Genetic mapping and QTL analysis in European hazelnut (Corylus avellana L.)

Association mapping and candidate gene identification for yield traits in European hazelnut (Corylus avellana L.)

European hazelnut (Corylus avellana L.) is an important nut crop due to its nutritional benefits, culinary uses, and economic value. Türkiye is the leading producer of hazelnut, followed by Italy and the United States. Quantitative trait locus studies offer promising opportunities for breeders and geneticists to identify genomic regions controlling desirable traits in hazelnut. A genome-wide association analysis was conducted with 5,567 single nucleotide polymorphisms on a Turkish core set of 86 hazelnut accessions, revealing 189 quantitative trait nucleotides (QTNs) associated with 22 of 31 traits (p < 2.9E-07). These QTNs were associated with plant and leaf, phenological, reproductive, nut, and kernel traits. Based on the close physical distance of QTNs associated with the same trait, we identified 23 quantitative trait loci. Furthermore, we identified 23 loci of multiple QTs comprising chromosome locations associated with more than one trait at the same position or in close proximity. A total of 159 candidate genes were identified for 189 QTNs, with 122 of them containing significant conserved protein domains. Some candidate matches to known proteins/domains were highly significant, suggesting that they have similar functions as their matches. This comprehensive study provides valuable insights for the development of breeding strategies and the improvement of hazelnut and enhances the understanding of the genetic architecture of complex traits by proposing candidate genes and potential functions.

A roadmap of tissue culture and biotechnology in European hazelnut (Corylus avellana L.)

The increasing interest in European hazelnut (Corylus avellana L.) cultivation registered in the last years has led to a significant increase in worldwide hazelnut growing areas, also involving regions characterized by a marginal presence of hazelnut orchards. Despite this increasement, world production still relies on the cultivation of few varieties, most of which are particularly suitable to the environment where they have been selected. Therefore, it is necessary to develop new cultivars with high environmental plasticity capable of providing constant and high-quality productions in the new environments and under the climatic change conditions of traditional growing areas. Over the years, many molecular markers for genetic breeding programs have been developed and omics sciences also provided further information about the genetics of this species. These data could be of support to the application of new plant breeding techniques (NPBTs), which would allow the development of cultivars with the desired characteristics in a shorter time than traditional techniques. However, the application of these methodologies is subordinated to the development of effective regeneration protocols which, to date, have been set up exclusively for seed-derived explants. A further aspect to be exploited is represented by the possibility of cultivating hazelnut cells and tissues in vitro to produce secondary metabolites of therapeutic interest. This review aims to consolidate the state of the art on biotechnologies and in vitro culture techniques applied on this species, also describing the various studies that over time allowed the identification of genomic regions that control traits of interest.

Marker-Assisted Selection in Breeding for Fruit Trait Improvement: A Review

Breeding fruit species is time-consuming and expensive. With few exceptions, trees are likely the worst species to work with in terms of genetics and breeding. Most are characterized by large trees, long juvenile periods, and intensive agricultural practice, and environmental variability plays an important role in the heritability evaluations of every single important trait. Although vegetative propagation allows for the production of a significant number of clonal replicates for the evaluation of environmental effects and genotype × environment interactions, the spaces required for plant cultivation and the intensity of work necessary for phenotypic surveys slow down the work of researchers. Fruit breeders are very often interested in fruit traits: size, weight, sugar and acid content, ripening time, fruit storability, and post-harvest practices, among other traits relevant to each individual species. The translation of trait loci and whole-genome sequences into diagnostic genetic markers that are effective and affordable for use by breeders, who must choose genetically superior parents and subsequently choose genetically superior individuals among their progeny, is one of the most difficult tasks still facing tree fruit geneticists. The availability of updated sequencing techniques and powerful software tools offered the opportunity to mine tens of fruit genomes to find out sequence variants potentially useful as molecular markers. This review is devoted to analysing what has been the role of molecular markers in assisting breeders in selection processes, with an emphasis on the fruit traits of the most important fruit crops for which examples of trustworthy molecular markers have been developed, such as the MDo.chr9.4 marker for red skin colour in apples, the CCD4-based marker CPRFC1, and LG3_13.146 marker for flesh colour in peaches, papayas, and cherries, respectively.

