A new method for studying population genetics of cyst nematodes based on Pool-Seq and genomewide allele frequency analysis

Genome scans for selection signatures identify candidate virulence genes for adaptation of the soybean cyst nematode to host resistance

Plant pathogens are constantly under selection pressure for host resistance adaptation. Soybean cyst nematode (SCN, Heterodera glycines) is a major pest of soybean primarily managed through resistant cultivars; however, SCN populations have evolved virulence in response to selection pressures driven by repeated monoculture of the same genetic resistance. Resistance to SCN is mediated by multiple epistatic interactions between Rhg (for resistance to H. glycines) genes. However, the identity of SCN virulence genes that confer the ability to overcome resistance remains unknown. To identify candidate genomic regions showing signatures of selection for increased virulence, we conducted whole genome resequencing of pooled individuals (Pool-Seq) from two pairs of SCN populations adapted on soybeans with Peking-type (rhg1-a, rhg2, and Rhg4) resistance. Population differentiation and principal component analysis-based approaches identified approximately 0.72-0.79 million SNPs, the frequency of which showed potential selection signatures across multiple genomic regions. Chromosomes 3 and 6 between population pairs showed the greatest density of outlier SNPs with high population differentiation. Conducting multiple outlier detection tests to identify overlapping SNPs resulted in a total of 966 significantly differentiated SNPs, of which 285 exon SNPs were mapped to 97 genes. Of these, six genes encoded members of known stylet-secreted effector protein families potentially involved in host defence modulation including venom-allergen-like, annexin, glutathione synthetase, SPRYSEC, chitinase, and CLE effector proteins. Further functional analysis of identified candidate genes will provide new insights into the genetic mechanisms by which SCN overcomes soybean resistance and inform the development of molecular markers for rapidly screening the virulence profile of an SCN-infested field.

Population Genomics of Pooled Samples: Unveiling Symbiont Infrapopulation Diversity and Host-Symbiont Coevolution

Microscopic symbionts represent crucial links in biological communities. However, they present technical challenges in high-throughput sequencing (HTS) studies due to their small size and minimal high-quality DNA yields, hindering our understanding of host-symbiont coevolution at microevolutionary and macroevolutionary scales. One approach to overcome those barriers is to pool multiple individuals from the same infrapopulation (i.e., individual host) and sequence them together (Pool-Seq), but individual-level information is then compromised. To simultaneously address both issues (i.e., minimal DNA yields and loss of individual-level information), we implemented a strategic Pool-Seq approach to assess variation in sequencing performance and categorize genetic diversity (single nucleotide polymorphisms (SNPs)) at both the individual-level and infrapopulation-level for microscopic feather mites. To do so, we collected feathers harboring mites (Proctophyllodidae: Amerodectes protonotaria) from four individual Prothonotary Warblers (Parulidae: Protonotaria citrea). From each of the four hosts (i.e., four mite infrapopulations), we conducted whole-genome sequencing on three extraction pools consisting of different numbers of mites (1 mite, 5 mites, and 20 mites). We found that samples containing pools of multiple mites had more sequencing reads map to the feather mite reference genome than did the samples containing only a single mite. Mite infrapopulations were primarily genetically structured by their associated individual hosts (not pool size) and the majority of SNPs were shared by all pools within an infrapopulation. Together, these results suggest that the patterns observed are driven by evolutionary processes occurring at the infrapopulation level and are not technical signals due to pool size. In total, despite the challenges presented by microscopic symbionts in HTS studies, this work highlights the value of both individual-level and infrapopulation-level sequencing toward our understanding of host-symbiont coevolution at multiple evolutionary scales.

Single nucleotide polymorphism markers in Heterorhabditis bacteriophora associated with virulence at low temperature

