs. In addition to characterizing more than 200 snoRNAs in the platypus genome, researchers led by Dr. Jürgen Schmitz of the University of Münster made a particularly intriguing finding: evolution of a novel, platypus-specific snoRNA dispersal process. “Small nucleolar RNAs are rather stationary and, thus, not found in high copy numbers within mammalian genomes,” describes Schmitz. “We discovered an exceptional chimera, however, of a housekeeping snoRNA and a more mobile retroposon-like, non-LTR transposable element (RTE)-related sequence, which facilitated the mobility of the novel ‘snoRTE’ and the distribution of more than 40,000 copies, several of them actively expressed.”

Reference: Murchison, E.P., Kheradpour, P., Sachidanandam, R., Smith, C., Hodges, E., Xuan, Z., Kellis, M., Grützner, F., Stark, A., and Hannon, G.J. Conservation of small RNA pathways in platypus. Genome Res. doi:10.1101/gr.73056.107.

Contact: Gregory J. Hannon, Ph.D., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. ( , +1-516-367-8847)

Reference: Schmitz, J., Zemann, A., Churakov, G., Kuhl, H., Grützner, F., Reinhardt, R., and Brosius, J. Retroposed SNOfall – A mammalian-wide comparison of platypus snoRNAs. Genome Res. doi:10.1101/gr.7177908.

Contact: Jürgen Schmitz, Ph.D., Institute of Experimental Pathology, University of Münster, Münster, Germany. ( , +49-251-8352133)

3. Bird-like sex-determination system of mammalian ancestors

Therian mammals possess an XY sex-determination system, where females have a pair of X chromosomes, and males harbor one X and one Y chromosome. However, the sex-determination system of platypus stands in stark contrast: the female has five different pairs of X chromosomes, while males have five X chromosomes and five Y chromosomes. In a study carried out at the Australian National University in Canberra, scientists used physical mapping of chromosomes to investigate homology and the evolutionary relationships between platypus and therian sex chromosomes.

“Surprisingly, we find that the platypus sex chromosomes are unrelated to the X or Y chromosomes of other mammals, and lack the mammalian sex determining gene SRY and the gene from which it evolved,” explains co-first author Dr. Paul Waters. “Instead, their sex chromosomes share extensive homology to those of birds.” Waters notes that this finding suggests our ancient mammal-like reptile ancestor may have had bird-like sex chromosomes and sex determination system.

Reference: Veyrunes, F., Waters, P.D., Miethke, P., Rens, W., McMillan, D., Alsop, A.E., Grützner, F., Deakin, J.E., Whittington, C.M., Schatzkamer, K., Kremitzki, C.L., Graves, T., Ferguson-Smith, M.A., Warren, W., Graves, J.A.M. Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes. Genome Res. doi:10.1101/gr.7101908.