Paternal leakage of plastids rescues inter-lineage hybrids in Silene nutans.

BACKGROUND AND AIMS: Organelle genomes are usually maternally inherited in angiosperms. However, biparental inheritance has been observed, especially in hybrids resulting from crosses between divergent genetic lineages. When it concerns the plastid genome, this exceptional mode of inheritance might rescue inter-lineage hybrids suffering from plastid-nuclear incompatibilities. Genetically differentiated lineages of Silene nutans exhibit strong postzygotic isolation owing to plastid-nuclear incompatibilities, highlighted by inter-lineage hybrid chlorosis and mortality. Surviving hybrids can exhibit variegated leaves, which might indicate paternal leakage of the plastid genome. We tested whether the surviving hybrids inherited the paternal plastid genome and survived thanks to paternal leakage. METHODS: We characterized the leaf phenotype (fully green, variegated or white) of 504 surviving inter-lineage hybrids obtained from a reciprocal cross experiment among populations of four genetic lineages (W1, W2, W3 and E1) of S. nutans from Western Europe and genotyped 560 leaf samples (both green and white leaves for variegated hybrids) using six lineage-specific plastid single nucleotide polymorphisms. KEY RESULTS: A high proportion of the surviving hybrids (≤98 %) inherited the paternal plastid genome, indicating paternal leakage. The level of paternal leakage depended on cross type and cross direction. The E1 and W2 lineages as maternal lineages led to the highest hybrid mortality and to the highest paternal leakage from W1 and W3 lineages in the few surviving hybrids. This was consistent with E1 and W2 lineages, which contained the most divergent plastid genomes. When W3 was the mother, more hybrids survived, and no paternal leakage was detected. CONCLUSIONS: By providing a plastid genome potentially more compatible with the hybrid nuclear background, paternal leakage has the potential to rescue inter-lineage hybrids from plastid-nuclear incompatibilities. This phenomenon might slow down the speciation process, provided hybrid survival and reproduction can occur in the wild.

The decoupled evolution of the organellar genomes of Silene nutans leads to distinct roles in the speciation process.

There is growing evidence that cytonuclear incompatibilities (i.e. disruption of cytonuclear coadaptation) might contribute to the speciation process. In a former study, we described the possible involvement of plastid-nuclear incompatibilities in the reproductive isolation between four lineages of Silene nutans (Caryophyllaceae). Because organellar genomes are usually cotransmitted, we assessed whether the mitochondrial genome could also be involved in the speciation process, knowing that the gynodioecious breeding system of S. nutans is expected to impact the evolutionary dynamics of this genome. Using hybrid capture and high-throughput DNA sequencing, we analyzed diversity patterns in the genic content of the organellar genomes in the four S. nutans lineages. Contrary to the plastid genome, which exhibited a large number of fixed substitutions between lineages, extensive sharing of polymorphisms between lineages was found in the mitochondrial genome. In addition, numerous recombination-like events were detected in the mitochondrial genome, loosening the linkage disequilibrium between the organellar genomes and leading to decoupled evolution. These results suggest that gynodioecy shaped mitochondrial diversity through balancing selection, maintaining ancestral polymorphism and, thus, limiting the involvement of the mitochondrial genome in evolution of hybrid inviability between S. nutans lineages.

What is the potential impact of genetic divergence of plastid ribosomal genes between Silene nutans lineages in hybrids? An in silico approach using the 3D structure of the plastid ribosome.

