Mutation rates can evolve through genetic drift, indirect selection due to genetic hitchhiking, or direct selection on the physicochemical cost of high fidelity. However, for many systems, it has been difficult to disentangle the relative impact of these forces empirically. In RNA viruses, an observed correlation between mutation rate and virulence has led many to argue that their extremely high mutation rates are advantageous because they may allow for increased adaptability. This argument has profound implications because it suggests that pathogenesis in many viral infections depends on rare or de novo mutations. Here, we present data for an alternative model whereby RNA viruses evolve high mutation rates as a byproduct of selection for increased replicative speed. We find that a poliovirus antimutator, 3DG64S, has a significant replication defect and that wild-type (WT) and 3DG64S populations have similar adaptability in 2 distinct cellular environments. Experimental evolution of 3DG64S under selection for replicative speed led to reversion and compensation of the fidelity phenotype. Mice infected with 3DG64S exhibited delayed morbidity at doses well above the lethal level, consistent with attenuation by slower growth as opposed to reduced mutational supply. Furthermore, compensation of the 3DG64S growth defect restored virulence, while compensation of the fidelity phenotype did not. Our data are consistent with the kinetic proofreading model for biosynthetic reactions and suggest that speed is more important than accuracy. In contrast with what has been suggested for many RNA viruses, we find that within-host spread is associated with viral replicative speed and not standing genetic diversity.

Pathogenic mutations in cyclin-dependent kinase-like 5 (CDKL5) result in CDKL5 deficiency disorder (CDD), a rare disease marked by early-life seizures, autistic behaviors, and intellectual disability. Although mouse models of CDD exhibit dendritic instability and alterations in synaptic scaffolding proteins, studies of glutamate receptor levels and function are limited. Here we used a novel mouse model of CDD, the Cdkl5 R59X knock-in mouse (R59X), to investigate changes in synaptic glutamate receptor subunits and functional consequences. Male mice were used for all experiments to avoid the confounding effects of X-inactivation that would be present in female heterozygous mice. We showed that adult male R59X mice recapitulated the behavioral outcomes observed in other mouse models of CDD, including social deficits and memory and learning impairments, and exhibited decreased latency to seizure upon pentylenetetrazol administration. Furthermore, we observed a specific increase in GluA2-lacking ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptors (AMPARs) in the adult R59X hippocampus, which is accompanied electrophysiologically by increased rectification ratio of AMPAR EPSCs and elevated early-phase long term potentiation (LTP). Finally, we showed that acute treatment with the GluA2-lacking AMPAR blocker IEM-1460 decreased AMPAR currents, and rescued social deficits, working memory impairments, and seizure behavior latency in R59X mice.

27Mutation rates can evolve through genetic drift, indirect selection due to genetic hitchhiking, or 28 direct selection on the physicochemical cost of high fidelity. However, for many systems, it has 29 been difficult to disentangle the relative impact of these forces empirically. In RNA viruses, an 30 observed correlation between mutation rate and virulence has led many to argue that their 31 extremely high mutation rates are advantageous, because they may allow for increased 32 adaptability. This argument has profound implications, as it suggests that pathogenesis in many 33 viral infections depends on rare or de novo mutations. Here we present data for an alternative 34 model whereby RNA viruses evolve high mutation rates as a byproduct of selection for 35 increased replicative speed. We find that a poliovirus antimutator, 3D G64S , has a significant 36 replication defect and that wild type and 3D G64S populations have similar adaptability in two 37 distinct cellular environments. Experimental evolution of 3D G64S under r-selection led to 38 reversion and compensation of the fidelity phenotype. Mice infected with 3D G64S exhibited 39 delayed morbidity at doses well above the LD 50 , consistent with attenuation by slower growth as 40 opposed to reduced mutational supply. Furthermore, compensation of the 3D G64S growth defect 41 restored virulence, while compensation of the fidelity phenotype did not. Our data are consistent 42 with the kinetic proofreading model for biosynthetic reactions and suggest that speed is more 43 important than accuracy. In contrast to what has been suggested for many RNA viruses, we find 44 that within host spread is associated with viral replicative speed and not standing genetic 45 diversity.46 47 3 mutation rates. Since most mutations are deleterious and mutation rates are tuned by natural 53 selection, we asked why hasn't the virus evolved to have a lower mutation rate? We used 54 experimental evolution and a murine infection model to show that RNA virus mutation rates may 55 actually be too high and are not necessarily adaptive. Rather, our data indicate that viral 56 mutation rates are driven higher as a result of selection for viruses with faster replication kinetics. 57We suggest that viruses have high mutation rates, not because they facilitate adaption, but 58 because it is hard to be both fast and accurate. 59 60 Introduction 61 Mutation is the ultimate source of genetic variation, and mutation rates can have a significant 62 impact on evolutionary rate [1-3]. The intraspecific variability in mutation rate in many viruses 63 and bacteria indicates that mutation rates have been optimized by natural selection [4-13]. 64Given that most mutations are deleterious, the burden of excess mutational load will select 65 against strains with abnormally high mutation rates [14][15][16][17]. This principle led to Sturtevant to 66 ask, "Why does the mutation rate not evolve to zero?" [18,19]. 68A large body of theoretical and experimental work suggests that the selective pressure for 69...