TY - JOUR AB - Domestication is a human‐induced selection process that imprints the genomes of domesticated populations over a short evolutionary time scale and that occurs in a given demographic context. Reconstructing historical gene flow, effective population size changes and their timing is therefore of fundamental interest to understand how plant demography and human selection jointly shape genomic divergence during domestication. Yet, the comparison under a single statistical framework of independent domestication histories across different crop species has been little evaluated so far. Thus, it is unclear whether domestication leads to convergent demographic changes that similarly affect crop genomes. To address this question, we used existing and new transcriptome data on three crop species of Solanaceae (eggplant, pepper and tomato), together with their close wild relatives. We fitted twelve demographic models of increasing complexity on the unfolded joint allele frequency spectrum for each wild/crop pair, and we found evidence for both shared and species‐specific demographic processes between species. A convergent history of domestication with gene flow was inferred for all three species, along with evidence of strong reduction in the effective population size during the cultivation stage of tomato and pepper. The absence of any reduction in size of the crop in eggplant stands out from the classical view of the domestication process; as does the existence of a “protracted period” of management before cultivation. Our results also suggest divergent management strategies of modern cultivars among species as their current demography substantially differs. Finally, the timing of domestication is species‐specific and supported by the few historical records available. AU - Arnoux, Stéphanie AU - Fraisse, Christelle AU - Sauvage, Christopher ID - 8928 IS - 2 JF - Journal of Evolutionary Biology SN - 1010061X TI - Genomic inference of complex domestication histories in three Solanaceae species VL - 34 ER - TY - JOUR AB - Marine environments are inhabited by a broad representation of the tree of life, yet our understanding of speciation in marine ecosystems is extremely limited compared with terrestrial and freshwater environments. Developing a more comprehensive picture of speciation in marine environments requires that we 'dive under the surface' by studying a wider range of taxa and ecosystems is necessary for a more comprehensive picture of speciation. Although studying marine evolutionary processes is often challenging, recent technological advances in different fields, from maritime engineering to genomics, are making it increasingly possible to study speciation of marine life forms across diverse ecosystems and taxa. Motivated by recent research in the field, including the 14 contributions in this issue, we highlight and discuss six axes of research that we think will deepen our understanding of speciation in the marine realm: (a) study a broader range of marine environments and organisms; (b) identify the reproductive barriers driving speciation between marine taxa; (c) understand the role of different genomic architectures underlying reproductive isolation; (d) infer the evolutionary history of divergence using model‐based approaches; (e) study patterns of hybridization and introgression between marine taxa; and (f) implement highly interdisciplinary, collaborative research programmes. In outlining these goals, we hope to inspire researchers to continue filling this critical knowledge gap surrounding the origins of marine biodiversity. AU - Faria, Rui AU - Johannesson, Kerstin AU - Stankowski, Sean ID - 9100 IS - 1 JF - Journal of Evolutionary Biology SN - 1010061X TI - Speciation in marine environments: Diving under the surface VL - 34 ER - TY - JOUR AB - Interspecific crossing experiments have shown that sex chromosomes play a major role in reproductive isolation between many pairs of species. However, their ability to act as reproductive barriers, which hamper interspecific genetic exchange, has rarely been evaluated quantitatively compared to Autosomes. This genome-wide limitation of gene flow is essential for understanding the complete separation of species, and thus speciation. Here, we develop a mainland-island model of secondary contact between hybridizing species of an XY (or ZW) sexual system. We obtain theoretical predictions for the frequency of introgressed alleles, and the strength of the barrier to neutral gene flow for the two types of chromosomes carrying multiple interspecific barrier loci. Theoretical predictions are obtained for scenarios where introgressed alleles are rare. We show that the same analytical expressions apply for sex chromosomes and autosomes, but with different sex-averaged effective parameters. The specific features of sex chromosomes (hemizygosity and absence of recombination in the heterogametic sex) lead to reduced levels of introgression on the X (or Z) compared to autosomes. This effect can be enhanced by certain types of sex-biased