DOCSLIB.ORG

Seasonally Fluctuating Selection Can Maintain Polymorphism at Many Loci

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Seasonally Fluctuating Selection Can Maintain Polymorphism at Many Loci Seasonally fluctuating selection can maintain PNAS PLUS polymorphism at many loci via segregation lift Meike J. Wittmanna,b,c,1, Alan O. Berglanda,d, Marcus W. Feldmana, Paul S. Schmidte, and Dmitri A. Petrova,1 aDepartment of Biology, Stanford University, Stanford, CA 94305; bFakultat¨ fur¨ Mathematik, Universitat¨ Wien, 1090 Wien, Austria; cFakultat¨ fur¨ Biologie, Universitat¨ Bielefeld, 33615 Bielefeld, Germany; dDepartment of Biology, University of Virginia, Charlottesville, VA 22904; and eDepartment of Biology, University of Pennsylvania, Philadelphia, PA 19104-6313 Edited by M. T. Clegg, University of California, Irvine, CA, and approved October 3, 2017 (received for review March 8, 2017) Most natural populations are affected by seasonal changes in (11) and genotypes (12). In fact, most organisms with multiple temperature, rainfall, or resource availability. Seasonally fluctuat- generations per year experience a particular type of temporal ing selection could potentially make a large contribution to main- heterogeneity: seasonality, for example, in temperature, rainfall, taining genetic polymorphism in populations. However, previous resource availability, or in the abundance of predators, com- theory suggests that the conditions for multilocus polymorphism petitors, or parasites. Even tropical populations usually experi- are restrictive. Here, we explore a more general class of mod- ence some seasonality. For example, flowering and fruiting in els with multilocus seasonally fluctuating selection in diploids. tropical forests is often synchronized within and between tree In these models, the multilocus genotype is mapped to fitness in species, leading to seasonal changes in food availability for ani- two steps. The first mapping is additive across loci and accounts mals (13). Often, there are life-history trade-offs across sea- for the relative contributions of heterozygous and homozygous sons (14, 15). For example, seasons with abundant resource loci—that is, dominance. The second step uses a nonlinear fit- supply might select for investment in reproduction, whereas ness function to account for the strength of selection and epis- stressful seasons may select for investment in survival. Since such tasis. Using mathematical analysis and individual-based simula- life-history traits are usually polygenic, many organisms should tions, we show that stable polymorphism at many loci is possible experience seasonally fluctuating selection at a large number if currently favored alleles are sufficiently dominant. This general of loci. EVOLUTION mechanism, which we call “segregation lift,” requires seasonal With discrete generations, the fates of genotypes under tem- changes in dominance, a phenomenon that may arise naturally porally fluctuating selection depend on their geometric mean fit- in situations with antagonistic pleiotropy and seasonal changes in nesses over time (16). In haploids, two alleles generally cannot the relative importance of traits for fitness. Segregation lift works coexist because one will have a higher geometric mean fitness best under diminishing-returns epistasis, is not affected by prob- and eventually go to fixation (ref. 16, but see refs. 17 and 18). lems of genetic load, and is robust to differences in parameters In diploids, polymorphism at a single locus is stable if heterozy- across loci and seasons. Under segregation lift, loci can exhibit gotes have the highest geometric mean fitness (“marginal over- conspicuous seasonal allele-frequency fluctuations, but often fluc- dominance”), although in any particular generation, one of the tuations may be small and hard to detect. An important direction homozygotes might be fittest (16, 19, 20). for future work is to formally test for segregation lift in empiri- cal data and to quantify its contribution to maintaining genetic Significance variation in natural populations. temporal heterogeneity j cyclical selection j genetic diversity j marginal A key question in evolutionary biology is: What maintains overdominance j balancing selection the abundant genetic variation observed in natural popula- tions? Many organisms experience some seasonality in their ver since biologists were first able to detect population habitats, and, if they have multiple generations per year, sea- Egenetic variation at the molecular level, they have been puz- sonally fluctuating selection is a potentially powerful mecha- zled by its abundance in natural populations (1). Dispute over nism to maintain polymorphism. However, previous research the underlying reasons gave rise to two scientific schools (2, 3). has argued that this occurs rarely. Inspired by recent empiri- Proponents of the “(neo)classical” school claim that the bulk of cal findings, we reevaluate the potential of seasonally fluctu- genetic variation is due to neutral or weakly deleterious muta- ating selection