Purifying selection and adaptive evolution proximate to the zoonosis of SARS-CoV-1 and SARS-CoV-2

Over the past two decades the pace of spillovers from animal viruses to humans has accelerated, with COVID-19 becoming the most deadly zoonotic disease in living memory. Prior to zoonosis, it is conceivable that the virus might largely be subjected to purifying selection, requiring no additional selective changes for successful zoonotic transmission. Alternatively, selective changes occurring in the reservoir species may coincidentally preadapt the virus for human-to-human transmission, facilitating spread upon cross-species exposure. Here we quantify changes in the genomes of SARS-CoV-2 and SARS-CoV-1 proximate to zoonosis to evaluate the selection pressures acting on the viruses. Application of molecular-evolutionary and population-genetic approaches to quantify site-specific selection within both SARS-CoV genomes revealed strong purifying selection across many genes at the time of zoonosis. Even in the viral surface-protein Spike that has been fast-evolving in humans, there is little evidence of positive selection proximate to zoonosis. Nevertheless, in SARS-CoV-2, NSP12, a core protein for viral replication, exhibited a region under adaptive selection proximate to zoonosis. Furthermore, in both SARS-CoV-1 and SARS-CoV-2, regions of adaptive selection proximate to zoonosis were found in ORF7a, a putative Major Histocompatibility Complex modulatory gene. These findings suggest that these replication and immunomodulatory proteins have played a previously underappreciated role in the adaptation of SARS coronaviruses to human hosts.