forces, but it remains overall small (except when alleles causing incompatibilities are recessive). We discuss these predictions in the light of empirical data comprising model-based tests of introgression and cline surveys in various biological systems. AU - Fraisse, Christelle AU - Sachdeva, Himani ID - 9168 IS - 2 JF - Genetics SN - 1943-2631 TI - The rates of introgression and barriers to genetic exchange between hybridizing species: Sex chromosomes vs autosomes VL - 217 ER - TY - JOUR AB - We present DILS, a deployable statistical analysis platform for conducting demographic inferences with linked selection from population genomic data using an Approximate Bayesian Computation framework. DILS takes as input single‐population or two‐population data sets (multilocus fasta sequences) and performs three types of analyses in a hierarchical manner, identifying: (a) the best demographic model to study the importance of gene flow and population size change on the genetic patterns of polymorphism and divergence, (b) the best genomic model to determine whether the effective size Ne and migration rate N, m are heterogeneously distributed along the genome (implying linked selection) and (c) loci in genomic regions most associated with barriers to gene flow. Also available via a Web interface, an objective of DILS is to facilitate collaborative research in speciation genomics. Here, we show the performance and limitations of DILS by using simulations and finally apply the method to published data on a divergence continuum composed by 28 pairs of Mytilus mussel populations/species. AU - Fraisse, Christelle AU - Popovic, Iva AU - Mazoyer, Clément AU - Spataro, Bruno AU - Delmotte, Stéphane AU - Romiguier, Jonathan AU - Loire, Étienne AU - Simon, Alexis AU - Galtier, Nicolas AU - Duret, Laurent AU - Bierne, Nicolas AU - Vekemans, Xavier AU - Roux, Camille ID - 9119 JF - Molecular Ecology Resources SN - 1755098X TI - DILS: Demographic inferences with linked selection by using ABC VL - 21 ER - TY - JOUR AB - Genetic variation segregates as linked sets of variants, or haplotypes. Haplotypes and linkage are central to genetics and underpin virtually all genetic and selection analysis. And yet, genomic data often lack haplotype information, due to constraints in sequencing technologies. Here we present “haplotagging”, a simple, low-cost linked-read sequencing technique that allows sequencing of hundreds of individuals while retaining linkage information. We apply haplotagging to construct megabase-size haplotypes for over 600 individual butterflies (Heliconius erato and H. melpomene), which form overlapping hybrid zones across an elevational gradient in Ecuador. Haplotagging identifies loci controlling distinctive high- and lowland wing color patterns. Divergent haplotypes are found at the same major loci in both species, while chromosome rearrangements show no parallelism. Remarkably, in both species the geographic clines for the major wing pattern loci are displaced by 18 km, leading to the rise of a novel hybrid morph in the centre of the hybrid zone. We propose that shared warning signalling (Müllerian mimicry) may couple the cline shifts seen in both species, and facilitate the parallel co-emergence of a novel hybrid morph in both co-mimetic species. Our results show the power of efficient haplotyping methods when combined with large-scale sequencing data from natural populations. AU - Meier, Joana I. AU - Salazar, Patricio A. AU - Kučka, Marek AU - Davies, Robert William AU - Dréau, Andreea AU - Aldás, Ismael AU - Power, Olivia Box AU - Nadeau, Nicola J. AU - Bridle, Jon R. AU - Rolian, Campbell AU - Barton, Nicholas H AU - McMillan, W. Owen AU - Jiggins, Chris D. AU - Chan, Yingguang Frank ID - 9375 IS - 25 JF - PNAS TI - Haplotype tagging reveals parallel formation of hybrid races in two butterfly species VL - 118 ER - TY - JOUR AB - Chromosomal inversions have long been recognized for their role in local adaptation. By suppressing recombination in heterozygous individuals, they can maintain coadapted gene complexes and protect them from homogenizing effects of gene flow. However, to fully understand their importance for local adaptation we need to know their influence on phenotypes under divergent selection. For this, the marine snail Littorina saxatilis provides an ideal study system. Divergent ecotypes adapted to wave action and crab predation occur in close proximity on intertidal shores with gene flow between them. Here, we used F2 individuals obtained from crosses between the ecotypes to test for associations between genomic regions and traits distinguishing the Crab‐/Wave‐adapted ecotypes including size, shape, shell thickness, and behavior. We show that most of these traits are influenced by two previously detected inversion regions that are divergent between ecotypes. We thus gain a better understanding of one important underlying mechanism responsible for the rapid and repeated formation of ecotypes: divergent selection acting on inversions. We also found that some inversions