to simultaneously maintain polymorphism at tions present at an equilibrium between mutation, genetic drift, many loci in the genome. We obtain a more general condition and selection. The neoclassical view admits that selection may for the maintenance of multilocus polymorphism by season- maintain alleles at intermediate frequency at some loci, but ally fluctuating selection. This condition may plausibly be sat- argues that such loci are exceedingly rare on a genomic scale isfied for many species and does not suffer from problems of (2). By contrast, the “balance” school posits that a substan- previous models. tial fraction of variation is maintained by some form of bal- ancing selection [with some controversy over the meaning of Author contributions: M.J.W., A.O.B., M.W.F., P.S.S., and D.A.P. designed research; M.J.W. performed analyses and simulations; A.O.B., M.W.F., and D.A.P. gave input on all aspects “substantial” (3)]—for example, heterozygote advantage (over- of the analyses; P.S.S. gave input on the paper; and M.J.W, A.O.B., M.W.F., and D.A.P. dominance), negative frequency-dependent selection, and spa- wrote the paper. tial or temporal variability in selection pressures (4). The authors declare no conflict of interest. Fifty years later, the debate has not been conclusively set- This article is a PNAS Direct Submission. tled (5, 6), although the majority view is that (nearly) neu- Published under the PNAS license. tral mutations cause most genetic variation, with overdomi- Data deposition: Source code underlying the analyses in this manuscript has been nance playing a relatively minor part, perhaps acting at only deposited in the figshare repository (available at https://doi.org/10.6084/m9.figshare. tens of loci per species (7–9). A mechanism considered more 5142262). common and powerful is spatial environmental heterogeneity. 1To whom correspondence may be addressed. Email: [email protected] Temporal heterogeneity, by contrast, is believed to be of lim- or [email protected]. ited importance (10), despite widespread temporal fluctuations This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. in the strength and direction of selection, both on phenotypes 1073/pnas.1702994114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1702994114 PNAS Early Edition j 1 of 10 Downloaded by guest on October 3, 2021 Extending these results to the multilocus case is nontrivial, alone is not sufficient to reconcile evidence from population and, so far, only two cases are well-understood: (i) multiplicative genomics and quantitative genetics (38). Thus, we need to recon- selection across loci and (ii) temporally fluctuating selection on sider the potential of temporally fluctuating selection to maintain a fully additive trait. Under multiplicative selection in an infinite multilocus polymorphism. population with free recombination, the allele-frequency dynam- As explained above, the conditions for multilocus polymor- ics at a focal locus are independent of those at other loci. Thus, phism under seasonally fluctuating selection have been examined polymorphism is stable if heterozygotes have the highest geo- mostly in two narrow cases. Here, we examine a more general metric mean fitness, as in the single-locus case. However, devi- class of seasonal selection models with various forms of domi- ations from multiplicative selection appear to be the rule. In nance and epistasis. Using deterministic mathematical analysis particular, beneficial mutations often exhibit diminishing-returns and stochastic simulations, we show that multilocus polymor- epistasis (21–23). Additionally, there is the potential problem of phism is possible if the currently favored allele at any time is genetic load. Genetic load is commonly defined as the differ- sufficiently dominant, with dominance measured by using a scale ence between the population’s average fitness and the fitness of on which contributions across loci are additive. This mechanism, the fittest possible genotype. Lewontin and Hubby (1) noticed which we call “segregation lift,” can maintain polymorphism at that this value can become unsustainably high if there is strong a large number of loci across the genome, is robust to many heterozygote advantage at many loci. This was a conundrum for model perturbations, and does not require single individuals to the neoclassical school, which was worried that with high genetic have too many offspring. Depending on the parameter values, load, single individuals would have to produce an astronomically allele-frequency fluctuations can be large and readily detectable, large number of offspring. Others have dismissed this concern,
Load more

Recommended publications
  • Purebred Dog Breeds Into the Twenty-First Century: Achieving Genetic Health for Our Dogs
