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1. Genomic signatures of selection in bats surviving white‐nose syndrome

   https://doi.org/10.1111/mec.15813  

   Gignoux‐Wolfsohn, Sarah A.; Pinsky, Malin L.; Kerwin, Kathleen; Herzog, Carl; Hall, MacKenzie; Bennett, Alyssa B.; Fefferman, Nina H.; Maslo, Brooke (February 2021, Molecular Ecology)

   Abstract Rapid evolution of advantageous traits following abrupt environmental change can help populations recover from demographic decline. However, for many introduced diseases affecting longer‐lived, slower reproducing hosts, mortality is likely to outpace the acquisition of adaptive de novo mutations. Adaptive alleles must therefore be selected from standing genetic variation, a process that leaves few detectable genomic signatures. Here, we present whole genome evidence for selection in bat populations that are recovering from white‐nose syndrome (WNS). We collected samples both during and after a WNS‐induced mass mortality event in two little brown bat populations that are beginning to show signs of recovery and found signatures of soft sweeps from standing genetic variation at multiple loci throughout the genome. We identified one locus putatively under selection in a gene associated with the immune system. Multiple loci putatively under selection were located within genes previously linked to host response to WNS as well as to changes in metabolism during hibernation. Results from two additional populations suggested that loci under selection may differ somewhat among populations. Through these findings, we suggest that WNS‐induced selection may contribute to genetic resistance in this slowly reproducing species threatened with extinction. 

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2. Introgressive Hybridization and Hypoxia Adaptation in High-Altitude Vertebrates

   https://doi.org/10.3389/fgene.2021.696484  

   Storz, Jay F.; Signore, Anthony V. (June 2021, Frontiers in Genetics)

   null (Ed.)

   In natural populations of animals, a growing body of evidence suggests that introgressive hybridization may often serve as an important source of adaptive genetic variation. Population genomic studies of high-altitude vertebrates have provided strong evidence of positive selection on introgressed allelic variants, typically involving a long-term highland species as the donor and a more recently arrived colonizing species as the recipient. In high-altitude humans and canids from the Tibetan Plateau, case studies of adaptive introgression involving the HIF transcription factor, EPAS1 , have provided insights into complex histories of ancient introgression, including examples of admixture from now-extinct source populations. In Tibetan canids and Andean waterfowl, directed mutagenesis experiments involving introgressed hemoglobin variants successfully identified causative amino acid mutations and characterized their phenotypic effects, thereby providing insights into the functional properties of selectively introgressed alleles. We review case studies of adaptive introgression in high-altitude vertebrates and we highlight findings that may be of general significance for understanding mechanisms of environmental adaptation involving different sources of genetic variation. 

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3. Drift, selection and adaptive variation in small populations of a threatened rattlesnake

   https://doi.org/10.1111/mec.15517  

   Ochoa, Alexander; Broe, Michael; Moriarty_Lemmon, Emily; Lemmon, Alan_R; Rokyta, Darin_R; Gibbs, H_Lisle (July 2020, Molecular Ecology)

   Abstract An important goal of conservation genetics is to determine if the viability of small populations is reduced by a loss of adaptive variation due to genetic drift. Here, we assessed the impact of drift and selection on direct measures of adaptive variation (toxin loci encoding venom proteins) in the eastern massasauga rattlesnake (Sistrurus catenatus), a threatened reptile that exists in small isolated populations. We estimated levels of individual polymorphism in 46 toxin loci and 1,467 control loci across 12 populations of this species, and compared the results with patterns of selection on the same loci following speciation ofS. catenatusand its closest relative, the western massasauga (S. tergeminus). Multiple lines of evidence suggest that both drift and selection have had observable impacts on standing adaptive variation. In support of drift effects, we found little evidence for selection on toxin variation within populations and a significant positive relationship between current levels of adaptive variation and long‐ and short‐term estimates of effective population size. However, we also observed levels of directional selection on toxin loci among populations that are broadly similar to patterns predicted from interspecific selection analyses that pre‐date the effects of recent drift, and that functional variation in these loci persists despite small short‐term effective sizes. This suggests that much of the adaptive variation present in populations may represent an example of “drift debt,” a nonequilibrium state where present‐day levels of variation overestimate the amount of functional genetic diversity present in future populations. 