The entomopathogenic nematode (EPN), Heterorhabditis bacteriophora, is an important biological control agent worldwide. Industrially produced EPN need to meet the climatic requirements for the control of pests in field agriculture in autumn and spring when temperatures are low. For this trait (virulence at low temperature), previous EPN improvement attempts relied on phenotypic selection and the selected trait had low stability. The use of molecular markers can increase the efficacy of EPN breeding by tracking traits associated with specific genotypes. To date, fewer than 200 polymorphic and reproducible sequence-tagged molecular markers in H. bacteriophora have been reported. Here, we enhanced the palette of highly polymorphic genetic markers for this EPN by applying genotyping by sequencing (GBS). By analysing 48 H. bacteriophora homozygous wild-type inbred lines from different origins, we determined 4894 single nucleotide polymorphisms (SNPs) with at least one polymorphism along the tested set. For validation, we designed robust PCR assays for seven SNPs, finding 95% correspondence with the expected genotypes along 294 analysed alleles. We phenotyped all lines for their virulence at low temperature (15°C) against mealworm and observed infectivity ranging from 38 to 80%. Further, we carried out association analyses between genotypic and phenotypic data and determined two SNPs yielding potential association with H. bacteriophora virulence at low temperature. The use of these candidate SNPs as breeding markers will speed up the generation of strains better adapted to low temperature in this species. The generated set of lines and SNP data are a versatile tool applicable for further traits in this EPN.

Selection of genome-wide SNPs for pooled allelotyping assays useful for population monitoring

Parasitic worms are serious pests of humans, livestock and crops worldwide. Multiple management strategies are employed in order to reduce their impact, and some of these may affect their genome and population allelic frequency distribution. The evolution of chemical resistance, ecological changes, and pest dispersal have allowed an increasing number of pests to become difficult to control with current management methods. Their lifestyle limits the use of ecological and individual-based management of populations. There is a need to develop rapid, affordable, and simple diagnostics to assess the efficacy of management strategies and delay the evolution of resistance to these strategies. This study presents a multi-locus, equal-representation, whole genome pooled SNPs selection approach as a monitoring tool for the ovine nematode parasite Haemonchus contortus. The SNP selection method used two reference genomes of different quality, then validated these SNPs against a high-quality recent genome assembly. From over 11 million high-quality SNPs identified, 334 SNPs were selected, of which 262 were species-specific, yielded similar allele frequencies when assessed as multiple individuals or as pools of individuals, and suitable to distinguish mixed nematode isolate pools from single isolate pools. As a proof-of-concept, 21 Australian H. contortus populations with various phenotypes and genotypes were screened. This analysis confirmed the overall low-level of genetic differentiation between populations collected from the field, but clearly identifying highly inbred populations, and populations showing genetic signatures associated with chemical resistance. The analysis showed that 66% of the SNPs were necessary for stability in assessing population genetic patterns, and SNP pairs did not show linkage according to allelic frequencies across the 21 populations. This method demonstrates that ongoing monitoring of parasite allelic frequencies and genetic changes can be achieved as a management assessment tool to identify drug-treatment failure, population incursions, and inbreeding signatures due to selection. The SNP selection method could also be applied to other parasite species.

Genome-wide patterns of genetic diversity, population structure and demographic history in mānuka (Leptospermum scoparium) growing on indigenous Māori land

Leptospermum scoparium J. R. Forst et G. Forst, known as mānuka by Māori, the indigenous people of Aotearoa (New Zealand), is a culturally and economically significant shrub species, native to New Zealand and Australia. Chemical, morphological and phylogenetic studies have indicated geographical variation of mānuka across its range in New Zealand, and genetic differentiation between New Zealand and Australia. We used pooled whole genome re-sequencing of 76 L. scoparium and outgroup populations from New Zealand and Australia to compile a dataset totalling ~2.5 million SNPs. We explored the genetic structure and relatedness of L. scoparium across New Zealand, and between populations in New Zealand and Australia, as well as the complex demographic history of this species. Our population genomic investigation suggests there are five geographically distinct mānuka gene pools within New Zealand, with evidence of gene flow occurring between these pools. Demographic modelling suggests three of these gene pools have undergone expansion events, whilst the evolutionary histories of the remaining two have been subjected to contractions. Furthermore, mānuka populations in New Zealand are genetically distinct from populations in Australia, with coalescent modelling suggesting these two clades diverged ~9-12 million years ago. We discuss the evolutionary history of this species and the benefits of using pool-seq for such studies. Our research will support the management and conservation of mānuka by landowners, particularly Māori, and the development of a provenance story for the branding of mānuka based products.

Comparative genomics of two inbred lines of the potato cyst nematode Globodera rostochiensis reveals disparate effector family-specific diversification patterns

BACKGROUND

Potato cyst nematodes belong to the most harmful pathogens in potato, and durable management of these parasites largely depends on host-plant resistances. These resistances are pathotype specific. The current Globodera rostochiensis pathotype scheme that defines five pathotypes (Ro1 - Ro5) is both fundamentally and practically of limited value. Hence, resistant potato varieties are used worldwide in a poorly informed manner.

