1. Department of Ecology & Evolutionary Biology, University of Toronto, Toronto, ON M5S 3B2, Canada.
2. Carl R. Woese Institute for Genomic Biology, University of Illinois, Urbana, IL 61801, USA.
3. Department of Mechanical & Industrial Engineering, University of Toronto, Toronto, ON M5S 3G8, Canada.
4. Centre for the Analysis of Genome Evolution & Function, University of Toronto, Toronto, ON M5S 3B2, Canada.
5. Radcliffe Institute for Advanced Study, Harvard University, Cambridge, MA 02138, USA.