Although mice are the most widely used model organism, genetic studies have suffered from limited mapping resolution due to extensive linkage disequilibrium (LD) that is characteristic of crosses among inbred strains. Carworth Farms White (CFW) mice are a commercially available outbred mouse population that exhibit rapid LD decay compared to other available mouse populations. We performed a genome-wide association study (GWAS) of behavioral, physiological and gene expression phenotypes using 1,200 male CFW mice. We used genotyping-by-sequencing (GBS) to obtain genotypes at 92,734 single nucleotide polymorphisms (SNPs). We also measured gene expression using RNA-Sequencing in three brain regions. Our study identified numerous behavioral, physiological and expression quantitative trait loci (QTLs). We integrated the behavioral QTL and eQTL results to implicate specific genes, including Azi2 in sensitivity to methamphetamine and Zmynd11 in anxiety-like behavior. The combination of CFW mice, GBS and RNA-Sequencing constitutes a powerful approach to GWAS in mice.

Even when trained under exactly the same conditions outbred male Sprague-Dawley (SD) rats vary in the form of the Pavlovian conditioned approach response (CR) they acquire. The form of the CR (i.e. sign-tracking vs. goal-tracking) predicts to what degree individuals attribute incentive salience to cues associated with food or drugs. However, we have noticed variation in the incidence of these two phenotypes in rats obtained from different vendors. In this study, we quantified sign- and goal-tracking behavior in a reasonably large sample of SD rats obtained from two vendors (Harlan or Charles River), as well as from individual colonies operated by both vendors. Our sample of rats acquired from Harlan had, on average, more sign-trackers than goal-trackers, and vice versa for our sample of rats acquired from Charles River. Furthermore, there were significant differences among colonies of the same vendor. Although it is impossible to rule out environmental variables, SD rats at different vendors and barriers may have reduced phenotypic heterogeneity as a result of genetic variables, such as random genetic drift or population bottlenecks. Consistent with this hypothesis, we identified marked population structure among colonies from Harlan. Therefore, despite sharing the same name, investigators should be aware that important genetic and phenotypic differences exist among SD rats from different vendors or even from different colonies of the same vendor. If used judiciously this can be an asset to experimental design, but it can also be a pitfall for those unaware of the issue.

The LG/J x SM/J advanced intercross line of mice (LG x SM AIL) is a multigenerational outbred population. High minor allele frequencies, a simple genetic background, and the fully sequenced LG and SM genomes make it a powerful population for genome-wide association studies. Here we use 1,063 AIL mice to identify 126 significant associations for 50 traits relevant to human health and disease. We also identify thousands of cis- and trans-eQTLs in the hippocampus, striatum, and prefrontal cortex of ~200 mice. We replicate an association between locomotor activity and Csmd1, which we identified in an earlier generation of this AIL, and show that Csmd1 mutant mice recapitulate the locomotor phenotype. Our results demonstrate the utility of the LG x SM AIL as a mapping population, identify numerous novel associations, and shed light on the genetic architecture of mammalian behavior.

Genome wide association analyses (GWAS) in model organisms have numerous advantages compared 2 to human GWAS, including the ability to use populations with well-defined genetic diversity, the ability to 3 collect tissue for gene expression analysis and the ability to perform experimental manipulations. We 4 examined behavioral, physiological, and gene expression traits in 1,063 male and female mice from a 5 50-generation intercross between two inbred strains (LG/J and SM/J). We used genotyping by 6 sequencing in conjunction with whole genome sequence data from the two founder strains to obtain 7 genotypes at 4.3 million SNPs. As expected, all alleles were common (mean MAF=0.35) and linkage 8 disequilibrium degraded rapidly, providing excellent power and sub-megabase mapping precision. We 9 identified 126 genome-wide significant loci for 50 traits and integrated this information with 7,081 cis-10 eQTLs and 1,476 trans-eQTLs identified in hippocampus, striatum and prefrontal cortex. We replicated 11 several loci that were identified using an earlier generation of this intercross, including an association 12 between locomotor activity and a locus containing a single gene, Csmd1. We also showed that Csmd1 13 mutant mice recapitulated the locomotor phenotype. Our results demonstrate the utility of this population, 14 identify numerous novel associations, and provide examples of replication in an independent cohort, 15 which is customary in human genetics, and replication by experimental manipulation, which is a unique 16 advantage of model organisms.

Muscle bulk in adult healthy humans is highly variable even after accounting for height, age and sex. Low muscle mass, due to fewer and/or smaller constituent muscle fibers, would exacerbate the impact of muscle loss occurring in aging or disease. Genetic variability substantially influences muscle mass differences, but causative genes remain largely unknown. In a genome-wide association study (GWAS) on appendicular lean mass (ALM) in a population of 85,750 middle-age (38-49 years) individuals from the UK Biobank (UKB) we found 182 loci associated with ALM (P<5x10-8). We replicated associations for 78% of these loci (P<5x10-8) with ALM in a population of 181,862 elderly (60-74 years) individuals from UKB. We also conducted a GWAS on hindlimb skeletal muscle mass of 1,867 mice from an advanced intercross between two inbred strains (LG/J and SM/J) which identified 23 quantitative trait loci. 38 positional candidates distributed across 5 loci overlapped between the two species. In vitro studies of positional candidates confirmed CPNE1 and STC2 as modifiers of myogenesis. Collectively, these findings shed light on the genetics of muscle mass variability in humans and identify targets for the development of interventions for treatment of muscle loss. The overlapping results between humans and the mouse model GWAS point to shared genetic mechanisms across species.

