1. Section of Integrative Biology and Institute for Cellular and Molecular Biology, The University of Texas at Austin, University Station C0930, Austin, TX 78712-0253, and Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA
2. Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322, USA
3. Department of Microbiology, Emory University, 1510 Clifton Road, Atlanta, GA 30322, USA