Whole-genome assembly of Corylus avellana cv'Tonda Gentile delle Langhe' using linked-reads (10X Genomics)

The European hazelnut (Corylus avellana L.; 2n = 2x = 22) is a worldwide economically important tree nut that is cross-pollinated due to sporophytic incompatibility. Therefore, any individual plant is highly heterozygous. Cultivars are clonally propagated using mound layering, rooted suckers, and micropropagation. In recent years, the interest in this crop has increased, due to a growing demand related to the recognized health benefits of nut consumption. C. avellana cv “Tonda Gentile delle Langhe” (“TGdL”) is well-known for its high kernel quality, and the premium price paid for this cultivar is an economic benefit for producers in northern Italy. Assembly of a high-quality genome is a difficult task in many plant species because of the high level of heterozygosity. We assembled a chromosome-level genome sequence of “TGdL” with a two-step approach. First, 10X Genomics Chromium Technology was used to create a high-quality sequence, which was then assembled into scaffolds with cv “Tombul” genome as the reference. Eleven pseudomolecules were obtained, corresponding to 11 chromosomes. A total of 11,046 scaffolds remained unplaced, representing 11% of the genome (46,504,161 bp). Gene prediction, performed with Maker-P software, identified 27,791 genes (AED ≤0.4 and 92% of BUSCO completeness), whose function was analyzed with BlastP and InterProScan software. To characterize “TGdL” specific genetic mechanisms, Orthofinder was used to detect orthologs between hazelnut and closely related species. The “TGdL” genome sequence is expected to be a powerful tool to understand hazelnut genetics and allow detection of markers/genes for important traits to be used in targeted breeding programs.

Mapping the Genetic Regions Responsible for Key Phenology-Related Traits in the European Hazelnut

An increasing interest in the cultivation of (European) hazelnut (Corylus avellana) is driving a demand to breed cultivars adapted to non-conventional environments, particularly in the context of incipient climate change. Given that plant phenology is so strongly determined by genotype, a rational approach to support these breeding efforts will be to identify quantitative trait loci (QTLs) and the genes underlying the basis for adaptation. The present study was designed to map QTLs for phenology-related traits, such as the timing of both male and female flowering, dichogamy, and the period required for nuts to reach maturity. The analysis took advantage of an existing linkage map developed from a population of F₁ progeny bred from the cross "Tonda Gentile delle Langhe" × "Merveille de Bollwiller," consisting in 11 LG. A total of 42 QTL-harboring regions were identified. Overall, 71 QTLs were detected, 49 on the TGdL map and 22 on the MB map; among these, 21 were classified as major; 13 were detected in at least two of the seasons (stable-major QTL). In detail, 20 QTLs were identified as contributing to the time of male flowering, 15 to time of female flowering, 25 to dichogamy, and 11 to time of nut maturity. LG02 was found to harbor 16 QTLs, while 15 QTLs mapped to LG10 and 14 to LG03. Many of the QTLs were clustered with one another. The major cluster was located on TGdL_02 and consisted of mainly major QTLs governing all the analyzed traits. A search of the key genomic regions revealed 22 candidate genes underlying the set of traits being investigated. Many of them have been described in the literature as involved in processes related to flowering, control of dormancy, budburst, the switch from vegetative to reproductive growth, or the morphogenesis of flowers and seeds.

Construction of a high-density genetic map and QTL mapping of leaf traits and plant growth in an interspecific F1 population of Catalpa bungei × Catalpa duclouxii Dode

BACKGROUND

Catalpa bungei is an important tree species used for timber in China and widely cultivated for economic and ornamental purposes. A high-density linkage map of C. bungei would be an efficient tool not only for identifying key quantitative trait loci (QTLs) that affect important traits, such as plant growth and leaf traits, but also for other genetic studies.

RESULTS

Restriction site-associated DNA sequencing (RAD-seq) was used to identify molecular markers and construct a genetic map. Approximately 280.77 Gb of clean data were obtained after sequencing, and in total, 25,614,295 single nucleotide polymorphisms (SNPs) and 2,871,647 insertions-deletions (InDels) were initially identified in the genomes of 200 individuals of a C. bungei (7080) × Catalpa duclouxii (16-PJ-3) F₁ population and their parents. Finally, 9072 SNP and 521 InDel markers that satisfied the requirements for constructing a genetic map were obtained. The integrated genetic map contained 9593 pleomorphic markers in 20 linkage groups and spanned 3151.63 cM, with an average distance between adjacent markers of 0.32 cM. Twenty QTLs for seven leaf traits and 13 QTLs for plant height at five successive time points were identified using our genetic map by inclusive composite interval mapping (ICIM). Q16-60 was identified as a QTL for five leaf traits, and three significant QTLs (Q9-1, Q18-66 and Q18-73) associated with plant growth were detected at least twice. Genome annotation suggested that a cyclin gene participates in leaf trait development, while the growth of C. bungei may be influenced by CDC48C and genes associated with phytohormone synthesis.

CONCLUSIONS

This is the first genetic map constructed in C. bungei and will be a useful tool for further genetic study, molecular marker-assisted breeding and genome assembly.