RESULTS

We generated two novel reference genomes of G. rostochiensis inbred lines derived from a Ro1 and a Ro5 population. These genome sequences comprise 173 and 189 scaffolds respectively, marking a ≈ 24-fold reduction in fragmentation as compared to the current reference genome. We provide copy number variations for 19 effector families. Four dorsal gland effector families were investigated in more detail. SPRYSECs, known to be implicated in plant defence suppression, constitute by far the most diversified family studied herein with 60 and 99 variants in Ro1 and Ro5 distributed over 18 and 26 scaffolds. In contrast, CLEs, effectors involved in feeding site induction, show strong physical clustering. The 10 and 16 variants cluster on respectively 2 and 1 scaffolds. Given that pathotypes are defined by their effectoromes, we pinpoint the disparate nature of the contributing effector families in terms of sequence diversification and loss and gain of variants.

CONCLUSIONS

Two novel reference genomes allow for nearly complete inventories of effector diversification and physical organisation within and between pathotypes. Combined with insights we provide on effector family-specific diversification patterns, this constitutes a basis for an effectorome-based virulence scheme for this notorious pathogen.

Genome Resources of Two Pathotypes of the Potato Cyst Nematode Globodera rostochiensis from New York

The potato cyst nematode Globodera rostochiensis is a regulated pest posing a serious threat to potato production worldwide. Although the endemic pathotype (Ro1) of G. rostochiensis has been confined to New York State for several decades as a result of quarantine regulations and management with resistant potato cultivars, a virulent pathotype, Ro2, has emerged, for which control measures are scarce. The ability to detect Ro2 early in fields is necessary to sustain the success of G. rostochiensis quarantine in the United States. Here, we report the comparative analysis of whole-genome sequences of multiple single-cyst-derived Ro1 and Ro2 lines, propagated from original field populations. The identified discriminant variants are good targets for developing molecular diagnostic tools for differentiating G. rostochiensis pathotypes in New York.

Genomic Analyses of Globodera pallida, A Quarantine Agricultural Pathogen in Idaho

Globodera pallida is among the most significant plant-parasitic nematodes worldwide, causing major damage to potato production. Since it was discovered in Idaho in 2006, eradication efforts have aimed to contain and eradicate G. pallida through phytosanitary action and soil fumigation. In this study, we investigated genome-wide patterns of G. pallida genetic variation across Idaho fields to evaluate whether the infestation resulted from a single or multiple introduction(s) and to investigate potential evolutionary responses since the time of infestation. A total of 53 G. pallida samples (~1,042,000 individuals) were collected and analyzed, representing five different fields in Idaho, a greenhouse population, and a field in Scotland that was used for external comparison. According to genome-wide allele frequency and fixation index (F_st) analyses, most of the genetic variation was shared among the G. pallida populations in Idaho fields pre-fumigation, indicating that the infestation likely resulted from a single introduction. Temporal patterns of genome-wide polymorphisms involving (1) pre-fumigation field samples collected in 2007 and 2014 and (2) pre- and post-fumigation samples revealed nucleotide variants (SNPs, single-nucleotide polymorphisms) with significantly differentiated allele frequencies indicating genetic differentiation. This study provides insights into the genetic origins and adaptive potential of G. pallida invading new environments.

Recent Advances in Population Genomics of Plant-Parasitic Nematodes

Plant-parasitic nematodes are a costly burden of crop production. Ubiquitous in nature, phytoparasitic nematodes are associated with nearly every important agricultural crop and represent a significant constraint on global food security. Population genetics is a key discipline in plant nematology to understand aspects of the life strategies of these parasites, in particular their modes of reproduction, geographic origins, evolutionary histories, and dispersion abilities. Advances in high-throughput sequencing technologies have enabled a recent but active effort in genomic analyses of plant-parasitic nematodes. Such genomic approaches applied to multiple populations are providing new insights into the molecular and evolutionary processes that underpin the establishment of these nematodes and into a better understanding of the genetic and mechanistic basis of their pathogenicity and adaptation to their host plants. In this review, we attempt to update information about genome resources and genotyping techniques useful for nematologists who are thinking about initiating population genomics or genome sequencing projects. This review is intended also to foster the development of population genomics in plant-parasitic nematodes through highlighting recent publications that illustrate the potential for this approach to identify novel molecular markers or genes of interest and improve our knowledge of the genome variability, pathogenicity, and evolutionary potential of plant-parasitic nematodes.

