Lifestyle Transitions in Fusarioid Fungi are Frequent and Lack Clear Genomic Signatures.

The fungal genus Fusarium (Ascomycota) includes well-known plant pathogens that are implicated in diseases worldwide, and many of which have been genome sequenced. The genus also encompasses other diverse lifestyles, including species found ubiquitously as asymptomatic-plant inhabitants (endophytes). Here, we produced structurally annotated genome assemblies for five endophytic Fusarium strains, including the first whole-genome data for Fusarium chuoi. Phylogenomic reconstruction of Fusarium and closely related genera revealed multiple and frequent lifestyle transitions, the major exception being a monophyletic clade of mutualist insect symbionts. Differential codon usage bias and increased codon optimisation separated Fusarium sensu stricto from allied genera. We performed computational prediction of candidate secreted effector proteins (CSEPs) and carbohydrate-active enzymes (CAZymes)-both likely to be involved in the host-fungal interaction-and sought evidence that their frequencies could predict lifestyle. However, phylogenetic distance described gene variance better than lifestyle did. There was no significant difference in CSEP, CAZyme, or gene repertoires between phytopathogenic and endophytic strains, although we did find some evidence that gene copy number variation may be contributing to pathogenicity. Large numbers of accessory CSEPs (i.e., present in more than one taxon but not all) and a comparatively low number of strain-specific CSEPs suggested there is a limited specialisation among plant associated Fusarium species. We also found half of the core genes to be under positive selection and identified specific CSEPs and CAZymes predicted to be positively selected on certain lineages. Our results depict fusarioid fungi as prolific generalists and highlight the difficulty in predicting pathogenic potential in the group.

Genomic signals of local adaptation and hybridization in Asian white birch.

Disentangling the numerous processes that affect patterns of genome-wide diversity in widespread tree species has important implications for taxonomy, conservation, and forestry. Here, we investigate the population genomic structure of Asian white birch (Betula platyphylla) in China and seek to explain it in terms of hybridization, demography and adaptation. We generate whole genome sequence data from 83 individuals across the species range in China. Combining this with an existing data set for 79 European and Russian white birches, we show a clear distinction between B. pendula and B. platyphylla, which have sometimes been lumped taxonomically. Genomic diversity of B. platyphylla in north-western China and Central Russia is affected greatly by hybridization with B. pendula. Excluding these hybridized populations, B. platyphylla in China has a linear distribution from north-eastern to south-western China, along the edge of the inland mountainous region. Within this distribution, three genetic clusters are found, which we model as long diverged with subsequent episodes of gene flow. Patterns of covariation between allele frequencies and environmental variables in B. platyphylla suggest the role of natural selection in the distribution of diversity at 7609 SNPs of which 3767 were significantly differentiated among the genetic clusters. The putative adaptive SNPs are distributed throughout the genome and span 1633 genic regions. Of these genic regions, 87 were previously identified as candidates for selective sweeps in Eurasian B. pendula. We use the 7609 environmentally associated SNPs to estimate the risk of nonadaptedness for each sequenced B. platyphylla individual under a scenario of future climate change, highlighting areas where populations may be under future threat from rising temperatures.

Maintenance and expansion of genetic and trait variation following domestication in a clonal crop.

Clonal propagation enables favourable crop genotypes to be rapidly selected and multiplied. However, the absence of sexual propagation can lead to low genetic diversity and accumulation of deleterious mutations, which may eventually render crops less resilient to pathogens or environmental change. To better understand this trade-off, we characterize the domestication and contemporary genetic diversity of Enset (Ensete ventricosum), an indigenous African relative of bananas (Musa) and a principal starch staple for 20 million Ethiopians. Wild enset reproduction occurs strictly by sexual outcrossing, but for cultivation, it is propagated clonally and associated with diversification and specialization into hundreds of named landraces. We applied tGBS sequencing to generate genome-wide genotypes for 192 accessions from across enset's cultivated distribution, and surveyed 1340 farmers on enset agronomic traits. Overall, reduced heterozygosity in the domesticated lineage was consistent with a domestication bottleneck that retained 37% of wild diversity. However, an excess of putatively deleterious missense mutations at low frequency present as heterozygotes suggested an accumulation of mutational load in clonal domesticated lineages. Our evidence indicates that the major domesticated lineages initially arose through historic sexual recombination associated with a domestication bottleneck, followed by the amplification of favourable genotypes through an extended period of clonal propagation. Among domesticated lineages, we found a significant phylogenetic signal for multiple farmer-identified food, nutrition and disease resistance traits and little evidence of contemporary recombination. The development of future-climate adapted genotypes may require crop breeding, but outcrossing risks exposing deleterious alleles as homozygotes. This trade-off may partly explain the ubiquity and persistence of clonal propagation over recent centuries of comparative climate stability.