Introduction: Following the integration of cyanobacteria into the eukaryotic cells, many genes were transferred from the plastid to the nucleus. As a result, plastid complexes are encoded both by plastid and nuclear genes. Tight co-adaptation is required between these genes as plastid and nuclear genomes differ in several characteristics, such as mutation rate and inheritance patterns. Among these are complexes from the plastid ribosome, composed of two main subunits: a large and a small one, both composed of nuclear and plastid gene products. This complex has been identified as a potential candidate for sheltering plastid-nuclear incompatibilities in a Caryophyllaceae species, Silene nutans. This species is composed of four genetically differentiated lineages, which exhibit hybrid breakdown when interlineage crosses are conducted. As this complex is composed of numerous interacting plastid-nuclear gene pairs, in the present study, the goal was to reduce the number of gene pairs that could induce such incompatibilities. Method: We used the previously published 3D structure of the spinach ribosome to further elucidate which of the potential gene pairs might disrupt plastid-nuclear interactions within this complex. After modeling the impact of the identified mutations on the 3D structure, we further focused on one strongly mutated plastid-nuclear gene pair: rps11-rps21. We used the centrality measure of the mutated residues to further understand if the modified interactions and associated modified centralities might be correlated with hybrid breakdown. Results and discussion: This study highlights that lineage-specific mutations in essential plastid and nuclear genes might disrupt plastid-nuclear protein interactions of the plastid ribosome and that reproductive isolation correlates with changes in residue centrality values. Because of this, the plastid ribosome might be involved in hybrid breakdown in this system.

Reproductive isolation among lineages of Silene nutans (Caryophyllaceae): A potential involvement of plastid-nuclear incompatibilities.

Early stages of speciation in plants might involve genetic incompatibilities between plastid and nuclear genomes, leading to inter-lineage hybrid breakdown due to the disruption between co-adapted plastid and nuclear genes encoding subunits of the same plastid protein complexes. We tested this hypothesis in Silene nutans, a gynodioecious Caryophyllaceae, where four distinct genetic lineages exhibited strong reproductive isolation among each other, resulting in chlorotic or variegated hybrids. By sequencing the whole gene content of the four plastomes through gene capture, and a large part of the nuclear genes encoding plastid subunits from RNAseq data, we searched for non-synonymous substitutions fixed in each lineage on both genomes. Lineages of S. nutans exhibited a high level of dN/dS ratios for plastid and nuclear genes encoding most plastid complexes, with a strong pattern of coevolution for genes encoding the subunits of ribosome and cytochrome b6/f that could explain the chlorosis of hybrids. Overall, relaxation of selection due to past bottlenecks and positive selection have driven the diversity pattern observed in S. nutans plastid complexes, leading to plastid-nuclear incompatibilities. We discuss the possible role of gynodioecy in the evolutionary dynamics of the plastomes through linked selection.

Dioecy Is Associated with High Genetic Diversity and Adaptation Rates in the Plant Genus Silene.

About 15,000 angiosperm species (∼6%) have separate sexes, a phenomenon known as dioecy. Why dioecious taxa are so rare is still an open question. Early work reported lower species richness in dioecious compared with nondioecious sister clades, raising the hypothesis that dioecy may be an evolutionary dead-end. This hypothesis has been recently challenged by macroevolutionary analyses that detected no or even positive effect of dioecy on diversification. However, the possible genetic consequences of dioecy at the population level, which could drive the long-term fate of dioecious lineages, have not been tested so far. Here, we used a population genomics approach in the Silene genus to look for possible effects of dioecy, especially for potential evidence of evolutionary handicaps of dioecy underlying the dead-end hypothesis. We collected individual-based RNA-seq data from several populations in 13 closely related species with different sexual systems: seven dioecious, three hermaphroditic, and three gynodioecious species. We show that dioecy is associated with increased genetic diversity, as well as higher selection efficacy both against deleterious mutations and for beneficial mutations. The results hold after controlling for phylogenetic inertia, differences in species census population sizes and geographic ranges. We conclude that dioecious Silene species neither show signs of increased mutational load nor genetic evidence for extinction risk. We discuss these observations in the light of the possible demographic differences between dioecious and self-compatible hermaphroditic species and how this could be related to alternatives to the dead-end hypothesis to explain the rarity of dioecy.

Cytonuclear Genetic Incompatibilities in Plant Speciation.