[Quarantine nematode species and pathotypes potentially dangerous for domestic potato production: populations diversity and the genetics of potato resistance]

Обзор посвящен проблеме потенциально опасных для отечественного картофелеводства каран- тинных видов и патотипов нематод. Картофель поражают более 30 видов паразитических нематод, однако в статье основное внимание уделено самым вредоносным, приносящим большой ущерб картофелеводству пред- ставителям родов Globodera, Ditylenchus, Nacobbus и Meloidogyne. Проанализированы фитопатологические и молекулярные методы идентификации видов и патотипов и основные достижения в изучении изменчивости популяций паразитических нематод картофеля. Показано, что, благодаря особенностям жизненного цикла не- матод и лабильности их геномов, генетическая изменчивость этих организмов очень велика, что создает угрозу образования новых патогенных генотипов паразита. Сведения о внутри- и межпопуляционной изменчивости нематод важны для изучения путей интродукции и распространения отдельных видов, а также поиска корреля- ций молекулярных маркеров с определенным патотипом. Филогенетические исследования, основанные на со- временных данных по генетической изменчивости популяций, позволили выявить комплексы видов у Globodera pallida (Stone) Behrens и Nacobbus aberrans (Thorne) Thorne & Allen (sensu lato), включающие криптические виды. К основным составляющим успешной защиты, предотвращающей массовое распространение паразитических нематод, относятся карантинные мероприятия, агротехнические приемы, биологические способы защиты и возделывание устойчивых сортов. Особое внимание в обзоре уделено вопросам селекции сортов картофеля с длительной устойчивостью к различным видам нематод, поскольку возделывание таких сортов – экологически наиболее безопасный и экономически выгодный способ предотвращения эпифитотий. В настоящее время до- стигнуты значительные успехи в генетической защите сортов картофеля, особенно в отношении цистообразую- щих нематод. Приведены сведения об источниках устойчивости картофеля к паразитическим нематодам, выде- ленных в коллекциях диких и культурных видов. Проанализированы данные об идентифицированных R-генах и QTL устойчивости, которые были интрогрессированы в селекционный материал с помощью различных методов и подходов. Представлены результаты изучения структурной и функциональной организации генов устойчиво- сти к цистообразующим нематодам картофеля. Рассмотрены результаты исследований по использованию моле- кулярных маркеров определенных генов в маркер-опосредованной селекции для создания новых устойчивых сортов, в том числе с групповой устойчивостью.

Genomics versus mtDNA for resolving stock structure in the silky shark (Carcharhinus falciformis)

Conservation genetic approaches for elasmobranchs have focused on regions of the mitochondrial genome or a handful of nuclear microsatellites. High-throughput sequencing offers a powerful alternative for examining population structure using many loci distributed across the nuclear and mitochondrial genomes. These single nucleotide polymorphisms are expected to provide finer scale and more accurate population level data; however, there have been few genomic studies applied to elasmobranch species. The desire to apply next-generation sequencing approaches is often tempered by the costs, which can be offset by pooling specimens prior to sequencing (pool-seq). In this study, we assess the utility of pool-seq by applying this method to the same individual silky sharks, Carcharhinus falciformis, previously surveyed with the mtDNA control region in the Atlantic and Indian Oceans. Pool-seq methods were able to recover the entire mitochondrial genome as well as thousands of nuclear markers. This volume of sequence data enabled the detection of population structure between regions of the Atlantic Ocean populations, undetected in the previous study (inter-Atlantic mitochondrial SNPs FST values comparison ranging from 0.029 to 0.135 and nuclear SNPs from 0.015 to 0.025). Our results reinforce the conclusion that sampling the mitochondrial control region alone may fail to detect fine-scale population structure, and additional sampling across the genome may increase resolution for some species. Additionally, this study shows that the costs of analyzing 4,988 loci using pool-seq methods are equivalent to the standard Sanger-sequenced markers and become less expensive when large numbers of individuals (>300) are analyzed.

