Journal of Heredity, 2022, 113, 491–499 https://doi.org/10.1093/jhered/esac037 Advance access publication 5 August 2022 Original Article Received January 19, 2022; Accepted August 2, 2022 Original Article Give and Take: Effects of Genetic Admixture on Mutation Load in Endangered Florida Panthers Alexander Ochoa 1, , David P. Onorato 2, , Melody E. Roelke-Parker 3, , Melanie Culver 4, , Robert R. Fitak 1, 1 Department of Biology and Genomics and Bioinformatics Cluster, University of Central Florida, Orlando, FL 32816, USA, 2 Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, Naples, FL 34114, USA, 3 Frederick National Laboratory for Cancer Research, Leidos Biomedical Research, Inc., Fredrick, MD 21701, USA, 4 U.S. Geological Survey, Arizona Cooperative Fish and Wildlife Research Unit, and School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721, USA Address correpondence to A. Ochoa at the address above, or e-mail: alexander.ochoa@ucf.edu. Corresponding Editor: Klaus-Peter Koepfi Abstract Genetic admixture is a biological event inherent to genetic rescue programs aimed at the long-term conservation of endangered wildlife. Although the success of such programs can be measured by the increase in genetic diversity and fitness of subsequent admixed individuals, predictions supporting admixture costs to fitness due to the introduction of novel deleterious alleles are necessary. Here, we analyzed nonsynonymous variation from conserved genes to quantify and compare levels of mutation load (i.e. proportion of deleterious alleles and genotypes carrying these alleles) among endangered Florida panthers and non-endangered Texas pumas. Specifically, we used canonical (i.e. non-admixed) Florida panthers, Texas pumas, and F 1 (canonical Florida × Texas) panthers dating from a genetic rescue program and Everglades National Park panthers with Central American ancestry resulting from an earlier admixture event. We found neither genetic drift nor selection significantly reduced overall proportions of deleterious alleles in the severely bottlenecked canonical Florida panthers. Nevertheless, the deleterious alleles identified were distributed into a disproportionately high number of homozygous genotypes due to close inbreeding in this group. Conversely, admixed Florida panthers (either with Texas or Central American ancestry) presented reduced levels of homozygous genotypes carrying deleterious alleles but increased levels of heterozygous genotypes carrying these variants relative to canonical Florida panthers. Although admixture is likely to alleviate the load of standing deleterious variation present in homozygous genotypes, our results suggest that introduced novel deleterious alleles (temporarily present in heterozygous state) in genetically rescued populations could potentially be expressed in subsequent generations if their effective sizes remain small. Key words: deleterious variation, genetic drift, genetic rescue, inbreeding, small populations, wildlife conservation Introduction Mutation load, defned as the proportion of deleterious alleles and genotypes carrying these alleles at the genome- wide level, has been used as a metric to infer the genetic health and extinction risk of species or populations that have experienced severe or consistent demographic bottlenecks (Robinson et al. 2019; Grossen et al. 2020), maintained long-term small effective sizes (Robinson et al. 2016), or been affected by recent anthropogenic activities (Yang et al. 2018; van der Valk et al. 2019). In bottlenecked populations, genetic drift is likely to eliminate a fraction of deleterious alleles (Glémin 2003; Mathur and DeWoody 2021; van der Valk et al. 2021), for these variants are often found at rel- atively low frequencies at the time of demographic decline (Mezmouk and Ross-Ibarra 2014; Perrier et al. 2017). By the same token, however, drift is also capable of increasing the frequency of another fraction of deleterious alleles (Romiguier et al. 2014; Renaut and Rieseberg 2015; Ochoa and Gibbs 2021), leading some of them to the point of fxa- tion (Benazzo et al. 2017). Despite recent studies suggesting that sustained drift could reduce overall proportions of deleterious alleles in small populations (Mathur and DeWoody 2021; Ochoa and Gibbs 2021), there remains a concern that persistent delete- rious alleles in these populations may be redistributed into a disproportionately high number of homozygous genotypes given genetic isolation, inbreeding, and ineffcient purifying selection (Hedrick and Garcia-Dorado 2016). Under such scenarios, individuals are prone to expressing a mosaic of del- eterious traits with negative effects on their ftness, collectively termed inbreeding depression (Frankham 1995; Hedrick and Kalinowski 2000; Charlesworth and Willis 2009). Consequently, the proportion of deleterious homozygotes at the genome-wide level refects minimum realized levels of mu- tation load (Mathur and DeWoody 2021), as deleterious alleles present in the heterozygous state could still manifest in the © The Author(s) 2022. 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