Genome sequence and genetic diversity of European ash trees.

Ash trees (genus Fraxinus, family Oleaceae) are widespread throughout the Northern Hemisphere, but are being devastated in Europe by the fungus Hymenoscyphus fraxineus, causing ash dieback, and in North America by the herbivorous beetle Agrilus planipennis. Here we sequence the genome of a low-heterozygosity Fraxinus excelsior tree from Gloucestershire, UK, annotating 38,852 protein-coding genes of which 25% appear ash specific when compared with the genomes of ten other plant species. Analyses of paralogous genes suggest a whole-genome duplication shared with olive (Olea europaea, Oleaceae). We also re-sequence 37 F. excelsior trees from Europe, finding evidence for apparent long-term decline in effective population size. Using our reference sequence, we re-analyse association transcriptomic data, yielding improved markers for reduced susceptibility to ash dieback. Surveys of these markers in British populations suggest that reduced susceptibility to ash dieback may be more widespread in Great Britain than in Denmark. We also present evidence that susceptibility of trees to H. fraxineus is associated with their iridoid glycoside levels. This rapid, integrated, multidisciplinary research response to an emerging health threat in a non-model organism opens the way for mitigation of the epidemic.

A chromosome-scale genome assembly of European hazel (Corylus avellana L.) reveals targets for crop improvement.

The European hazelnut (Corylus avellana L.) is a tree crop of economic importance worldwide, but especially for northern Turkey, where the majority of production takes place. Hazelnut production is currently challenged by environmental stresses, such as a recent outbreak of severe powdery mildew disease; furthermore, allergy to hazelnuts is an increasing health concern in some regions. In order to provide a foundation for using the available hazelnut genetic resources for crop improvement, we produced a fully assembled genome sequence and annotation for a hazelnut species, from C. avellana cv. 'Tombul', one of the most important Turkish varieties. A hybrid sequencing strategy, combining short reads, long reads and proximity ligation methods, enabled us to resolve heterozygous regions and produce a high-quality 370-Mb assembly that agrees closely with cytogenetic studies and genetic maps of the 11 C. avellana chromosomes, and covers 97.8% of the estimated genome size. The genome includes 27 270 high-confidence protein-coding genes, over 20 000 of which were functionally annotated based on homology with known plant proteins. We focused particularly on gene families encoding hazelnut allergens, and the Mildew resistance Locus O (MLO) proteins that are an important susceptibility factor for powdery mildew. The complete assembly enabled us to differentiate between members of these families and to identify homologues that may be important in mildew disease and hazelnut allergy. These findings provide examples of how the genome can be used to guide research and to develop effective strategies for crop improvement in C. avellana.

The challenge of demonstrating contemporary natural selection on polygenic quantitative traits in the wild.

In a From the Cover article in this issue of Molecular Ecology, Ashraf et al. (2022) apply genomic prediction methods, devised by breeders to inform artificial selection, to understand the genetic component of variation in highly polygenic quantitative traits in Soay sheep (Figure 1). These methods have allowed them to investigate the effects of contemporary natural selection on genetic variation underlying these traits in the wild (Hunter et al., 2022). Genomic prediction approaches promise to enhance our understanding of the evolution of highly polygenic quantitative traits in the wild and may allow us to document concrete examples of their natural selection in real time in systems that would otherwise be intractable.

Resolving phylogeny and polyploid parentage using genus-wide genome-wide sequence data from birch trees.