contributed to more than one trait suggesting that they may contain several loci involved in adaptation, consistent with the hypothesis that suppression of recombination within inversions facilitates differentiation in the presence of gene flow. AU - Koch, Eva L. AU - Morales, Hernán E. AU - Larsson, Jenny AU - Westram, Anja M AU - Faria, Rui AU - Lemmon, Alan R. AU - Lemmon, E. Moriarty AU - Johannesson, Kerstin AU - Butlin, Roger K. ID - 9394 IS - 3 JF - Evolution Letters TI - Genetic variation for adaptive traits is associated with polymorphic inversions in Littorina saxatilis VL - 5 ER - TY - JOUR AB - Humans conceptualize the diversity of life by classifying individuals into types we call ‘species’1. The species we recognize influence political and financial decisions and guide our understanding of how units of diversity evolve and interact. Although the idea of species may seem intuitive, a debate about the best way to define them has raged even before Darwin2. So much energy has been devoted to the so-called ‘species problem’ that no amount of discourse will ever likely solve it2,3. Dozens of species concepts are currently recognized3, but we lack a concrete understanding of how much researchers actually disagree and the factors that cause them to think differently1,2. To address this, we used a survey to quantify the species problem for the first time. The results indicate that the disagreement is extensive: two randomly chosen respondents will most likely disagree on the nature of species. The probability of disagreement is not predicted by researcher experience or broad study system, but tended to be lower among researchers with similar focus, training and who study the same organism. Should we see this diversity of perspectives as a problem? We argue that we should not. AU - Stankowski, Sean AU - Ravinet, Mark ID - 9392 IS - 9 JF - Current Biology SN - 09609822 TI - Quantifying the use of species concepts VL - 31 ER - TY - GEN AB - Chromosomal inversion polymorphisms, segments of chromosomes that are flipped in orientation and occur in reversed order in some individuals, have long been recognized to play an important role in local adaptation. They can reduce recombination in heterozygous individuals and thus help to maintain sets of locally adapted alleles. In a wide range of organisms, populations adapted to different habitats differ in frequency of inversion arrangements. However, getting a full understanding of the importance of inversions for adaptation requires confirmation of their influence on traits under divergent selection. Here, we studied a marine snail, Littorina saxatilis, that has evolved ecotypes adapted to wave exposure or crab predation. These two types occur in close proximity on different parts of the shore. Gene flow between them exists in contact zones. However, they exhibit strong phenotypic divergence in several traits under habitat-specific selection, including size, shape and behaviour. We used crosses between these ecotypes to identify genomic regions that explain variation in these traits by using QTL analysis and variance partitioning across linkage groups. We could show that previously detected inversion regions contribute to adaptive divergence. Some inversions influenced multiple traits suggesting that they contain sets of locally adaptive alleles. Our study also identified regions without known inversions that are important for phenotypic divergence. Thus, we provide a more complete overview of the importance of inversions in relation to the remaining genome. AU - Koch, Eva AU - Morales, Hernán E. AU - Larsson, Jenny AU - Westram, Anja M AU - Faria, Rui AU - Lemmon, Alan R. AU - Lemmon, E. Moriarty AU - Johannesson, Kerstin AU - Butlin, Roger K. ID - 12987 TI - Data from: Genetic variation for adaptive traits is associated with polymorphic inversions in Littorina saxatilis ER - TY - JOUR AB - Antibiotic concentrations vary dramatically in the body and the environment. Hence, understanding the dynamics of resistance evolution along antibiotic concentration gradients is critical for predicting and slowing the emergence and spread of resistance. While it has been shown that increasing the concentration of an antibiotic slows resistance evolution, how adaptation to one antibiotic concentration correlates with fitness at other points along the gradient has not received much attention. Here, we selected populations of Escherichia coli at several points along a concentration gradient for three different antibiotics, asking how rapidly resistance evolved and whether populations became specialized to the antibiotic concentration they were selected on. Populations selected at higher concentrations evolved resistance more slowly but exhibited equal or higher fitness across the whole gradient. Populations selected at lower concentrations evolved resistance rapidly, but overall fitness in the presence of antibiotics was lower. However, these populations readily adapted to higher concentrations upon subsequent selection. Our results indicate that