    Purebred Dog Breeds into the Twenty-First Century: Achieving Genetic Health for Our Dogs BY JEFFREY BRAGG WHAT IS A CANINE BREED? What is a breed? To put the question more precisely, what are the necessary conditions that enable us to say with conviction, "this group of animals constitutes a distinct breed?" In the cynological world, three separate approaches combine to constitute canine breeds. Dogs are distinguished first by ancestry , all of the individuals descending from a particular founder group (and only from that group) being designated as a breed. Next they are distinguished by purpose or utility, some breeds existing for the purpose of hunting particular kinds of game,others for the performance of particular tasks in cooperation with their human masters, while yet others owe their existence simply to humankind's desire for animal companionship. Finally dogs are distinguished by typology , breed standards (whether written or unwritten) being used to describe and to recognize dogs of specific size, physical build, general appearance, shape of head, style of ears and tail, etc., which are said to be of the same breed owing to their similarity in the foregoing respects. The preceding statements are both obvious and known to all breeders and fanciers of the canine species. Nevertheless a correct and full understanding of these simple truisms is vital to the proper functioning of the entire canine fancy and to the health and well being of the animals which are the object of that fancy. It is my purpose in this brief to elucidate the interrelationship of the above three approaches, to demonstrate how distortions and misunderstandings of that interrelationship now threaten the health of all of our dogs and the very existence of the various canine breeds, and to propose reforms which will restore both balanced breed identity and genetic health to CKC breeds.
    [Show full text]
  • Review of the Current State of Genetic Testing - a Living Resource
    Review of the Current State of Genetic Testing - A Living Resource Prepared by Liza Gershony, DVM, PhD and Anita Oberbauer, PhD of the University of California, Davis Editorial input by Leigh Anne Clark, PhD of Clemson University July, 2020 Contents Introduction .................................................................................................................................................. 1 I. The Basics ......................................................................................................................................... 2 II. Modes of Inheritance ....................................................................................................................... 7 a. Mendelian Inheritance and Punnett Squares ................................................................................. 7 b. Non-Mendelian Inheritance ........................................................................................................... 10 III. Genetic Selection and Populations ................................................................................................ 13 IV. Dog Breeds as Populations ............................................................................................................. 15 V. Canine Genetic Tests ...................................................................................................................... 16 a. Direct and Indirect Tests ................................................................................................................ 17 b. Single
    [Show full text]
  • Balancing Selection at the Prion Protein Gene Consistent with Prehistoric Kurulike Epidemics Simon Mead,1 Michael P
    Balancing Selection at the Prion Protein Gene Consistent with Prehistoric Kurulike Epidemics Simon Mead,1 Michael P. H. Stumpf,2 Jerome Whitfield,1,3 Jonathan A. Beck,1 Mark Poulter,1 Tracy Campbell,1 James Uphill,1 David Goldstein,2 Michael Alpers,1,3,4 Elizabeth M. C. Fisher,1 John Collinge1* 1Medical Research Council, Prion Unit, and Department of Neurodegenerative Disease, Institute of Neurology, University College, Queen Square, London WC1N 3BG, UK. 2Department of Biology (Galton Laboratory), University College London, Gower Street, London WC1E 6BT, UK. 3Institute of Medical Research, Goroka, EHP, Papua New Guinea. 4Curtin University of Technology, Perth, WA, Australia. *To whom correspondence should be addressed. E-mail: [email protected] Kuru is an acquired prion disease largely restricted to the cannibalism imposed by the Australian authorities in the mid- Fore linguistic group of the Papua New Guinea Highlands 1950s led to a decline in kuru incidence, and although rare which was transmitted during endocannibalistic feasts. cases still occur these are all in older individuals and reflect Heterozygosity for a common polymorphism in the the long incubation periods possible in human prion human prion protein gene (PRNP) confers relative diseasekuru has not been recorded in any individual born after the late 1950s (4). resistance to prion diseases. Elderly survivors of the kuru A coding polymorphism at codon 129 of PRNP is a strong epidemic, who had multiple exposures at mortuary feasts, susceptibility factor for human prion diseases. Methionine are, in marked contrast to younger unexposed Fore, homozygotes comprise 37% of the UK population whereas predominantly PRNP 129 heterozygotes.