Numerous plant genera have a history including frequent hybridisation and polyploidisation (allopolyploidisation), which means that their phylogeny is a network of reticulate evolution that cannot be accurately depicted as a bifurcating tree with a single tip per species. The genus Betula, which contains many ecologically important tree species, is a case in point. We generated genome-wide sequence reads for 27 diploid and 36 polyploid Betula species or subspecies using restriction site associated DNA (RAD) sequences. These reads were assembled into contigs with a mean length of 675 bp. We reconstructed the evolutionary relationships among diploid Betula species using both supermatrix (concatenation) and species tree methods. We identified the closest diploid relatives of the polyploids according to the relative rates at which reads from polyploids mapped to contigs from different diploid species within a concatenated reference sequence. By mapping reads from allopolyploids to their different putative diploid relatives we assembled contigs from the putative sub-genomes of allopolyploid taxa. We used these to build new phylogenies that included allopolyploid sub-genomes as separate tips. This approach yielded a highly evidenced phylogenetic hypothesis for the genus Betula, including the complex reticulate origins of the majority of its polyploid taxa. Our phylogeny divides the genus into two well supported clades, which, interestingly, differ in their seed-wing morphology. We therefore propose to split Betula into two subgenera.

The origin of Darwin's "abominable mystery".

The phrase "Darwin's abominable mystery" is frequently used with reference to a range of outstanding questions about the evolution of the plant group today known as the angiosperms. Here, I seek to more fully understand what prompted Darwin to coin the phrase in 1879, and the meaning he attached to it, by surveying the systematics, paleobotanical records, and phylogenetic hypotheses of his time. In the light of this historical research, I argue that Darwin was referring to the origin only of a subset of what are today called angiosperms: a (now obsolete) group equivalent to the "dicotyledons" of the Hooker and Bentham system. To Darwin and his contemporaries, the dicotyledons' fossil record began abruptly and with great diversity in the Cretaceous, whereas the gymnosperms and monocotyledons were thought to have fossil records dating back to the Carboniferous or beyond. Based on their morphology, the dicotyledons were widely seen by botanists in Darwin's time (unlike today) as more similar to the gymnosperms than to the monocotyledons. Thus, morphology seemed to point to gymnosperm progenitors of dicotyledons, but this hypothesis made the monocotyledons, given their (at the time) apparently longer fossil record, difficult to place. Darwin had friendly disagreements about the mystery of the dicotyledons' abrupt appearance in the fossil record with others who thought that their evolution must have been more rapid than his own gradualism would allow. But the mystery may have been made "abominable" to him because it was seen by some contemporary paleobotanists, most notably William Carruthers, the Keeper of Botany at the British Museum, as evidence for divine intervention in the history of life. Subsequent developments in plant systematics and paleobotany after 1879 meant that Darwin's letter was widely understood to be referring to the abrupt appearance of all angiosperms when it was published in 1903, a meaning that has been attached to it ever since.

Population structure of Betula albosinensis and Betula platyphylla: evidence for hybridization and a cryptic lineage.

BACKGROUND AND AIMS: Differences in local abundance and ploidy level are predicted to impact the direction of introgression between species. Here, we tested these hypotheses on populations of Betula albosinensis (red birch) and Betula platyphylla (white birch) which were thought to differ in ploidy level, the former being tetraploid and the latter diploid. METHODS: We sampled 391 birch individuals from nine localities in China, and classified them into species based on leaf morphology. Twelve nuclear microsatellite markers were genotyped in each sample, and analysed using principal coordinates analysis and STRUCTURE software. We compared the effects of two different methods of scoring polyploid genotypes on population genetic analyses. We analysed the effect of ploidy, local species abundance and latitude on levels of introgression between the species. KEY RESULTS: Leaf morphology divided our samples into red and white birch, but genetic analyses unexpectedly revealed two groups within red birch, one of which was tetraploid, as expected, but the other of which appeared to have diploid microsatellite genotypes. Five individuals were identified as early-generation hybrids or backcrosses between white birch and red birch and five were identified between red birch and 'diploid' red birch. Cline analysis showed that levels of admixture were not significantly correlated with latitude. Estimated genetic differentiation among species was not significantly different between determined tetraploid and undetermined tetraploid genotypes. CONCLUSIONS: Limited hybridization and gene flow have occurred between red birch and white birch. Relative species abundance and ploidy level do not impact the direction of introgression between them, as genetic admixture is roughly symmetrical. We unexpectedly found populations of apparently diploid red birch and this taxon may be a progenitor of allotetraploid red birch populations. Incomplete lineage sorting may explain patterns of genetic admixture between apparently diploid and allotetraploid red birch.