resistance management strategies must account not only for the rates of resistance evolution but also for the fitness of evolved strains. AU - Lagator, Mato AU - Uecker, Hildegard AU - Neve, Paul ID - 9410 IS - 5 JF - Biology letters TI - Adaptation at different points along antibiotic concentration gradients VL - 17 ER - TY - JOUR AB - A key step in understanding the genetic basis of different evolutionary outcomes (e.g., adaptation) is to determine the roles played by different mutation types (e.g., SNPs, translocations and inversions). To do this we must simultaneously consider different mutation types in an evolutionary framework. Here, we propose a research framework that directly utilizes the most important characteristics of mutations, their population genetic effects, to determine their relative evolutionary significance in a given scenario. We review known population genetic effects of different mutation types and show how these may be connected to different evolutionary outcomes. We provide examples of how to implement this framework and pinpoint areas where more data, theory and synthesis are needed. Linking experimental and theoretical approaches to examine different mutation types simultaneously is a critical step towards understanding their evolutionary significance. AU - Berdan, Emma L. AU - Blanckaert, Alexandre AU - Slotte, Tanja AU - Suh, Alexander AU - Westram, Anja M AU - Fragata, Inês ID - 9470 IS - 12 JF - Molecular Ecology SN - 09621083 TI - Unboxing mutations: Connecting mutation types with evolutionary consequences VL - 30 ER - TY - JOUR AB - Aims: Mass antigen testing programs have been challenged because of an alleged insufficient specificity, leading to a large number of false positives. The objective of this study is to derive a lower bound of the specificity of the SD Biosensor Standard Q Ag-Test in large scale practical use. Methods: Based on county data from the nationwide tests for SARS-CoV-2 in Slovakia between 31.10.–1.11. 2020 we calculate a lower confidence bound for the specificity. As positive test results were not systematically verified by PCR tests, we base the lower bound on a worst case assumption, assuming all positives to be false positives. Results: 3,625,332 persons from 79 counties were tested. The lowest positivity rate was observed in the county of Rožňava where 100 out of 34307 (0.29%) tests were positive. This implies a test specificity of at least 99.6% (97.5% one-sided lower confidence bound, adjusted for multiplicity). Conclusion: The obtained lower bound suggests a higher specificity compared to earlier studies in spite of the underlying worst case assumption and the application in a mass testing setting. The actual specificity is expected to exceed 99.6% if the prevalence in the respective regions was non-negligible at the time of testing. To our knowledge, this estimate constitutes the first bound obtained from large scale practical use of an antigen test. AU - Hledik, Michal AU - Polechova, Jitka AU - Beiglböck, Mathias AU - Herdina, Anna Nele AU - Strassl, Robert AU - Posch, Martin ID - 9816 IS - 7 JF - PLoS ONE TI - Analysis of the specificity of a COVID-19 antigen test in the Slovak mass testing program VL - 16 ER - TY - JOUR AB - This paper analyses the conditions for local adaptation in a metapopulation with infinitely many islands under a model of hard selection, where population size depends on local fitness. Each island belongs to one of two distinct ecological niches or habitats. Fitness is influenced by an additive trait which is under habitat‐dependent directional selection. Our analysis is based on the diffusion approximation and accounts for both genetic drift and demographic stochasticity. By neglecting linkage disequilibria, it yields the joint distribution of allele frequencies and population size on each island. We find that under hard selection, the conditions for local adaptation in a rare habitat are more restrictive for more polygenic traits: even moderate migration load per locus at very many loci is sufficient for population sizes to decline. This further reduces the efficacy of selection at individual loci due to increased drift and because smaller populations are more prone to swamping due to migration, causing a positive feedback between increasing maladaptation and declining population sizes. Our analysis also highlights the importance of demographic stochasticity, which exacerbates the decline in numbers of maladapted populations, leading to population collapse in the rare habitat at significantly lower migration than predicted by deterministic arguments. AU - Szep, Eniko AU - Sachdeva, Himani AU - Barton, Nicholas H ID - 9252 IS - 5 JF - Evolution KW - Genetics KW - Ecology KW - Evolution KW - Behavior and Systematics KW - General Agricultural and Biological Sciences SN - 0014-3820 TI - Polygenic local adaptation in metapopulations: A stochastic eco‐evolutionary model VL - 75 ER - TY - JOUR AB - If there are no constraints on the process of speciation, then the number of species might be expected to