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Background: Nonadditivity in fitness effects from two or more mutations, termed epistasis, can result in compensation of deleterious mutations or negation of beneficial mutations. Recent evidence shows the importance of epistasis in individual evolutionary pathways. However, an unresolved question in molecular evolution is how often and how significantly fitness effects change in alternative genetic backgrounds. Results: To answer this question, we quantified the effects of all single mutations and double mutations between all positions in the IgG-binding domain of protein G (GB1). By observing the first two steps of all possible evolutionary pathways using this fitness profile, we were able to characterize the extent and magnitude of pairwise epistasis throughout an entire protein molecule. Furthermore, we developed a novel approach to quantitatively determine the effects of single mutations on structural stability (DDG U ). This enabled determination of the importance of stability effects in functional epistasis. Conclusions: Our results illustrate common biophysical mechanisms for occurrences of positive and negative epistasis. Our results show pervasive positive epistasis within a conformationally dynamic network of residues. The stability analysis shows that significant negative epistasis, which is more common than positive epistasis, mostly occurs between combinations of destabilizing mutations. Furthermore, we show that although significant positive epistasis is rare, many deleterious mutations are beneficial in at least one alternative mutational background. The distribution of conditionally beneficial mutations throughout the domain demonstrates that the functional portion of sequence space can be significantly expanded by epistasis.
Article A Comprehensive Biophysical Description of Pairwise Epistasis throughout an Entire Protein Domain

showing signs of disease in southern China during the period from 2013-2015. The nucleotide and amino acid sequences from the isolated field strains were compared to 42 published references. Nucleotide homologies ranged from 63.1-99.9% and amino acid homologies ranging from 60.2-100%. At least seven IBV genotypes were co-circulating in commercial chicken farms in southern China. The IBV isolates were genetically diverse and underwent continuing evolution. The QX-type, TW I-type, and 4/91-type were the most common genotypes during the three-year observation period and accounted for 88.8% of the isolated strains. Notably, the prevalence of the TW I-type strains has been increasing in recent years and has become the most common genotype in China. The emergence of variant IBV strains can be attributed to recombination. Serologic analysis and antigenic 3D cartography of 4 reference and 14 field isolated strains indicated the surveyed IBVs had diverse serology types and that the serotype of the isolated QX-type and TW I-type strains was distinct from the vaccines strains. Therefore, long-term continuing surveillance is necessary for IBV prevention and control. Avian infectious bronchitis (IB) is an acute and highly contagious disease caused by the avian infectious bronchitis virus (IBV). IB causes substantial economic losses throughout the poultry industry worldwide. IBV affects chickens of all ages causing upper respiratory disease, nephritic syndromes, and decreased egg production 1 . IBV has been circulating in China since 1982. The epidemiology of IBV in China is complicated due to the large geographical area, and multiple IBV genotypes and serotypes are known to coexist 2 . Our previous epidemiology monitoring during 2004 to 2012, found at least seven genotypes of IBV circulating. In that study, the QX-like IBV genotype that causes nephritis had become predominant in China 3-6 . Although IBV vaccines are widely used and most commercial chickens have been vaccinated, there are still frequent outbreaks of IB that cause clinical disease and production problems 7-9 . For example, widespread use of vaccines derived from Massachusetts (Mass) serotypes such as H120 and H52 did not prevent frequent outbreaks of IB in chicken flocks 10, 11 . IBV belongs to the genus Gammacoronavirus, family Coronaviridae, and order Nidovirales. Its genome is linear, non-segmented, positive-sense, single-stranded RNA that is approximately 27.6 kilobases (kb) in length. As with other coronaviruses, the IBV S1 glycoprotein contains virus neutralizing epitopes and serotype-specific sequences 12, 13 . The S1 protein is involved in infectivity and plays an important role in attachment to the host cell, as well as induction of neutralizing antibodies 14 . Given the importance of the S1 protein virus/host interactions, IBV genetic analysis and research of IBV has been primarily based on the S1 gene 15 . Variation in the S1 protein is the basis for genotype and serotype classification, defines antigen characteristics, contributes to emergence of IBV variants, and poor vaccine protection 13, 16 . S1 variability is due in part to the lack of proofreading by the RNA-dependent RNA polymerase (RDRP) that introduces mutations into the viral genome during replication. Variation is also introduced by the specific template switching mechanism used by coronaviruses, which frequently leads to genetic recombination 17, 18 . At least 30 serotypes of IBV have been identified worldwide. Vaccine cross-protection between serotypes is minimal 19 . Therefore, phylogenetic and genetic analyses have become tools for monitoring the molecular evolution of IBV and provide a fast and accurate method for genotype classification. In some studies, the genotype analyses have been used to predict IBV serotype and used to inform vaccine selection 20 . In this study, long-term surveillance was conducted with the aim of identifying IBV strains isolated from commercial chicken farms in south China. During 2013-2015, IBV strains were isolated from flocks with clinical outbreaks. Genetic, phylogenetic, and recombination analyses were performed to study the genetic diversity of the S1 gene in field isolated IBV strains. Cross-neutralization tests and antigenic cartography were performed to determine the serological and antigenic relationship between a part of the isolated and vaccine strains.
Epidemiology and characterization of avian infectious bronchitis virus strains circulating in southern China during the period from 2013-2015 OPEN

Rheumatoid arthritis (RA) is a chronic and genetically complex inflammatory disorder that leads to erosive destruction of peripheral joints 1-5 . The use of animal models mimicking RA, such as pristane-induced arthritis (PIA) in rats, should facilitate its genetic analysis. Pristane is a non-immunogenic synthetic oil that, after a single subcutaneous injection into DA rats, induces arthritis restricted to peripheral joints with a chronic relapsing disease course 6 . To identify genes involved in the control of chronic arthritis, we made crosses between susceptible DA rats and resistant E3 rats and analysed the progeny with microsatellite markers covering the entire rat genome. Our results show that different arthritis phenotypes are associated with different chromosomal loci. Loci on chromosomes 4 and 6 (Pia2 and Pia3) influence arthritis onset, whereas a locus on chromosome 12 (Pia4) is associated with severity and joint erosion. We found that chronicity is associated with a different set of loci, one on chromosome 4 and the other on chromosome 14 (Pia5, Pia6) . These findings demonstrate for the first time that different phases of a chronic self-perpetuative disease which mimics RA are associated with distinct sets of genes.

Genetic control of arthritis onset, severity and chronicity in a model for rheumatoid arthritis in rats