DepthFinder: a tool to determine the optimal read depth for reduced-representation sequencing

MOTIVATION

Identification of DNA sequence variations such as single nucleotide polymorphisms (SNPs) is a fundamental step toward genetic studies. Reduced-representation sequencing methods have been developed as alternatives to whole genome sequencing to reduce costs and enable the analysis of many more individual. Amongst these methods, restriction site associated sequencing (RSAS) methodologies have been widely used for rapid and cost-effective discovery of SNPs and for high-throughput genotyping in a wide range of species. Despite the extensive improvements of the RSAS methods in the last decade, the estimation of the number of reads (i.e. read depth) required per sample for an efficient and effective genotyping remains mostly based on trial and error.

RESULTS

Herein we describe a bioinformatics tool, DepthFinder, designed to estimate the required read counts for RSAS methods. To illustrate its performance, we estimated required read counts in six different species (human, cattle, spruce budworm, salmon, barley and soybean) that cover a range of different biological (genome size, level of genome complexity, level of DNA methylation and ploidy) and technical (library preparation protocol and sequencing platform) factors. To assess the prediction accuracy of DepthFinder, we compared DepthFinder-derived results with independent datasets obtained from an RSAS experiment. This analysis yielded estimated accuracies of nearly 94%. Moreover, we present DepthFinder as a powerful tool to predict the most effective size selection interval in RSAS work. We conclude that DepthFinder constitutes an efficient, reliable and useful tool for a broad array of users in different research communities.

AVAILABILITY AND IMPLEMENTATION

https://bitbucket.org/jerlar73/DepthFinder.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

NanoGBS: A Miniaturized Procedure for GBS Library Preparation

High-throughput reduced-representation sequencing (RRS)-based genotyping methods, such as genotyping-by-sequencing (GBS), have provided attractive genotyping solutions in numerous species. Here, we present NanoGBS, a miniaturized and eco-friendly method for GBS library construction. Using acoustic droplet ejection (ADE) technology, NanoGBS libraries were constructed in tenfold smaller volumes compared to standard methods (StdGBS) and leading to a reduced use of plastics of up to 90%. A high-quality DNA library and SNP catalogue were obtained with extensive overlap (96%) in SNP loci and 100% agreement in genotype calls compared to the StdGBS dataset with a high level of accuracy (98.5%). A highly multiplexed pool of GBS libraries (768-plex) was sequenced on a single Ion Proton PI chip and yielded enough SNPs (~4K SNPs; 1.5 SNP per cM, on average) for many high-volume applications. Combining NanoGBS library preparation and increased multiplexing can dramatically reduce (72%) genotyping cost per sample. We believe that this approach will greatly facilitate the adoption of marker applications where extremely high throughputs are required and cost is still currently limiting.

Current Status of Potato Cyst Nematodes in North America

The potato cyst nematodes (PCNs) Globodera rostochiensis and Globodera pallida are internationally recognized quarantine pests. Although not widely distributed in either the United States or Canada, both are present and are regulated by the national plant protection organizations (NPPOs) of each country. G. rostochiensis was first discovered in New York in the 1940s, and G. pallida was first detected in a limited area of Idaho in 2006. In Canada, G. rostochiensis and G. pallida were first detected in Newfoundland in 1962 and 1977, respectively, and further detections of G. rostochiensis occurred in British Columbia and Québec, most recently in 2006. Adherence to a stringent NPPO-agreed-upon phytosanitary program has prevented the spread of PCNs to other potato-growing areas in both countries. The successful research and regulatory PCN programs in both countries rely on a network of state, federal, university, and private industry cooperatorspursuing a common goal of containment, management/eradication, and regulation. The regulatory and research efforts of these collaborative groups spanning from the 1940s to the present are highlighted in this review.

"Piler Dirt" Survey for the Sampling and Detection of Potato Cyst Nematodes

Potato cyst nematodes are a significant threat to potato production worldwide and have important economic impacts due to yield losses but also because of the expenses associated with regulation procedures. In order to reduce the sampling labor, an alternative strategy named the "Piler Dirt" that collects the soil carried with potato tubers during their transfer to storage was proposed. The method showed a better sensitivity than the reference method to detect fields infested with G. rostochiensis. The quantification of the number of cysts per kilogram of soil was proportional between the two methods at low and moderate population densities (R² = 0.885) but no correlations were found at high density. However, the quantity of soil generated by the method was exceedingly large to be treated by diagnostic labs. It was shown that subsampling six aliquots, each equivalent to 5,000 cm³/ha, from the total quantity of soil generated by the Piler Dirt method, resulted in a probability of 97% to detect infested fields, 95% of the time in our dataset. Overall, Piler Dirt appears as a good compromise to reduce labor time and cost without significantly affecting sensitivity. However, it will be challenging to implement because it needs to be done simultaneously with harvest and will require the participation of farmers during a busy period.