Comparison of different annotation tools for characterization of the complete chloroplast genome of Corylus avellana cv Tombul

BACKGROUND

Several bioinformatics tools have been designed for assembly and annotation of chloroplast (cp) genomes, making it difficult to decide which is most useful and applicable to a specific case. The increasing number of plant genomes provide an opportunity to accurately obtain cp genomes from whole genome shotgun (WGS) sequences. Due to the limited genetic information available for European hazelnut (Corylus avellana L.) and as part of a genome sequencing project, we analyzed the complete chloroplast genome of the cultivar 'Tombul' with multiple annotation tools.

RESULTS

Three different annotation strategies were tested, and the complete cp genome of C. avellana cv Tombul was constructed, which was 161,667 bp in length, and had a typical quadripartite structure. A large single copy (LSC) region of 90,198 bp and a small single copy (SSC) region of 18,733 bp were separated by a pair of inverted repeat (IR) regions of 26,368 bp. In total, 125 predicted functional genes were annotated, including 76 protein-coding, 25 tRNA, and 4 rRNA unique genes. Comparative genomics indicated that the cp genome sequences were relatively highly conserved in species belonging to the same order. However, there were still some variations, especially in intergenic regions, that could be used as molecular markers for analyses of phylogeny and plant identification. Simple sequence repeat (SSR) analysis showed that there were 83 SSRs in the cp genome of cv Tombul. Phylogenetic analysis suggested that C. avellana cv Tombul had a close affinity to the sister group of C. fargesii and C. chinensis, and then a closer evolutionary relationship with Betulaceae family than other species of Fagales.

CONCLUSION

In this study, the complete cp genome of Corylus avellana cv Tombul, the most widely cultivated variety in Turkey, was obtained and annotated, and additionally phylogenetic relationships were predicted among Fagales species. Our results suggest a very accurate assembly of chloroplast genome from next generation whole genome shotgun (WGS) sequences. Enhancement of taxon sampling in Corylus species provide genomic insights into phylogenetic analyses. The nucleotide sequences of cv Tombul cp genomes can provide comprehensive genetic insight into the evolution of genus Corylus.

Genetic diversity and structure in the northern populations of European hazelnut (Corylus avellana L.)

European hazelnut (Corylus avellana L.) is a strictly cross-pollinated diploid tree species, which has its northernmost populations in Fennoscandia, and it was one of the first species to recolonize northern Europe after the last ice age. Hazelnut produces edible nuts in Finland but nowadays they are underutilized as food, and currently no breeding programmes exist. In the present study, 300 hazelnut specimens were collected from 20 different locations (= populations) in Finland, and they were genetically analyzed using nine simple sequence repeat (SSR) markers. Most of the genetic diversity existed within populations (83%). According to different genetic analyses (STRUCTURE, principal coordinates analysis, and clustering), a general lack of structure was observed, suggesting extensive gene flow among hazelnuts between 17 investigated populations. However, genetic structuring was clearly observed in three populations: Hakavuori, Mustiala, and Pähkinämäki, which might have become isolated due to geographical barriers that kept them separate, diminishing gene flow from other populations. Studying the diversity of European hazelnut is of great interest for understanding population genetics of a species distributed in its marginal areas in the north, and the results are also valuable for further uses in plant conservation, selection, and possible future breeding actions in Finland.

Analysis of European hazelnut (Corylus avellana) reveals loci for cultivar improvement and the effects of domestication and selection on nut and kernel traits

Turkey is a rich source of European hazelnut (Corylus avellana) germplasm with nearly 400 accessions in the national collection. This genetic material encompasses cultivars, landraces and wild genotypes which were characterized for 12 nut and 13 kernel traits over 2 years in the 1990s. Analysis of these attributes revealed both the positive and negative impacts that human selection and breeding have had on hazelnut. Thus, while selection has resulted in larger nuts and kernels, cultivars have fewer nuts per cluster and kernels with larger internal cavities. Breeding has also resulted in a propensity for cultivars to have higher proportions of double kernels and empty nuts, two traits which reduce quality and yield. In addition, it is clear that while selection has successfully increased hazelnut fat content it has not impacted overall flavor, a much more complex trait. The nut and kernel phenotypic data were combined with genotypic data from 406 simple sequence repeat marker alleles for association mapping of the quantitative trait loci (QTL) for the traits. A total of 78 loci were detected in the population with the highest proportions for nut (24%) and kernel (26%) appearance parameters followed by quality (19%), shell thickness (16%) and yield-related (15%) traits. It is hoped that some of the identified QTL will be useful for future breeding of hazelnut for improved nut and kernel yield and quality.