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4. Broad concordance in the spatial distribution of adaptive and neutral genetic variation across an elevational gradient in deer mice

   https://doi.org/10.1093/molbev/msab161  

   Schweizer, Rena M; Jones, Matthew R; Bradburd, Gideon S; Storz, Jay F; Senner, Nathan R; Wolf, Cole; Cheviron, Zachary A (May 2021, Molecular Biology and Evolution)

   Sork, Victoria (Ed.)

   Abstract When species are continuously distributed across environmental gradients, the relative strength of selection and gene flow shape spatial patterns of genetic variation, potentially leading to variable levels of differentiation across loci. Determining whether adaptive genetic variation tends to be structured differently than neutral variation along environmental gradients is an open and important question in evolutionary genetics. We performed exome-wide population genomic analysis on deer mice sampled along an elevational gradient of nearly 4000 m of vertical relief. Using a combination of selection scans, genotype-environment associations, and geographic cline analyses, we found that a large proportion of the exome has experienced a history of altitude-related selection. Elevational clines for nearly 30% of these putatively adaptive loci were shifted significantly up- or down-slope of clines for loci that did not bear similar signatures of selection. Many of these selection targets can be plausibly linked to known phenotypic differences between highland and lowland deer mice, although the vast majority of these candidates have not been reported in other studies of highland taxa. Together, these results suggest new hypotheses about the genetic basis of physiological adaptation to high-altitude, and the spatial distribution of adaptive genetic variation along environmental gradients. 

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5. Unique Genomic and Phenotypic Responses to Extreme and Variable pH Conditions in Purple Urchin Larvae

   https://doi.org/10.1093/icb/icaa072  

   Garrett, April D; Brennan, Reid S; Steinhart, Anya L; Pelletier, Aubrey M; Pespeni, Melissa H (May 2020, Integrative and Comparative Biology)

   Synopsis Environmental variation experienced by a species across space and time can promote the maintenance of genetic diversity that may be adaptive in future global change conditions. Selection experiments have shown that purple sea urchin, Strongylocentrotus purpuratus, populations have adaptive genetic variation for surviving pH conditions at the “edge” (pH 7.5) of conditions experienced in nature. However, little is known about whether populations have genetic variation for surviving low-pH events beyond those currently experienced in nature or how variation in pH conditions affects organismal and genetic responses. Here, we quantified survival, growth, and allele frequency shifts in experimentally selected developing purple sea urchin larvae in static and variable conditions at three pH levels: pH 8.1 (control), pH 7.5 (edge-of-range), and pH 7.0 (extreme). Variable treatments recovered body size relative to static treatments, but resulted in higher mortality, suggesting a potential tradeoff between survival and growth under pH stress. However, within each pH level, allele frequency changes were overlapping between static and variable conditions, suggesting a shared genetic basis underlying survival to mean pH regardless of variability. In contrast, genetic responses to pH 7.5 (edge) versus pH 7.0 (extreme) conditions were distinct, indicating a unique genetic basis of survival. In addition, loci under selection were more likely to be in exonic regions than regulatory, indicating that selection targeted protein-coding variation. Loci under selection in variable pH 7.5 conditions, more similar to conditions periodically experienced in nature, performed functions related to lipid biosynthesis and metabolism, while loci under selection in static pH 7.0 conditions performed functions related to transmembrane and mitochondrial processes. While these results are promising in that purple sea urchin populations possess genetic variation for surviving extreme pH conditions not currently experienced in nature, they caution that increased acidification does not result in a linear response but elicits unique physiological stresses and survival mechanisms. 

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