match the number of available niches and this number might be indefinitely large. One possible constraint is the opportunity for allopatric divergence. In 1981, Felsenstein used a simple and elegant model to ask if there might also be genetic constraints. He showed that progress towards speciation could be described by the build‐up of linkage disequilibrium among divergently selected loci and between these loci and those contributing to other forms of reproductive isolation. Therefore, speciation is opposed by recombination, because it tends to break down linkage disequilibria. Felsenstein then introduced a crucial distinction between “two‐allele” models, which are subject to this effect, and “one‐allele” models, which are free from the recombination constraint. These fundamentally important insights have been the foundation for both empirical and theoretical studies of speciation ever since. AU - Butlin, Roger K. AU - Servedio, Maria R. AU - Smadja, Carole M. AU - Bank, Claudia AU - Barton, Nicholas H AU - Flaxman, Samuel M. AU - Giraud, Tatiana AU - Hopkins, Robin AU - Larson, Erica L. AU - Maan, Martine E. AU - Meier, Joana AU - Merrill, Richard AU - Noor, Mohamed A. F. AU - Ortiz‐Barrientos, Daniel AU - Qvarnström, Anna ID - 9374 IS - 5 JF - Evolution KW - Genetics KW - Ecology KW - Evolution KW - Behavior and Systematics KW - General Agricultural and Biological Sciences SN - 0014-3820 TI - Homage to Felsenstein 1981, or why are there so few/many species? VL - 75 ER - TY - GEN AB - This paper analyzes the conditions for local adaptation in a metapopulation with infinitely many islands under a model of hard selection, where population size depends on local fitness. Each island belongs to one of two distinct ecological niches or habitats. Fitness is influenced by an additive trait which is under habitat-dependent directional selection. Our analysis is based on the diffusion approximation and accounts for both genetic drift and demographic stochasticity. By neglecting linkage disequilibria, it yields the joint distribution of allele frequencies and population size on each island. We find that under hard selection, the conditions for local adaptation in a rare habitat are more restrictive for more polygenic traits: even moderate migration load per locus at very many loci is sufficient for population sizes to decline. This further reduces the efficacy of selection at individual loci due to increased drift and because smaller populations are more prone to swamping due to migration, causing a positive feedback between increasing maladaptation and declining population sizes. Our analysis also highlights the importance of demographic stochasticity, which exacerbates the decline in numbers of maladapted populations, leading to population collapse in the rare habitat at significantly lower migration than predicted by deterministic arguments. AU - Szep, Eniko AU - Sachdeva, Himani AU - Barton, Nicholas H ID - 13062 TI - Supplementary code for: Polygenic local adaptation in metapopulations: A stochastic eco-evolutionary model ER - TY - JOUR AB - A primary roadblock to our understanding of speciation is that it usually occurs over a timeframe that is too long to study from start to finish. The idea of a speciation continuum provides something of a solution to this problem; rather than observing the entire process, we can simply reconstruct it from the multitude of speciation events that surround us. But what do we really mean when we talk about the speciation continuum, and can it really help us understand speciation? We explored these questions using a literature review and online survey of speciation researchers. Although most researchers were familiar with the concept and thought it was useful, our survey revealed extensive disagreement about what the speciation continuum actually tells us. This is due partly to the lack of a clear definition. Here, we provide an explicit definition that is compatible with the Biological Species Concept. That is, the speciation continuum is a continuum of reproductive isolation. After outlining the logic of the definition in light of alternatives, we explain why attempts to reconstruct the speciation process from present‐day populations will ultimately fail. We then outline how we think the speciation continuum concept can continue to act as a foundation for understanding the continuum of reproductive isolation that surrounds us. AU - Stankowski, Sean AU - Ravinet, Mark ID - 9383 IS - 6 JF - Evolution SN - 0014-3820 TI - Defining the speciation continuum VL - 75 ER - TY - CHAP AB - Hybrid zones are narrow geographic regions where different populations, races or interbreeding species meet and mate, producing mixed ‘hybrid’ offspring. They are relatively common and can be found in a diverse range of organisms and environments. The study of hybrid zones has played an important role in our understanding of the origin of species, with hybrid zones having been described as ‘natural laboratories’. This is because they allow us to study,in situ, the conditions