High density SNP mapping and QTL analysis for time of leaf budburst in Corylus avellana L

The growing area of European hazelnut (Corylus avellana L.) is increasing, as well as the number of producing countries, and there is a pressing need for new improved cultivars. Hazelnut conventional breeding process is slow, due to the length of juvenile phase and the high heterozygosity level. The development of genetic linkage maps and the identification of molecular markers tightly linked to QTL (quantitative trait loci) of agronomic interest are essential tools for speeding up the selection of seedlings carrying desired traits through marker-assisted selection. The objectives of this study were to enrich a previous linkage map and confirm QTL related to time of leaf budburst, using an F₁ population obtained by crossing Tonda Gentile delle Langhe with Merveille de Bollwiller. Genotyping-by-Sequencing was used to identify a total of 9,999 single nucleotide polymorphism markers. Well saturated linkage maps were constructed for each parent using the double pseudo-testcross mapping strategy. A reciprocal translocation was detected in Tonda Gentile delle Langhe between two non-homologous chromosomes. Applying a bioinformatic approach, we were able to disentangle ‘pseudo-linkage’ between markers, removing markers around the translocation breakpoints and obtain a linear order of the markers for the two chromosomes arms, for each linkage group involved in the translocation. Twenty-nine QTL for time of leaf budburst were identified, including a stably expressed region on LG_02 of the Tonda Gentile delle Langhe map. The stability of these QTL and their coding sequence content indicates promise for the identification of specific chromosomal regions carrying key genes involved in leaf budburst.

Comparative Genomics and Phylogenetic Analysis Revealed the Chloroplast Genome Variation and Interspecific Relationships of Corylus (Betulaceae) Species

Corylus L. is an economically and phylogenetically important genus in the family Betulaceae. Taxonomic and phylogenetic relationships of Corylus species have long been controversial for lack of effective molecular markers. In this study, the complete chloroplast (cp) genomes of six Corylus species were assembled and characterized using next-generation sequencing. We compared the genome features, repeat sequences, sequence divergence, and constructed the phylogenetic relationships of the six Corylus species. The results indicated that Corylus cp genomes were typical of the standard double-stranded DNA molecule, ranging from 160,445 base pairs (bp) (C. ferox var. thibetca) to 161,621 bp (C. yunnanensis) in length. Each genome contained a pair of inverted repeats (IRs), a large single-copy (LSC) region and a small single-copy (SSC) region. Each of the six cp genomes possessed 113 unique genes arranged in the same order, including 80 protein-coding, 29 tRNA, and 4 rRNA genes. C. yunnanensis contained the highest number of repeat sequences, and the richest SSRs in six cp genomes were A/T mononucleotides. Comparative analyses of six Corylus cp genomes revealed four hotspot regions (trnH-psbA, rpoB-trnC, trnF-ndhJ, and rpl32-trnL) that could be used as potential molecular markers. Phylogenetic analyses of the complete chloroplast genomes and 80 protein-coding genes exhibited nearly identical topologies that strongly supported the monophyly of Corylus and simultaneously revealed the generic relationships among Betulaceae. The availability of these genomes can offer valuable genetic information for further taxonomy, phylogeny, and species delimitation in Corylus or even Betulaceae plants.

In silico development and characterization of tri-nucleotide simple sequence repeat markers in hazelnut (Corylus avellana L.)

Plant genomes are now sequenced rapidly and inexpensively. In silico approaches allow efficient development of simple sequence repeat markers, also known as microsatellite markers, from these sequences. A search of the genome sequence of 'Jefferson' hazelnut (Corylus avellana L.) identified 8,708 tri-nucleotide simple sequence repeats with at least five repeat units, and stepwise removal of the less promising sequences led to the development of 150 polymorphic markers. Fragments in the 'Jefferson' sequence containing tri-nucleotide repeats were used as references and aligned with genomic sequences from seven other cultivars. Following in silico alignment, sequences that showed variation in number of repeat units were selected and primer pairs were designed for 243 of them. Screening on agarose gels identified 173 as polymorphic. Removal of duplicate and previously published sequences reduced the number to 150, for which fluorescent primers and capillary electrophoresis were used for amplicon sizing. These were characterized using 50 diverse hazelnut accessions. Of the 150, 132 generated the expected one or two alleles per accession while 18 amplified more than two amplicons in at least one accession. Diversity parameters of the 132 marker loci averaged 4.73 for number of alleles, 0.51 for expected heterozygosity (He), 0.49 for observed heterozygosity (Ho), 0.46 for polymorphism information content (PIC), and 0.04 for frequency of null alleles. The clustering of the 50 accessions in a dendrogram constructed from the 150 markers confirmed the wide genetic diversity and presence of three of the four major geographic groups: Central European, Black Sea, and Spanish-Italian. In the mapping population, 105 loci segregated, of which 101 were assigned to a linkage group (LG), with positions well-dispersed across all 11 LGs. These new markers will be useful for cultivar fingerprinting, diversity studies, genome comparisons, mapping, and alignment of the linkage map with the genome sequence and physical map.