    [Show full text]
  • Strong Balancing Selection at HLA Loci: Evidence from Segregation In
    Proc. Natl. Acad. Sci. USA Vol. 94, pp. 12452–12456, November 1997 Evolution Strong balancing selection at HLA loci: Evidence from segregation in South Amerindian families (heterozygote advantageymaternal–fetal interactionymajor histocompatibility complexypolymorphismyreciprocal matings) FRANCIS L. BLACK† AND PHILIP W. HEDRICK‡§ †Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, CT 06520; and ‡Department of Biology, Arizona State University, Tempe, AZ 85287-1501 Edited by Margaret G. Kidwell, University of Arizona, Tucson, AZ, and approved September 3, 1997 (received for review July 23, 1997) ABSTRACT The genotypic proportions for major histo- The reduced proportion of HLA-A and HLA-B homozygotes compatibility complex loci, HLA-A and HLA-B, of progeny in observed in population samples of South Amerindians (8) needs families in 23 South Amerindian tribes in which segregation an s value of 0.425 to explain the deficiency (12). These results for homozygotes and heterozygotes could occur are examined. may not necessarily be in conflict if by chance the observations in Overall, there is a large deficiency of homozygotes compared refs. 8 and 9 are high values that would average away with lower with Mendelian expectations (for HLA-A, 114 observed and values in the past. In fact, there has probably been strong selection 155.50 expected and for HLA-B 110 observed and 144.75 in recent generations, if not the present one (3, 13). expected), consistent with strong balancing selection favoring Here, we present data showing a large excess of heterozy- heterozygotes. There is no evidence that these deficiencies gotes (a large deficiency of homozygotes) compared with were associated with particular alleles or with the age of the Mendelian expectations in 510 offspring from parents with individuals sampled.
    [Show full text]
  • Balancing Selection at a Premature Stop Mutation in the Myostatin Gene Underlies A
    bioRxiv preprint doi: https://doi.org/10.1101/442012; this version posted October 12, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 1 Balancing selection at a premature stop mutation in the myostatin gene underlies a 2 recessive leg weakness syndrome in pigs 3 Oswald Matika1, Diego Robledo1, Ricardo Pong-Wong1, Stephen C. Bishop1@, Valentina 4 Riggio1, Heather Finlayson1, Natalie R. Lowe1, Annabelle E. Hoste2, Grant A. Walling2, 5 Alan L. Archibald1, John A. Woolliams1‡, and Ross D. Houston1‡ 6 1The Roslin Institute and Royal (Dick) School of Veterinary Studies, The University of Edinburgh, 7 Midlothian, EH25 9RG, UK 8 2JSR Genetics, Southburn, Driffield, East Yorkshire, YO25 9ED, UK. 9 @Deceased 10 ‡ These authors contributed equally to this work 11 ABSTRACT 12 Balancing selection provides a plausible explanation for the maintenance of deleterious 13 alleles at moderate frequency in livestock, including lethal recessives exhibiting 14 heterozygous advantage in carriers. In the current study, a leg weakness syndrome 15 causing mortality of piglets in a commercial line showed monogenic recessive 16 inheritance, and a region on chromosome 15 associated with the syndrome was identified 17 by homozygosity mapping. Whole genome resequencing of cases and controls identified 18 a mutation coding for a premature stop codon within exon 3 of the porcine Myostatin 19 (MSTN) gene, similar to those causing a double-muscling phenotype observed in several 20 mammalian species.