Quantitative genetic studies in model organisms, particularly in mice, have been extremely successful in identifying chromosomal regions that are associated with a wide variety of behavioral and other traits. However, it is now widely understood that identification of the underlying genes will be far more challenging. In the last few years, a variety of populations have been utilized in an effort to more finely map these chromosomal regions with the goal of identifying specific genes. The common property of these newer populations is that linkage disequilibrium spans relatively short distances, which permits fine-scale mapping resolution. This review focuses on advanced intercross lines (AILs) which are the simplest such population. As originally proposed in 1995 by Darvasi and Soller, an AIL is the product of intercrossing two inbred strains beyond the F2 generation. Unlike recombinant inbred strains, AILs are maintained as outbred populations; brother-sister matings are specifically avoided. Each generation of intercrossing beyond the F2 further degrades linkage disequilibrium between adjacent makers, which allows for fine scale mapping of quantitative trait loci (QTLs). Advances in genotyping technology and techniques for the statistical analysis of AILs have permitted rapid advances in the application of AILs. We review some of the analytical issues and available software, including QTLRel, EMMA, EMMAX, GEMMA, TASSEL, GRAMMAR, WOMBAT, Mendel and others.

30Sprague Dawley (SD) rats are one of the most commonly used outbred rat strains. Despite 31 this, the genetic characteristics of SD are poorly understood. We collected behavioral data from 32 4,625 SD rats acquired predominantly from two commercial vendors, Charles River Laboratories 33 and Harlan Sprague Dawley Inc. Using double-digest genotyping-by-sequencing (ddGBS), we 34 obtained dense, high-quality genotypes at 234,887 SNPs across 4,061 rats. This genetic data 35 allowed us to characterize the variation present in Charles River vs. Harlan SD rats. We found that 36 the two populations are highly diverged (FST > 0.4). We also used these data to perform a genome-37 wide association study (GWAS) of Pavlovian conditioned approach (PavCA), which assesses the 38 propensity for rats to attribute motivational value to discrete, reward-associated cues. Due to the 39 genetic divergence between rats from Charles River and Harlan, we performed two separate 40 GWAS by fitting a linear mixed model that accounted for within vendor population structure and 41 using meta-analysis to jointly analyze the two studies. We identified 18 independent loci that were 42 significantly associated with one or more metrics used to describe PavCA; we also identified 3 43 loci that were body weight, which was only measured in a subset of the rats. The genetic 44 characterization of SD rats is a valuable resource for the rat community that can be used to inform 45 future study design. 46 AuthorSummary 47 Outbred Sprague Dawley rats are among the most commonly used rats for neuroscience, 48 physiology and pharmacological research. SD rats are sold by several commercial vendors, 49 including Charles River Laboratories and Harlan Sprague Dawley Inc. (now Envigo). Despite their 50wide spread use, little is known about the genetic diversity of SD. We genotyped more than 4,000 51 2 SD rats, which we used to characterize genetic differences between SD rats from Charles River 52 Laboratories and Harlan. Our analysis revealed that the two SD colonies are highly divergent. We 53 also performed a genome-wide association study (GWAS) for Pavlovian conditioned approach 54 (PavCA), which assesses the propensity for rats to attribute motivational value to discrete, reward-55 associated cues. Our results demonstrate that, despite sharing an identical name, SD rats are 56 obtained from different vendors are genetically very different. We conclude that results obtained 57 using SD rats should not be presented without also carefully noting the vendor. 58 Introduction 59Rats are among the most commonly used organisms for experimental psychology and 60 biomedical research. Whereas research using mice makes extensive use of inbred strains, in rats, 61 it is more common to use commercially available outbred populations. Among the commercially 62 available outbred rat populations, the Sprague Dawley strain (SD) is one of the most widely used. 63SD rats are distributed by several vendors. Each vendor has multiple breeding locations, and each 64 breeding locatio...

Sprague Dawley (SD) rats are among the most widely used outbred laboratory rat populations. Despite this, the genetic characteristics of SD rats have not been clearly described, and SD rats are rarely used for experiments aimed at exploring genotype-phenotype relationships. In order to use SD rats to perform a genome-wide association study (GWAS), we collected behavioral data from 4,625 SD rats that were predominantly obtained from two commercial vendors, Charles River Laboratories and Harlan Sprague Dawley Inc. Using double-digest genotyping-by-sequencing (ddGBS), we obtained dense, high-quality genotypes at 291,438 SNPs across 4,061 rats. This genetic data allowed us to characterize the variation present in Charles River vs. Harlan SD rats. We found that the two populations are highly diverged (FST > 0.4). Furthermore, even for rats obtained from the same vendor, there was strong population structure across breeding facilities and even between rooms at the same facility. We performed multiple separate GWAS by fitting a linear mixed model that accounted for population structure and using meta-analysis to jointly analyze all cohorts. Our study examined Pavlovian conditioned approach (PavCA) behavior, which assesses the propensity for rats to attribute incentive salience to reward-associated cues. We identified 46 significant associations for the various metrics used to define PavCA. The surprising degree of population structure among SD rats from different sources has important implications for their use in both genetic and non-genetic studies.