Microsatellite markers reveal two genetic groups in European populations of the carrot cyst nematode Heterodera carotae

The cyst nematode Heterodera carotae, which parasitizes carrot roots, has been recorded in many countries in Europe (Italy, The Netherlands, Switzerland, France, Denmark, …), in South Africa and in North America (Canada, USA). To date, there is a lack of knowledge about the genetic structure of the populations of this economically important nematode. The aim of this work was to study the structuration of the genetic diversity of the carrot cyst nematode at the European scale. We have developed a set of thirteen polymorphic microsatellite markers and used it to genotype seventeen European populations of H. carotae coming from France, Switzerland, Italy, Denmark and one non-European population from Canada. As previously showed for other cyst nematode species, the H. carotae populations were characterised by a strong heterozygote deficit. A Bayesian clustering analysis revealed two distinct genetic clusters, with one group located in the north of Europe and a second one located in the south of Europe. Moreover, our results highlighted rather limited gene flow at small spatial scale and some events of long distance migration. This first investigation of the genetic diversity of H. carotae populations would be useful to develop sustainable control strategies.

Molecular Assessment of the Introduction and Spread of Potato Cyst Nematode, Globodera rostochiensis, in Victoria, Australia

Potato cyst nematodes (PCN) are damaging soilborne quarantine pests of potato in many parts of the world. There are two recognized species, Globodera pallida and G. rostochiensis, with only the latter species-the golden cyst nematode-present in Australia. PCN was first discovered in Australia in 1986 in Western Australia, where it was subsequently eradicated and area freedom for market access was reinstated. In Victoria, PCN was first detected in 1991 east of Melbourne. Since then, it has been found in a small number of localized regions to the south and east. Strict quarantine controls have been in place since each new detection. It has previously been speculated that there were multiple separate introductions of PCN into Victoria. Our study utilized a historic (years 2001 to 2014) PCN cyst reference collection to examine genetic variability of Victorian PCN populations to investigate potential historical origins and subsequent changes in the populations that might inform patterns of spread. DNA was extracted from single larvae dissected from eggs within cysts and screened using nine previously described polymorphic microsatellite markers in two multiplex polymerase chain reaction assays. Sequence variation of the internal transcribed spacer region of the DNA was also assessed and compared with previously published data. A hierarchical sampling strategy was used, comparing variability of larvae within cysts, within paddocks, and between local regions. This sampling revealed very little differentiation between Victorian populations, which share the same microsatellite allelic variation, with differences between local regions probably reflecting changes in allele frequencies over time. Our molecular assessment supports a probable single localized introduction into Victoria followed by limited spread to nearby areas. The Australian PCN examined appear genetically distinct from populations previously sampled worldwide; thus, any new exotic incursions, potentially bringing in additional PCN pathotypes, should be easily differentiated from existing established local PCN populations.

Host Range and Genetic Characterization of Ditylenchus dipsaci Populations from Eastern Canada

The stem and bulb nematode, Ditylenchus dipsaci, is a plant-parasitic nematode affecting over 500 plant species worldwide. Since 2012, garlic producers from Ontario and Quebec have been particularly affected with economic losses caused by this pest. Reproduction of D. dipsaci on a particular host depends on its biological race, and races are unknown for these populations from eastern Canada. As a polyphagous pest, D. dipsaci can possibly be a threat and have negative impact on many crops grown in Quebec, such as field and vegetable crops (e.g., onion). In this study, the host range of four populations of D. dipsaci from Quebec and Ontario was determined in a greenhouse experiment using 11 crops. Garlic, onion, and green onion showed high susceptibility to the nematode, whereas reproduction on potato was poor. No reproduction was observed on corn, soybean, barley, alfalfa, mustard, carrot, and lettuce. These crops could therefore be used as rotational crops in a control program. Thirty-two populations of D. dipsaci were also genetically characterized using genotyping-by-sequencing. The comparison of allele frequencies at 481 loci showed that most of the populations had a genotype similar to a reference population from northern Ontario. However, a sample from eastern Quebec exhibited a distinct genotype and will require further phenotyping in a greenhouse to preclude the possibility of a different race.