    [Show full text]
  • Hitch-Hiking to a Locus Under Balancing Selection: High Sequence Diversity and Low Population Subdivision at the S-Locus Genomic Region in Arabidopsis Halleri
    Genet. Res., Camb. (2008), 90, pp. 37–46. f 2008 Cambridge University Press 37 doi:10.1017/S0016672307008932 Printed in the United Kingdom Hitch-hiking to a locus under balancing selection: high sequence diversity and low population subdivision at the S-locus genomic region in Arabidopsis halleri MARIA VALERIA RUGGIERO, BERTRAND JACQUEMIN, VINCENT CASTRIC AND XAVIER VEKEMANS* Universite´ des Sciences et Technologies de Lille 1, Laboratoire de ge´ne´tique et e´volution des populations ve´ge´tales, CNRS UMR 8016, 59655 Villeneuve d’Ascq, France (Received 8 September 2007 and in revised form 15 October 2007) Summary Hitch-hiking to a site under balancing selection is expected to produce a local increase in nucleotide polymorphism and a decrease in population differentiation compared with the background genomic level, but empirical evidence supporting these predictions is scarce. We surveyed molecular diversity at four genes flanking the region controlling self-incompatibility (the S-locus) in samples from six populations of the herbaceous plant Arabidopsis halleri, and compared their polymorphism with sequences from five control genes unlinked to the S-locus. As a preliminary verification, the S-locus flanking genes were shown to co-segregate with SRK, the gene involved in the self-incompatibility reaction at the pistil level. In agreement with theory, our results demonstrated a significant peak of nucleotide diversity around the S-locus as well as a significant decrease in population genetic structure in the S-locus region compared with both control genes and a set of seven unlinked microsatellite markers. This is consistent with the theoretical expectation that balancing selection is increasing the effective migration rate in subdivided populations.
    [Show full text]
  • Genomic Insights Into Positive Selection
    Review TRENDS in Genetics Vol.22 No.8 August 2006 Genomic insights into positive selection Shameek Biswas and Joshua M. Akey Department of Genome Sciences, University of Washington, 1705 NE Pacific, Seattle, WA 98195, USA The traditional way of identifying targets of adaptive more utilitarian benefits, each target of positive selection evolution has been to study a few loci that one has a story to tell about the historical forces and events hypothesizes a priori to have been under selection. that have shaped the history of a population. This approach is complicated because of the confound- Several genome-wide analyses for positive selection ing effects that population demographic history and have been performed in a variety of species. In this review, selection have on patterns of DNA sequence variation. In we summarize some of the recent studies, primarily principle, multilocus analyses can facilitate robust focusing on humans, critically evaluate what genome- inferences of selection at individual loci. The deluge of wide scans for selection are and are not likely to find and large-scale catalogs of genetic variation has stimulated suggest future avenues of research. A brief overview of many genome-wide scans for positive selection in statistical methods used to detect deviations from several species. Here, we review some of the salient neutrality is summarized in Box 1. For more detailed observations of these studies, identify important chal- discussions, see Refs [6,7]. lenges ahead, consider the limitations of genome-wide scans for selection and discuss the potential significance Thinking genomically of a comprehensive understanding of genomic patterns Positive selection perturbs patterns of genetic variation of selection for disease-related research.