Genome-wide epigenetic variation among ash trees differing in susceptibility to a fungal disease.

BACKGROUND: European ash trees (Fraxinus excelsior) are currently threatened by ash dieback (ADB) caused by the fungus Hymenoscyphus fraxineus but a small percentage of the population possesses natural low susceptibility. The genome of a European ash tree has recently been sequenced. Here, we present whole genome DNA methylation data for two F. excelsior genotypes with high susceptibility to ADB, and two genotypes with low susceptibility, each clonally replicated. We also include two genotypes of Manchurian ash (F. mandshurica), an ash species which has co-evolved with H. fraxineus and also has low susceptibility to ADB. RESULTS: In F. excelsior, we find an average methylation level of 76.2% in the CG context, 52.0% in the CHG context, and 13.9% in the CHH context; similar levels to those of tomato. We find higher methylation in transposable elements as opposed to non-mobile elements, and high densities of Non-Differentially Methylation Positions (N-DMPs) in genes with housekeeping functions. Of genes putatively duplicated in whole genome duplication (WGD) events, an average of 25.9% are differentially methylated in at least one cytosine context, potentially indicative of unequal silencing. Variability in methylation patterns exists among clonal replicates, and this is only slightly less than the variability found between different genotypes. Of twenty genes previously found to have expression levels associated with ADB susceptibility, we find only two of these have differential methylation between high and low susceptibility F. excelsior trees. In addition, we identify 1683 significant Differentially Methylated Regions (DMRs) (q-value< 0.001) between the high and low susceptibility genotypes of F. excelsior trees, of which 665 remain significant when F. mandshurica samples are added to the low susceptibility group. CONCLUSIONS: We find a higher frequency of differentially methylated WGD-derived gene duplicates in ash than other plant species previously studied. We also identify a set of genes with differential methylation between genotypes and species with high versus low susceptibility to ADB. This provides valuable foundational data for future work on the role that epigenetics may play in gene dosage compensation and susceptibility to ADB in ash.

Genetic diversity maintained among fragmented populations of a tree undergoing range contraction.

Dwarf birch (Betula nana) has a widespread boreal distribution but has declined significantly in Britain where populations are now highly fragmented. We analyzed the genetic diversity of these fragmented populations using markers that differ in mutation rate: conventional microsatellites markers (PCR-SSRs), RADseq generated transition and transversion SNPs (RAD-SNPs), and microsatellite markers mined from RADseq reads (RAD-SSRs). We estimated the current population sizes by census and indirectly, from the linkage-disequilibrium found in the genetic surveys. The two types of estimate were highly correlated. Overall, we found genetic diversity to be only slightly lower in Britain than across a comparable area in Scandinavia where populations are large and continuous. While the ensemble of British fragments maintain diversity levels close to Scandinavian populations, individually they have drifted apart and lost diversity; particularly the smaller populations. An ABC analysis, based on coalescent models, favors demographic scenarios in which Britain maintained high levels of genetic diversity through post-glacial re-colonization. This diversity has subsequently been partitioned into population fragments that have recently lost diversity at a rate corresponding to the current population-size estimates. We conclude that the British population fragments retain sufficient genetic resources to be the basis of conservation and re-planting programmes. Use of markers with different mutation rates gives us greater confidence and insight than one marker set could have alone, and we suggest that RAD-SSRs are particularly useful as high mutation-rate marker set with a well-specified ascertainment bias, which are widely available yet often neglected in existing RAD datasets.

Unidirectional diploid-tetraploid introgression among British birch trees with shifting ranges shown by restriction site-associated markers.

Hybridization may lead to introgression of genes among species. Introgression may be bidirectional or unidirectional, depending on factors such as the demography of the hybridizing species, or the nature of reproductive barriers between them. Previous microsatellite studies suggested bidirectional introgression between diploid Betula nana (dwarf birch) and tetraploid B. pubescens (downy birch) and also between B. pubescens and diploid B. pendula (silver birch) in Britain. Here, we analyse introgression among these species using 51 237 variants in restriction site-associated (RAD) markers in 194 individuals, called with allele dosages in the tetraploids. In contrast to the microsatellite study, we found unidirectional introgression into B. pubescens from both of the diploid species. This pattern fits better with the expected nature of the reproductive barrier between diploids and tetraploids. As in the microsatellite study, introgression into B. pubescens showed clear clines with increasing introgression from B. nana in the north and from B. pendula in the south. Unlike B. pendula alleles, introgression of B. nana alleles was found far from the current area of sympatry or allopatry between B. nana and B. pubescens. This pattern fits a shifting zone of hybridization due to Holocene reduction in the range of B. nana and expansion in the range of B. pubescens.