Temporal changes in genetic diversity and forage yield of perennial ryegrass in monoculture and in combination with red clover in swards

Agricultural grasslands are often cultivated as mixtures of grasses and legumes, and an extensive body of literature is available regarding interspecific interactions, and how these relate to yield and agronomic performance. However, knowledge of the impact of intraspecific diversity on grassland functioning is scarce. We investigated these effects during a 4-year field trial established with perennial ryegrass (Lolium perenne) and red clover (Trifolium pratense). We simulated different levels of intraspecific functional diversity by sowing single cultivars or by combining cultivars with contrasting growth habits, in monospecific or bispecific settings (i.e. perennial ryegrass whether or not in combination with red clover). Replicate field plots were established for seven seed compositions. We determined yield parameters and monitored differences in genetic diversity in the ryegrass component among seed compositions, and temporal changes in the genetic composition and genetic diversity at the within plot level. The composition of cultivars of both species affected the yield and species abundance. In general, the presence of clover had a positive effect on the yield. The cultivar composition of the ryegrass component had a significant effect on the yield, both in monoculture, and in combination with clover. For the genetic analyses, we validated empirically that genotyping-by-sequencing of pooled samples (pool-GBS) is a suitable method for accurate measurement of population allele frequencies, and obtained a dataset of 22,324 SNPs with complete data. We present a method to investigate the temporal dynamics of cultivars in seed mixtures grown under field conditions, and show how cultivar abundances vary during subsequent years. We screened the SNP panel for outlier loci, putatively under selection during the cultivation period, but none were detected.

Genetic diversity and phylogeny of South African Meloidogyne populations using genotyping by sequencing

Meloidogyne species cause great crop losses worldwide. Although genetic host plant resistance is an effective control strategy to minimize damage caused by Meloidogyne, some resistant genes are ineffective against virulent species such as Meloidogyne enterolobii. Detailed knowledge about the genetic composition of Meloidogyne species is thus essential. This study focused on genotyping-by-sequencing (GBS) and Pool-Seq to elucidate the genetic relation between South African M. enterolobii, M. incognita and M. javanica populations. Hence, 653 common single nucleotide polymorphisms (SNPs) were identified and used to compare these species at genetic level. Allele frequencies of 34 SNPs consistently differed between the three Meloidogyne species studied. Principal component and phylogenetic analyses grouped the M. enterolobii populations in one clade, showing a distant relation to the M. javanica populations. These two species also shared genetic links with the M. incognita populations studied. GBS has been used successfully in this study to identify SNPs that discriminated among the three Meloidogyne species investigated. Alleles, only occurring in the genome of M. enterolobii and located in genes involved in virulence in other animal species (e.g. a serine/threonine phosphatase and zinc finger) have also been identified, accentuating the value of GBS in future studies of this nature.

Genome Scans Reveal Homogenization and Local Adaptations in Populations of the Soybean Cyst Nematode

Determining the adaptive potential of alien invasive species in a new environment is a key concern for risk assessment. As climate change is affecting local climatic conditions, widespread modifications in species distribution are expected. Therefore, the genetic mechanisms underlying local adaptations must be understood in order to predict future species distribution. The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is a major pathogen of soybean that was accidentally introduced in most soybean-producing countries. In this study, we explored patterns of genetic exchange between North American populations of SCN and the effect of isolation by geographical distance. Genotyping-by-sequencing was used to sequence and compare 64 SCN populations from the United States and Canada. At large scale, only a weak correlation was found between genetic distance (Wright's fixation index, FST) and geographic distance, but local effects were strong in recently infested states. Our results also showed a high level of genetic differentiation within some populations, allowing them to adapt to new environments and become established in new soybean-producing areas. Bayesian genome scan methods identified 15 loci under selection for climatic or geographic co-variables. Among these loci, two non-synonymous mutations were detected in SMAD-4 (mothers against decapentaplegic homolog 4) and DOP-3 (dopamine receptor 3). High-impact variants linked to these loci by genetic hitchhiking were also highlighted as putatively involved in local adaptation of SCN populations to new environments. Overall, it appears that strong selective pressure by resistant cultivars is causing a large scale homogenization with virulent populations.

