1 Fitness barriers limit reversion of a proofreading-deficient coronavirus 1 2 Kevin W. Graepel1,3, Maria L. Agostini1,3, Xiaotao Lu2, Nicole R. Sexton4, and Mark R. 3 Denison1,2,3# 4 5 1Department of Pathology, Microbiology, and Immunology; 2Department of Pediatrics; 6 3Vanderbilt Institute for Infection, Immunology and Inflammation (VI4), Vanderbilt University 7 Medical Center, Nashville, TN, USA. 4Department of Microbiology, Immunology, and 8 Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, 9 Fort Collins, CO, USA. 10 11 #Address correspondence to