Molecular phylogeny and genome size evolution of the genus Betula (Betulaceae).

BACKGROUND AND AIMS: Betula L. (birch) is a genus of approx. 60 species, subspecies or varieties with a wide distribution in the northern hemisphere, of ecological and economic importance. A new classification of Betula has recently been proposed based on morphological characters. This classification differs somewhat from previously published molecular phylogenies, which may be due to factors such as convergent evolution, hybridization, incomplete taxon sampling or misidentification of samples. While chromosome counts have been made for many species, few have had their genome size measured. The aim of this study is to produce a new phylogenetic and genome size analysis of the genus. METHODS: Internal transcribed spacer (ITS) regions of nuclear ribosomal DNA were sequenced for 76 Betula samples verified by taxonomic experts, representing approx. 60 taxa, of which approx. 24 taxa have not been included in previous phylogenetic analyses. A further 49 samples from other collections were also sequenced, and 108 ITS sequences were downloaded from GenBank. Phylogenetic trees were built for these sequences. The genome sizes of 103 accessions representing nearly all described species were estimated using flow cytometry. KEY RESULTS: As expected for a gene tree of a genus where hybridization and allopolyploidy occur, the ITS tree shows clustering, but not resolved monophyly, for the morphological subgenera recently proposed. Most sections show some clustering, but species of the dwarf section Apterocaryon are unusually scattered. Betula corylifolia (subgenus Nipponobetula) unexpectedly clusters with species of subgenus Aspera Unexpected placements are also found for B. maximowicziana, B. bomiensis, B. nigra and B. grossa Biogeographical disjunctions were found within Betula between Europe and North America, and also disjunctions between North-east and South-west Asia. The 2C-values for Betula ranged from 0·88 to 5·33 pg, and polyploids are scattered widely throughout the ITS phylogeny. Species with large genomes tend to have narrow ranges. CONCLUSIONS: Betula grossa may have formed via allopolyploidization between parents in subgenus Betula and subgenus Aspera. Betula bomiensis may also be a wide allopolyploid. Betula corylifolia may be a parental species of allopolyploids in the subsection Chinenses Placements of B. maximowicziana, B. michauxii and B. nigra need further investigation. This analysis, in line with previous studies, suggests that section Apterocaryon is not monophyletic and thus dwarfism has evolved repeatedly in different lineages of Betula Polyploidization has occurred many times independently in the evolution of Betula.

Molecular footprints of the Holocene retreat of dwarf birch in Britain.

Past reproductive interactions among incompletely isolated species may leave behind a trail of introgressed alleles, shedding light on historical range movements. Betula pubescens is a widespread native tetraploid tree species in Britain, occupying habitats intermediate to those of its native diploid relatives, B. pendula and B. nana. Genotyping 1134 trees from the three species at 12 microsatellite loci, we found evidence of introgression from both diploid species into B. pubescens, despite the ploidy difference. Surprisingly, introgression from B. nana, a dwarf species whose present range is highly restricted in northern, high-altitude peat bogs, was greater than introgression from B. pendula, which is morphologically similar to B. pubescens and has a substantially overlapping range. A cline of introgression from B. nana was found extending into B. pubescens populations far to the south of the current B. nana range. We suggest that this genetic pattern is a footprint of a historical decline and/or northwards shift in the range of B. nana populations due to climate warming in the Holocene. This is consistent with pollen records that show a broader, more southerly distribution of B. nana in the past. Ecological niche modelling predicts that B. nana is adapted to a larger range than it currently occupies, suggesting additional factors such as grazing and hybridization may have exacerbated its decline. We found very little introgression between B. nana and B. pendula, despite both being diploid, perhaps because their distributions in the past have rarely overlapped. Future conservation of B. nana may partly depend on minimization of hybridization with B. pubescens, and avoidance of planting B. pendula near B. nana populations.