Due to the endosymbiotic origin of organelles, a pattern of coevolution and coadaptation between organellar and nuclear genomes is required for proper cell function. In this review, we focus on the impact of cytonuclear interaction on the reproductive isolation of plant species. We give examples of cases where species exhibit barriers to reproduction which involve plastid-nuclear or mito-nuclear genetic incompatibilities, and describe the evolutionary processes at play. We also discuss potential mechanisms of hybrid fitness recovery such as paternal leakage. Finally, we point out the possible interplay between plant mating systems and cytonuclear coevolution, and its consequence on plant speciation.

Congruent population genetic structures and divergence histories in anther-smut fungi and their host plants Silene italica and the Silene nutans species complex.

Study of the congruence of population genetic structure between hosts and pathogens gives important insights into their shared phylogeographical and coevolutionary histories. We studied the population genetic structure of castrating anther-smut fungi (genus Microbotryum) and of their host plants, the Silene nutans species complex, and the morphologically and genetically closely related Silene italica, which can be found in sympatry. Phylogeographical population genetic structure related to persistence in separate glacial refugia has been recently revealed in the S. nutans plant species complex across Western Europe, identifying several distinct lineages. We genotyped 171 associated plant-pathogen pairs of anther-smut fungi and their host plant individuals using microsatellite markers and plant chloroplastic single nucleotide polymorphisms. We found clear differentiation between fungal populations parasitizing S. nutans and S. italica plants. The population genetic structure of fungal strains parasitizing the S. nutans plant species complex mirrored the host plant genetic structure, suggesting that the pathogen was isolated in glacial refugia together with its host and/or that it has specialized on the plant genetic lineages. Using random forest approximate Bayesian computation (ABC-RF), we found that the divergence history of the fungal lineages on S. nutans was congruent with that previously inferred for the host plant and probably occurred with ancient but no recent gene flow. Genome sequences confirmed the genetic structure and the absence of recent gene flow between fungal genetic lineages. Our analyses of individual host-pathogen pairs contribute to a better understanding of co-evolutionary histories between hosts and pathogens in natural ecosystems, in which such studies remain scarce.

Complete Sequence, Multichromosomal Architecture and Transcriptome Analysis of the Solanum tuberosum Mitochondrial Genome.

Mitochondrial genomes (mitogenomes) in higher plants can induce cytoplasmic male sterility and be somehow involved in nuclear-cytoplasmic interactions affecting plant growth and agronomic performance. They are larger and more complex than in other eukaryotes, due to their recombinogenic nature. For most plants, the mitochondrial DNA (mtDNA) can be represented as a single circular chromosome, the so-called master molecule, which includes repeated sequences that recombine frequently, generating sub-genomic molecules in various proportions. Based on the relevance of the potato crop worldwide, herewith we report the complete mtDNA sequence of two S. tuberosum cultivars, namely Cicero and Désirée, and a comprehensive study of its expression, based on high-coverage RNA sequencing data. We found that the potato mitogenome has a multi-partite architecture, divided in at least three independent molecules that according to our data should behave as autonomous chromosomes. Inter-cultivar variability was null, while comparative analyses with other species of the Solanaceae family allowed the investigation of the evolutionary history of their mitogenomes. The RNA-seq data revealed peculiarities in transcriptional and post-transcriptional processing of mRNAs. These included co-transcription of genes with open reading frames that are probably expressed, methylation of an rRNA at a position that should impact translation efficiency and extensive RNA editing, with a high proportion of partial editing implying frequent mis-targeting by the editing machinery.

Evolution of Young Sex Chromosomes in Two Dioecious Sister Plant Species with Distinct Sex Determination Systems.