    [Show full text]
  • Balancing Selection on Allorecognition Genes in the Colonial Ascidian Botryllus Schlosseri
    Developmental and Comparative Immunology 69 (2017) 60e74 Contents lists available at ScienceDirect Developmental and Comparative Immunology journal homepage: www.elsevier.com/locate/dci Balancing selection on allorecognition genes in the colonial ascidian Botryllus schlosseri * Marie L. Nydam a, , Emily E. Stephenson a, b, Claire E. Waldman a, Anthony W. De Tomaso c a Division of Science and Mathematics, Centre College, 600 W. Walnut Street, Danville, KY 40422, United States b Centre for Infectious Disease Research, P.O. Box 34681, Lusaka, 10101, Zambia c Department of Molecular, Cellular, and Developmental Biology, University of California Santa Barbara, Santa Barbara, CA 93106, United States article info abstract Article history: Allorecognition is the capability of an organism to recognize its own or related tissues. The colonial Received 24 November 2016 ascidian Botryllus schlosseri, which comprises five genetically distinct and divergent species (Clades A-E), Received in revised form contains two adjacent genes that control allorecognition: fuhcsec and fuhctm. These genes have been 22 December 2016 characterized extensively in Clade A and are highly polymorphic. Using alleles from 10 populations Accepted 22 December 2016 across the range of Clade A, we investigated the type and strength of selection maintaining this variation. Available online 24 December 2016 Both fuhc genes exhibit higher within-population variation and lower population differentiation mea- sures (FST) than neutral loci. The fuhc genes contain a substantial number of codons with >95% posterior Keywords: > sec tm Botryllus schlosseri probability of dN/dS 1. fuhc and fuhc also have polymorphisms shared between Clade A and Clade E Allorecognition that were present prior to speciation (trans-species polymorphisms).
    [Show full text]
  • Hybridization with Mountain Hares Increases the Functional Allelic Repertoire in Brown Hares Jaakko L
    www.nature.com/scientificreports OPEN Hybridization with mountain hares increases the functional allelic repertoire in brown hares Jaakko L. O. Pohjoismäki 1*, Craig Michell 1, Riikka Levänen1 & Steve Smith 2 Brown hares (Lepus europaeus Pallas) are able to hybridize with mountain hares (L. timidus Linnaeus) and produce fertile ofspring, which results in cross-species gene fow. However, not much is known about the functional signifcance of this genetic introgression. Using targeted sequencing of candidate loci combined with mtDNA genotyping, we found the ancestral genetic diversity in the Finnish brown hare to be small, likely due to founder efect and range expansion, while gene fow from mountain hares constitutes an important source of functional genetic variability. Some of this variability, such as the alleles of the mountain hare thermogenin (uncoupling protein 1, UCP1), might have adaptive advantage for brown hares, whereas immunity-related MHC alleles are reciprocally exchanged and maintained via balancing selection. Our study ofers a rare example where an expanding species can increase its allelic variability through hybridization with a congeneric native species, ofering a route to shortcut evolutionary adaptation to the local environmental conditions. Understanding of the ecological and evolutionary processes underlying species’ dispersal and range limits is a fundamental theme in evolutionary biology 1. Te leading edge of the species range expansion is typically popu- lated by few individuals, which are also living at the boundary of their specifc habitat preferences. Te genetic landscape of these boundary populations is infuenced by founder efect as well as low population densities resulting in increased genetic drif and further loss of genetic diversity 2.
    [Show full text]
  • Detecting Ancient Balancing Selection: Methods and Application to Human Katherine Siewert University of Pennsylvania, [email protected]
    University of Pennsylvania ScholarlyCommons Publicly Accessible Penn Dissertations 2019 Detecting Ancient Balancing Selection: Methods And Application To Human Katherine Siewert University of Pennsylvania, [email protected] Follow this and additional works at: https://repository.upenn.edu/edissertations Part of the Bioinformatics Commons, Evolution Commons, and the Genetics Commons Recommended Citation Siewert, Katherine, "Detecting Ancient Balancing Selection: Methods And Application To Human" (2019). Publicly Accessible Penn Dissertations. 3313. https://repository.upenn.edu/edissertations/3313 This paper is posted at ScholarlyCommons. https://repository.upenn.edu/edissertations/3313 For more information, please contact [email protected]. Detecting Ancient Balancing Selection: Methods And Application To Human Abstract Balancing selection can maintain genetic variation in a population over long evolutionary time periods. Identifying genomic loci under this type of selection not only elucidates selective pressures and adaptations but can also help interpret common genetic variation contributing to disease. Summary statistics which capture signatures in the site frequency spectrum are frequently used to scan the genome to detect loci showing evidence of balancing selection. However, these approaches have limited power because they rely on imprecise signatures such as a general excess of heterozygosity or number of genetic variants. A second class of statistics, based on likelihoods, have higher power but are often computationally prohibitive. In addition, a majority of methods in both classes require a high-quality sequenced outgroup, which is unavailable for many species of interest. Therefore, there is a need for a well-powered and widely-applicable statistical approach to detect balancing selection. Theory suggests that long-term balancing selection will result in a genealogy with very long internal branches.