Genome sequence of dwarf birch (Betula nana) and cross-species RAD markers.

New sequencing technologies allow development of genome-wide markers for any genus of ecological interest, including plant genera such as Betula (birch) that have previously proved difficult to study due to widespread polyploidy and hybridization. We present a de novo reference genome sequence assembly, from 66× short read coverage, of Betula nana (dwarf birch) - a diploid that is the keystone woody species of subarctic scrub communities but of conservation concern in Britain. We also present 100 bp PstI RAD markers for B. nana and closely related Betula tree species. Assembly of RAD markers in 15 individuals by alignment to the reference B. nana genome yielded 44-86k RAD loci per individual, whereas de novo RAD assembly yielded 64-121k loci per individual. Of the loci assembled by the de novo method, 3k homologous loci were found in all 15 individuals studied, and 35k in 10 or more individuals. Matching of RAD loci to RAD locus catalogues from the B. nana individual used for the reference genome showed similar numbers of matches from both methods of RAD locus assembly but indicated that the de novo RAD assembly method may overassemble some paralogous loci. In 12 individuals hetero-specific to B. nana 37-47k RAD loci matched a catalogue of RAD loci from the B. nana individual used for the reference genome, whereas 44-60k RAD loci aligned to the B. nana reference genome itself. We present a preliminary study of allele sharing among species, demonstrating the utility of the data for introgression studies and for the identification of species-specific alleles.

Tapping Culture Collections for Fungal Endophytes: First Genome Assemblies for Three Genera and Five Species in the Ascomycota.

The Ascomycota form the largest phylum in the fungal kingdom and show a wide diversity of lifestyles, some involving associations with plants. Genomic data are available for many ascomycetes that are pathogenic to plants, but endophytes, which are asymptomatic inhabitants of plants, are relatively understudied. Here, using short- and long-read technologies, we have sequenced and assembled genomes for 15 endophytic ascomycete strains from CABI's culture collections. We used phylogenetic analysis to refine the classification of taxa, which revealed that 7 of our 15 genome assemblies are the first for the genus and/or species. We also demonstrated that cytometric genome size estimates can act as a valuable metric for assessing assembly "completeness", which can easily be overestimated when using BUSCOs alone and has broader implications for genome assembly initiatives. In producing these new genome resources, we emphasise the value of mining existing culture collections to produce data that can help to address major research questions relating to plant-fungal interactions.

Monkeying around with ploidy.

Inferences of whole genome duplication (WGD) events accompany the annotation of every newly sequenced plant genome, but much remains unknown about the evolutionary processes and pathways relating to WGD (Soltis et al. 2010). What ecological, biogeographical and genetic factors cause WGD to occur in nature? How does WGD affect gene expression? How do genomes evolve after WGD? New species that have arisen recently through WGD are good places to seek answers to such questions. These could be relatively common in nature, but reliably demonstrating their recent origin requires documentary evidence, which can be very hard to come by. Thus far, records of species introductions and meticulous botanizing have demonstrated six new natural allopolyploids in just four genera: Tragopogon miscellus and T. mirus, Senecio cambrensis and S. eboracensis, Spartina anglica and Cardamine schultzii (Abbott & Rieseberg 2012; Ainouche et al. 2009; Soltis & Soltis 2009). It is risky to generalize about a universal feature of plant evolution from such a small sample; more examples are needed, in different genera. It is therefore of considerable interest that Mario Vallejo-Marin of University of Stirling has this year named a new allopolyploid species of monkey flower, Mimulus peregrinus, and presented evidence that it is <140 years old (Vallejo-Marin 2012). This discovery is particularly timely as the monkey flower genus is developing rapidly as a model system for ecological genetics (Wu et al. 2008), and in the current issue of Molecular Ecology, Jennifer Modliszewski and John Willis of Duke University present new data showing high genetic diversity in another recently discovered monkey flower allopolyploid, M. sookensis (Modliszewski & Willis 2012).

Next-generation sequencing and genome evolution in allopolyploids.