Trait variation in response to varying winter temperatures, diversity patterns and signatures of selection along the latitudinal distribution of the widespread grassland plant Arrhenatherum elatius

Across Europe, genetic diversity can be expected to decline toward the North because of stochastic and selective effects which may imply diminished phenotypic variation and less potential for future genetic adaptations to environmental change. Understanding such latitudinal patterns can aid provenance selection for breeding or assisted migration approaches. In an experiment simulating different winter temperatures, we assessed quantitative trait variation, genetic diversity, and differentiation for natural populations of the grass Arrhenatherum elatius originating from a large latitudinal gradient. In general, populations from the North grew smaller and had a lower flowering probability. Toward the North, the absolute plastic response to the different winter conditions as well as heritability for biomass production significantly declined. Genetic differentiation in plant height and probability of flowering were very strong and significantly higher than under neutral expectations derived from SNP data, suggesting adaptive differentiation. Differentiation in biomass production did not exceed but mirrored patterns for neutral genetic differentiation, suggesting that migration‐related processes caused the observed clinal trait variation. Our results demonstrate that genetic diversity and trait differentiation patterns for A. elatius along a latitudinal gradient are likely shaped by both local selection and genetic drift.

The genome of the yellow potato cyst nematode, Globodera rostochiensis, reveals insights into the basis of parasitism and virulence

Background

The yellow potato cyst nematode, Globodera rostochiensis, is a devastating plant pathogen of global economic importance. This biotrophic parasite secretes effectors from pharyngeal glands, some of which were acquired by horizontal gene transfer, to manipulate host processes and promote parasitism. G. rostochiensis is classified into pathotypes with different plant resistance-breaking phenotypes.

Results

We generate a high quality genome assembly for G. rostochiensis pathotype Ro1, identify putative effectors and horizontal gene transfer events, map gene expression through the life cycle focusing on key parasitic transitions and sequence the genomes of eight populations including four additional pathotypes to identify variation. Horizontal gene transfer contributes 3.5 % of the predicted genes, of which approximately 8.5 % are deployed as effectors. Over one-third of all effector genes are clustered in 21 putative ‘effector islands’ in the genome. We identify a dorsal gland promoter element motif (termed DOG Box) present upstream in representatives from 26 out of 28 dorsal gland effector families, and predict a putative effector superset associated with this motif. We validate gland cell expression in two novel genes by in situ hybridisation and catalogue dorsal gland promoter element-containing effectors from available cyst nematode genomes. Comparison of effector diversity between pathotypes highlights correlation with plant resistance-breaking.

Conclusions

These G. rostochiensis genome resources will facilitate major advances in understanding nematode plant-parasitism. Dorsal gland promoter element-containing effectors are at the front line of the evolutionary arms race between plant and parasite and the ability to predict gland cell expression a priori promises rapid advances in understanding their roles and mechanisms of action.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-016-0985-1) contains supplementary material, which is available to authorized users.

A metagenetic approach to determine the diversity and distribution of cyst nematodes at the level of the country, the field and the individual

Distinct populations of the potato cyst nematode (PCN) Globodera pallida exist in the UK that differ in their ability to overcome various sources of resistance. An efficient method for distinguishing between populations would enable pathogen‐informed cultivar choice in the field. Science and Advice for Scottish Agriculture (SASA) annually undertake national DNA diagnostic tests to determine the presence of PCN in potato seed and ware land by extracting DNA from soil floats. These DNA samples provide a unique resource for monitoring the distribution of PCN and further interrogation of the diversity within species.

We identify a region of mitochondrial DNA descriptive of three main groups of G. pallida present in the UK and adopt a metagenetic approach to the sequencing and analysis of all SASA samples simultaneously. Using this approach, we describe the distribution of G. pallida mitotypes across Scotland with field‐scale resolution. Most fields contain a single mitotype, one‐fifth contain a mix of mitotypes, and less than 3% contain all three mitotypes. Within mixed fields, we were able to quantify the relative abundance of each mitotype across an order of magnitude. Local areas within mixed fields are dominated by certain mitotypes and indicate towards a complex underlying ‘pathoscape’. Finally, we assess mitotype distribution at the level of the individual cyst and provide evidence of ‘hybrids’. This study provides a method for accurate, quantitative and high‐throughput typing of up to one thousand fields simultaneously, while revealing novel insights into the national genetic variability of an economically important plant parasite.