In the last decade, progress has been made in methods to identify the sex determination system in plants. This gives the opportunity to study sex chromosomes that arose independently at different phylogenetic scales, and thus allows the discovery and the understanding of early stages of sex chromosome evolution. In the genus Silene, sex chromosomes have evolved independently in at least two clades from a nondioecious ancestor, the Melandrium and Otites sections. In the latter, sex chromosomes could be younger than in the section Melandrium, based on phylogenetic studies and as no heteromorphic sex chromosomes have been detected. This section might also exhibit lability in sex determination, because male heterogamy and female heterogamy have been suggested to occur.In this study, we investigated the sex determination system of two dioecious species in the section Otites (Silene otites and its close relative Silene pseudotites). Applying the new probabilistic method SEX-DETector on RNA-seq data from cross-controlled progenies, we inferred their most likely sex determination system and a list of putative autosomal and sex-linked contigs. We showed that the two phylogenetically close species differed in their sex determination system (XY versus ZW) with sex chromosomes that derived from two different pairs of autosomes. We built a genetic map of the sex chromosomes and showed that both pairs exhibited a large region with lack of recombination. However, the sex-limited chromosomes exhibited no strong degeneration. Finally, using the "ancestral" autosomal expression of sex-linked orthologs of nondioecious S. nutans, we found a slight signature of dosage compensation in the heterogametic females of S. otites.

Chloroplastic and nuclear diversity of wild beets at a large geographical scale: Insights into the evolutionary history of the Beta section.

Historical demographic processes and mating systems are believed to be major factors in the shaping of the intraspecies genetic diversity of plants. Among Caryophyllales, the Beta section of the genus Beta, within the Amaranthaceae/Chenopodiaceae alliance, is an interesting study model with species and subspecies (Beta macrocarpa, Beta patula, Beta vulgaris maritima and B.v. adanensis) differing in geographical distribution and mating system. In addition, one of the species, B. macrocarpa, mainly diploid, varies in its level of ploidy with a tetraploid cytotype described in the Canary Islands and in Portugal. In this study, we analyzed the nucleotide diversity of chloroplastic and nuclear sequences on a representative sampling of species and subspecies of the Beta section (except B. patula). Our objectives were (1) to assess their genetic relationships through phylogenetic and multivariate analyses, (2) relate their genetic diversity to their mating system, and (3) reconsider the ploidy status and the origin of the Canarian Beta macrocarpa.

CMS-G from Beta vulgaris ssp. maritima is maintained in natural populations despite containing an atypical cytochrome c oxidase.

While mitochondrial mutants of the respiratory machinery are rare and often lethal, cytoplasmic male sterility (CMS), a mitochondrially inherited trait that results in pollen abortion, is frequently encountered in wild populations. It generates a breeding system called gynodioecy. In Beta vulgaris ssp. maritima, a gynodioecious species, we found CMS-G to be widespread across the distribution range of the species. Despite the sequencing of the mitochondrial genome of CMS-G, the mitochondrial sterilizing factor causing CMS-G is still unknown. By characterizing biochemically CMS-G, we found that the expression of several mitochondrial proteins is altered in CMS-G plants. In particular, Cox1, a core subunit of the cytochrome c oxidase (complex IV), is larger but can still assemble into complex IV. However, the CMS-G-specific complex IV was only detected as a stabilized dimer. We did not observe any alteration of the affinity of complex IV for cytochrome c; however, in CMS-G, complex IV capacity is reduced. Our results show that CMS-G is maintained in many natural populations despite being associated with an atypical complex IV. We suggest that the modified complex IV could incur the associated cost predicted by theoretical models to maintain gynodioecy in wild populations.

Lineages of Silene nutans developed rapid, strong, asymmetric postzygotic reproductive isolation in allopatry.