PREMISE OF THE STUDY: Hybridization and polyploidization (allopolyploidy) are ubiquitous in the evolution of plants, but tracing the origins and subsequent evolution of the constituent genomes of allopolyploids has been challenging. Genome doubling greatly complicates genetic analyses, and this has long hindered investigation in that most allopolyploid species are "nonmodel" organisms. However, recent advances in sequencing and genomics technologies now provide unprecedented opportunities to analyze numerous genetic markers in multiple individuals in any organism. METHODS: Here we review the application of next-generation sequencing technologies to the study of three aspects of allopolyploid genome evolution: duplicated gene loss and expression in two recently formed Tragopogon allopolyploids, intergenomic interactions and chromosomal evolution in Tragopogon miscellus, and repetitive DNA evolution in Nicotiana allopolyploids. KEY RESULTS: For the first time, we can explore on a genomic scale the evolutionary processes that are ongoing in natural allopolyploids and not be restricted to well-studied crops and genetic models. CONCLUSIONS: These approaches can be easily and inexpensively applied to many other plant species-making any evolutionarily provocative system a new "model" system.

Unraveling Genetic Diversity Amongst European Hazelnut (Corylus avellana L.) Varieties in Turkey.

European hazelnut (Corylus avellana) is a diploid (2n = 22), monecious and wind-pollinated species, extensively cultivated for its nuts. Turkey is the world-leading producer of hazelnut, supplying 70-80% of the world's export capacity. Hazelnut is mostly grown in the Black Sea Region, and maintained largely through clonal propagation. Understanding the genetic variation between hazelnut varieties, and defining variety-specific and disease resistance-associated alleles, would facilitate hazelnut breeding in Turkey. Widely grown varieties 'Karafindik' (2), 'Sarifindik' (5), and 'Yomra' (2) were collected from Akçakoca in the west, while 'Tombul' (8), 'Çakildak' (3), 'Mincane' (2), 'Allahverdi' (2), 'Sivri' (4), and 'Palaz' (5) were collected from Ordu and Giresun provinces in the east (numbers in parentheses indicate sample sizes for each variety). Powdery mildew resistant and susceptible hazelnut genotypes were collected from the field gene bank and heavily infected orchards in Giresun. Every individual was subjected to double digest restriction enzyme-associated DNA sequencing (ddRAD-seq) and a RADtag library was created. RADtags were aligned to the 'Tombul' reference genome, and Stacks software used to identify polymorphisms. 101 private and six common alleles from nine hazelnut varieties, four private from resistants and only one from susceptible were identified for diagnosis of either a certain hazelnut variety or powdery mildew resistance. Phylogenetic analysis and population structure calculations indicated that 'Mincane', 'Sarifindik', 'Tombul', 'Çakildak', and 'Palaz' were genetically close to each other; however, individuals within every varietal group were found in different sub-populations. Our findings indicated that years of clonal propagation of some preferred varieties across the Black Sea Region has resulted in admixed sub-populations and great genetic diversity within each variety. This impedes the development of a true breeding variety. For example, 'Tombul' is the most favored Turkish variety because of its high quality nuts, but an elite 'Tombul' line does not yet exist. This situation continues due to the lack of a breed protection program for commercially valuable hazelnut varieties. This study provides molecular markers suitable for establishing such a program.

Tissue-specific silencing of homoeologs in natural populations of the recent allopolyploid Tragopogon mirus.

Recent years have seen rapid advances in our knowledge of the transcriptomic consequences of allopolyploidy, primarily through the study of polyploid crops and model systems. However, few studies have distinguished between homoeologs and between tissues, and still fewer have examined young natural allopolyploid populations of independent origin, whose parental species are still present in the same location. Here, we examined the expression of 13 homoeolog pairs in seven tissues of 10 plants of allotetraploid Tragopogon mirus from two natural populations formed by independent polyploidizations between Tragopogon dubius and Tragopogon porrifolius c. 40 generations ago. We compare these with patterns of expression in the diploid parental species from the same locality. Of the 910 assays in T. mirus, 576 (63%) showed expression of both homoeologs, 63 (7%) showed no expression of either homoeolog, 186 (20%) showed nonexpression of one homoeolog across all tissues of a plant, and 72 (8%) showed non-expression of a homoeolog in a particular tissue within a plant. We found two cases of reciprocal tissue-specific expression between homoeologs, potentially indicative of subfunctionalization. Our study shows that tissue-specific silencing, and even apparent subfunctionalization, can arise rapidly in the early generations of natural allopolyploidy.