and evolutionary forces that enable divergent taxa to remain distinct despite some ongoing gene exchange between them. AU - Stankowski, Sean AU - Shipilina, Daria AU - Westram, Anja M ID - 14984 SN - 9780470016176 T2 - Encyclopedia of Life Sciences TI - Hybrid Zones VL - 2 ER - TY - DATA AB - Here are the research data underlying the publication " Effects of fine-scale population structure on inbreeding in a long-term study of snapdragons (Antirrhinum majus)." Further information are summed up in the README document. AU - Surendranadh, Parvathy AU - Arathoon, Louise S AU - Baskett, Carina AU - Field, David AU - Pickup, Melinda AU - Barton, Nicholas H ID - 9192 TI - Effects of fine-scale population structure on the distribution of heterozygosity in a long-term study of Antirrhinum majus ER - TY - JOUR AB - The growth of snail shells can be described by simple mathematical rules. Variation in a few parameters can explain much of the diversity of shell shapes seen in nature. However, empirical studies of gastropod shell shape variation typically use geometric morphometric approaches, which do not capture this growth pattern. We have developed a way to infer a set of developmentally descriptive shape parameters based on three-dimensional logarithmic helicospiral growth and using landmarks from two-dimensional shell images as input. We demonstrate the utility of this approach, and compare it to the geometric morphometric approach, using a large set of Littorina saxatilis shells in which locally adapted populations differ in shape. Our method can be modified easily to make it applicable to a wide range of shell forms, which would allow for investigations of the similarities and differences between and within many different species of gastropods. AU - Larsson, J. AU - Westram, Anja M AU - Bengmark, S. AU - Lundh, T. AU - Butlin, R. K. ID - 7651 IS - 163 JF - Journal of The Royal Society Interface SN - 1742-5689 TI - A developmentally descriptive method for quantifying shape in gastropod shells VL - 17 ER - TY - CHAP AB - Inversions are chromosomal rearrangements where the order of genes is reversed. Inversions originate by mutation and can be under positive, negative or balancing selection. Selective effects result from potential disruptive effects on meiosis, gene disruption at inversion breakpoints and, importantly, the effects of inversions as modifiers of recombination rate: Recombination is strongly reduced in individuals heterozygous for an inversion, allowing for alleles at different loci to be inherited as a ‘block’. This may lead to a selective advantage whenever it is favourable to keep certain combinations of alleles associated, for example under local adaptation with gene flow. Inversions can cover a considerable part of a chromosome and contain numerous loci under different selection pressures, so that the resulting overall effects may be complex. Empirical data from various systems show that inversions may have a prominent role in local adaptation, speciation, parallel evolution, the maintenance of polymorphism and sex chromosome evolution. AU - Westram, Anja M AU - Faria, Rui AU - Butlin, Roger AU - Johannesson, Kerstin ID - 9123 SN - 9780470016176 T2 - eLS TI - Inversions and Evolution ER - TY - GEN AB - The Mytilus complex of marine mussel species forms a mosaic of hybrid zones, found across temperate regions of the globe. This allows us to study "replicated" instances of secondary contact between closely-related species. Previous work on this complex has shown that local introgression is both widespread and highly heterogeneous, and has identified SNPs that are outliers of differentiation between lineages. Here, we developed an ancestry-informative panel of such SNPs. We then compared their frequencies in newly-sampled populations, including samples from within the hybrid zones, and parental populations at different distances from the contact. Results show that close to the hybrid zones, some outlier loci are near to fixation for the heterospecific allele, suggesting enhanced local introgression, or the local sweep of a shared ancestral allele. Conversely, genomic cline analyses, treating local parental populations as the reference, reveal a globally high concordance among loci, albeit with a few signals of asymmetric introgression. Enhanced local introgression at specific loci is consistent with the early transfer of adaptive variants after contact, possibly including asymmetric bi-stable variants (Dobzhansky-Muller incompatibilities), or haplotypes loaded with fewer deleterious mutations. Having escaped one barrier, however, these variants can be trapped or delayed at the next barrier, confining the introgression locally. These results shed light on the decay of species barriers during phases of contact. AU - Simon, Alexis AU - Fraisse, Christelle AU - El Ayari, Tahani AU - Liautard-Haag, Cathy AU - Strelkov, Petr AU - Welch, John AU - Bierne, Nicolas ID - 13073 TI - How do species barriers decay? concordance and local introgression in mosaic hybrid zones of mussels ER -