Reproductive isolation can rise either as a consequence of genomic divergence in allopatry or as a byproduct of divergent selection in parapatry. To determine whether reproductive isolation in gynodioecious Silene nutans results from allopatric divergence or from ecological adaptation following secondary contact, we investigated the pattern of postzygotic reproductive isolation and hybridization in natural populations using two phylogeographic lineages, western (W1) and eastern (E1). Experimental crosses between the lineages identified strong, asymmetric postzygotic isolation between the W1 and the E1 lineages, independent of geographic overlap. The proportion of ovules fertilized, seeds aborted, and seeds germinated revealed relatively little effect on the fitness of hybrids. In contrast, hybrid mortality was high and asymmetric: while half of the hybrid seedlings with western lineage mothers died, nearly all hybrid seedlings with E1 mothers died. This asymmetric mortality mirrored the proportion of chlorotic seedlings, and is congruent with cytonuclear incompatibility. We found no evidence of hybridization between the lineages in regions of co-occurrence using nuclear and plastid markers. Together, our results are consistent with the hypothesis that strong postzygotic reproductive isolation involving cytonuclear incompatibilities arose in allopatry. We argue that the dynamics of cytonuclear gynodioecy could facilitate the evolution of reproductive isolation.

When is it worth being a self-compatible hermaphrodite? Context-dependent effects of self-pollination on female advantage in gynodioecious Silene nutans.

In gynodioecious plant species with nuclear-cytoplasmic sex determination, females and hermaphrodites plants can coexist whenever female have higher seed fitness than hermaphrodites. Although the effect of self fertilization on seed fitness in hermaphrodites has been considered theoretically, this effect is far from intuitive, because it can either increase the relative seed fitness of the females (if it leads hermaphrodites to produce inbred, low quality offspring) or decrease it (if it provides reproductive assurance to hermaphrodites). Hence, empirical investigation is needed to document whether relative seed fitness varies with whether pollen is or is not limiting to seed production. In the current study, we measured fruit set and seed production in both females and hermaphrodites and the selfing rate in hermaphrodites in two experimental patches that differed in sex ratios in the gynodioecious plant Silene nutans. We found an impact of plant gender, patch, and their interaction, with females suffering from stronger pollen limitation when locally frequent. In the most pollen-limited situation, the selfing rate of hermaphrodites increased and provided hermaphrodites with a type of reproductive assurance that is not available to females. By integrating both the beneficial (reproductive assurance) and costly effects (through inbreeding depression) of self-pollination, we showed that whether females did or did not exhibit higher seed fitness depended on the degree of pollen limitation on seed production.

Cytoplasmic male sterility and mitochondrial metabolism in plants.

Cytoplasmic male sterility (CMS) is a common feature encountered in plant species. It is the result of a genomic conflict between the mitochondrial and the nuclear genomes. CMS is caused by mitochondrial encoded factors which can be counteracted by nuclear encoded factors restoring male fertility. Despite extensive work, the molecular mechanism of male sterility still remains unknown. Several studies have suggested the involvement of respiration on the disruption of pollen production through an energy deficiency. By comparing recent works on CMS and respiratory mutants, we suggest that the "ATP hypothesis" might not be as obvious as previously suggested.

A scenario of mitochondrial genome evolution in maize based on rearrangement events.

BACKGROUND: Despite their monophyletic origin, animal and plant mitochondrial genomes have been described as exhibiting different modes of evolution. Indeed, plant mitochondrial genomes feature a larger size, a lower mutation rate and more rearrangements than their animal counterparts. Gene order variation in animal mitochondrial genomes is often described as being due to translocation and inversion events, but tandem duplication followed by loss has also been proposed as an alternative process. In plant mitochondrial genomes, at the species level, gene shuffling and duplicate occurrence are such that no clear phylogeny has ever been identified, when considering genome structure variation. RESULTS: In this study we analyzed the whole sequences of eight mitochondrial genomes from maize and teosintes in order to comprehend the events that led to their structural features, i.e. the order of genes, tRNAs, rRNAs, ORFs, pseudogenes and non-coding sequences shared by all mitogenomes and duplicate occurrences. We suggest a tandem duplication model similar to the one described in animals, except that some duplicates can remain. This model enabled us to develop a manual method to deal with duplicates, a recurrent problem in rearrangement analyses. The phylogenetic tree exclusively based on rearrangement and duplication events is congruent with the tree based on sequence polymorphism, validating our evolution model. CONCLUSIONS: This study suggests more similarity than usually reported between plant and animal mitochondrial genomes in their mode of evolution. Further work will consist of developing new tools in order to automatically look for signatures of tandem duplication events in other plant mitogenomes and evaluate the occurrence of this process on a larger scale.