    [Show full text]
  • Divergent Allele Advantage Provides a Quantitative Model for Maintaining Alleles with a Wide Range of Intrinsic Merits
    HIGHLIGHTED ARTICLE | INVESTIGATION Divergent Allele Advantage Provides a Quantitative Model for Maintaining Alleles with a Wide Range of Intrinsic Merits Thorsten Stefan,*,†,1,2 Louise Matthews,* Joaquin M. Prada,*,3 Colette Mair,*,4 Richard Reeve,* and Michael J. Stear*,5 *Boyd Orr Centre for Population and Ecosystem Health, Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, G12 8QQ, United Kingdom and †Institute of Applied Mathematics and Statistics, University of Hohenheim, 70593 Stuttgart, Germany ORCID IDs: 0000-0002-0599-6412 (T.S.); 0000-0001-5054-1348 (M.J.S.) ABSTRACT The Major Histocompatibility Complex (MHC) is the most genetically diverse region of the genome in most vertebrates. Some form of balancing selection is necessary to account for the extreme diversity, but the precise mechanism of balancing selection is unknown. Due to the way MHC molecules determine immune recognition, overdominance (also referred to as heterozygote advantage) has been suggested as the main driving force behind this unrivalled diversity. However, both theoretical results and simulation models have shown that overdominance in its classical form cannot maintain large numbers of alleles unless all alleles confer unrealistically similar levels of fitness. There is increasing evidence that heterozygotes containing genetically divergent alleles allow for broader antigen presentation to immune cells, providing a selective mechanism for MHC polymorphism. By framing competing models of overdominance within a general framework, we show that a model based on Divergent Allele Advantage (DAA) provides a superior mechanism for maintaining alleles with a wide range of intrinsic merits, as intrinsically less-fit MHC alleles that are more divergent can survive under DAA.
    [Show full text]
  • Lessons Learned from the Dog Genome
    Review TRENDS in Genetics Vol.23 No.11 Lessons learned from the dog genome Robert K. Wayne1 and Elaine A. Ostrander2 1 Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 91302, USA 2 Cancer Genetics Branch, National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892, USA Extensive genetic resources and a high-quality genome sequencing of the dog genome and related genomic sequence position the dog as an important model resources, powerful new approaches for uncovering the species for understanding genome evolution, popu- genetic basis of phenotypic variation and disease are emer- lation genetics and genes underlying complex phenoty- ging. ‘Selective sweep’ approaches are particularly prom- pic traits. Newly developed genomic resources have ising in the search for the genomic signal of positive expanded our understanding of canine evolutionary selection [5–7]. Furthermore, canine genomic resources history and dog origins. Domestication involved genetic have a wide variety of applications including systematics, contributions from multiple populations of gray wolves population genetics, endangered species conservation as probably through backcrossing. More recently, the well as gene mapping of specific phenotypic traits [4,8]. advent of controlled breeding practices has segregated genetic variability into distinct dog breeds that possess The evolutionary history of dog-like carnivores specific phenotypic traits. Consequently, genome-wide The dog family is a phenotypically diverse group including association and selective sweep scans now allow the 35 closely related extant species [9,10]. This recent radi- discovery of genes underlying breed-specific character- ation has resulted in an evolutionary branching pattern istics. The dog is finally emerging as a novel resource for that is often difficult to resolve because many branches are studying the genetic basis of complex traits, including closely spaced in time.
    [Show full text]