Fine-scale geographical structure of genetic diversity in inland wild beet populations.

Introgression arising from crop-to-wild gene flow provides novel sources of genetic variation in plant species complexes. Hybridization within the Beta vulgaris species complex is of immediate concern; crop lineages (B. vulgaris ssp. vulgaris) hybridize easily with their wild relatives (B. vulgaris ssp. maritima) thereby threatening wild beet gene diversity with genetic swamping. Hybridization 'hotspots' occur in European seed production areas because inland ruderal wild beets occur and reproduce in sympatry with cultivated beets. We studied gene flow occurring between seed-producing cultivars and ruderal wild B. vulgaris in southwestern France to determine whether feral beets, arising from unharvested cultivated seed, represent an opportunity for crop-to-wild gene flow. We surveyed 42 inland ruderal beet populations located near seed production fields for nucleo-cytoplasmic variation and used a cytoplasmic marker diagnostic of cultivated lines. Occurrence of cultivated-type cytoplasm within ruderal populations clearly reflected events of crop seed escape. However, we found no genetic signatures of nuclear cultivated gene introgression, which suggests past introgression of cultivated cytoplasm into a wild nuclear background through seed escape rather than recent direct pollen flow. Overall, patterns of genetic structure suggested that inland ruderal wild beet populations act as a metapopulation, with founding events involving a few sib groups, followed by low rates of seed or pollen gene flow after populations are established. Altogether, our results indicate that a long-lived seed bank plays a key role in maintaining cultivated-type cytoplasm in the wild and highlight the need for careful management of seed production areas where wild and cultivated relatives co-occur.

Sex ratio variation among gynodioecious populations of sea beet: can it be explained by negative frequency-dependent selection?

This study is devoted to assess sex ratio variation among 33 populations of the gynodioecious Beta vulgaris ssp. maritima in Brittany (France) and to explore the causes of this variation. We showed that three different CMS (cytoplasmic male sterility) cytotypes occurred in populations, but strongly differed for their frequencies and the frequency of their associated nuclear restorer alleles (which counteract the effect of CMS and restore male fertility). No correlation was found between CMS and restorer frequencies within populations, which has been previously interpreted as a result of stochasticity. However, neutral genetic variation did not indicate recent population bottlenecks in studied populations. Moreover, no significant correlation was found between female frequency or variance and current population size. Consequently, stochastic processes could not be the major cause of sex ratio variation. Alternatively, empirical estimations of the variation of females, CMS genes and nuclear restorer allele's frequencies were compared to theoretical predictions based on a frequency-dependent selection model of gynodioecy. In particular, we showed that an absence of correlation between CMS and restorer frequencies could also occur without stochasticity. The large variation of sex ratio in Beta vulgaris could thus be explained by frequency-dependent selection acting on CMS genes and restorer alleles.

The effect of breeding system on polymorphism in mitochondrial genes of Silene.

Gynodioecy is a breeding system characterized by the co-occurrence of hermaphrodite and female individuals, generally as the result of nuclear-cytoplasmic interactions. The question remains whether the genetic factors controlling gynodioecy are maintained in species over long evolutionary timescales by balancing selection or are continually arising and being replaced in epidemic sweeps. If balancing selection maintains these factors, then neutral cytoplasmic diversity should be greater in gynodioecious than hermaphroditic species. In contrast, epidemic sweeps of factors controlling gynodioecy should decrease cytoplasmic diversity in gynodioecious relative to hermaphroditic species. We took a comparative approach in which we sequenced two mitochondrial genes, cytochrome b (cob) and cytochrome oxidase (cox1), for multiple populations of several hermaphroditic, gynodioecious, and dioecious species in the genus Silene. Breeding system was predictive of polymorphism. Gynodioecious species harbor many old haplotypes while hermaphroditic and dioecious species have little to no nucleotide diversity. The genealogical structure of neither gene departed from neutral expectations. Taken together, our results suggest that balancing selection acts on cytoplasmic male-sterility factors in several gynodioecious species in the genus.

Extensive variation in synonymous substitution rates in mitochondrial genes of seed plants.

BACKGROUND: It has long been known that rates of synonymous substitutions are unusually low in mitochondrial genes of flowering and other land plants. Although two dramatic exceptions to this pattern have recently been reported, it is unclear how often major increases in substitution rates occur during plant mitochondrial evolution and what the overall magnitude of substitution rate variation is across plants. RESULTS: A broad survey was undertaken to evaluate synonymous substitution rates in mitochondrial genes of angiosperms and gymnosperms. Although most taxa conform to the generality that plant mitochondrial sequences evolve slowly, additional cases of highly accelerated rates were found. We explore in detail one of these new cases, within the genus Silene. A roughly 100-fold increase in synonymous substitution rate is estimated to have taken place within the last 5 million years and involves only one of ten species of Silene sampled in this study. Examples of unusually slow sequence evolution were also identified. Comparison of the fastest and slowest lineages shows that synonymous substitution rates vary by four orders of magnitude across seed plants. In other words, some plant mitochondrial lineages accumulate more synonymous change in 10,000 years than do others in 100 million years. Several perplexing cases of gene-to-gene variation in sequence divergence within a plant were uncovered. Some of these probably reflect interesting biological phenomena, such as horizontal gene transfer, mitochondrial-to-nucleus transfer, and intragenomic variation in mitochondrial substitution rates, whereas others are likely the result of various kinds of errors. CONCLUSION: The extremes of synonymous substitution rates measured here constitute by far the largest known range of rate variation for any group of organisms. These results highlight the utility of examining absolute substitution rates in a phylogenetic context rather than by traditional pairwise methods. Why substitution rates are generally so low in plant mitochondrial genomes yet occasionally increase dramatically remains mysterious.

Spatial analysis of nuclear and cytoplasmic DNA diversity in wild sea beet (Beta vulgaris ssp. maritima) populations: do marine currents shape the genetic structure?

Patterns of seed dispersal in the wild sea beet (Beta vulgaris ssp. maritima) are predicted to be influenced by marine currents because populations are widely distributed along the European Atlantic coast. We investigated the potential influence of marine currents on the pattern of spatial genetic structuring in natural populations of sea beet. Populations were located along the French coasts of the Anglo-Norman gulf that features peculiar marine currents in the Channel. Thirty-three populations were sampled, among which 23 were continental and 10 were insular populations located in Jersey, Guernsey and Chausey, for a total of 1224 plants genotyped. To validate the coastal topography influence and the possibility of marine current orientated gene flow on the genetic features of sea beet populations, we assessed patterns of genetic structuring of cytoplasmic and nuclear diversity by: (i) searching for an isolation-by-distance (IBD) pattern using spatial autocorrelation tools; (ii) using the Monmonier algorithm to identify genetic boundaries in the area studied; and (iii) performing assignment tests that are based on multilocus genotype information to ascertain population membership of individuals. Our results showed a highly contrasted cytoplasmic and nuclear genetic differentiation and highlighted the peculiar situation of island populations. Beyond a classical isolation-by-distance due to short-range dispersal, genetic barriers fitting the orientation of marine currents were clearly identified. This suggests the occurrence of long-distance seed dispersal events and an asymmetrical gene flow separating the eastern and western part of the Anglo-Norman gulf.