Corylus belong to Betulaceae, all bearing edible nuts which collectively known as hazelnuts, are widely used in food processing and the manufacture of confectionery products including chocolate, candy and biscuits. Hazelnuts together with cashews, almonds and walnuts is known as the four major nuts. Hazelnuts not only popular for its flavor, but also have the benefits for human cardiovascular health as the high oleic acid content in nuts. While, the biosynthesis of oleic acid in hazelnuts have not been systematically studied. In addition, the most widely cultivated species (European hazelnut, Corylus avellana) is usually susceptible to diseases such as Eastern filbert blight, and identifying key genes related to stress response from other highly disease-resistant hazelnut species is a feasible way to solve this dilemma. Meanwhile, the six genus within Betulaceae containing a strikingly chromosome number variation, and their evolutionary history is unclear. Therefore, the highly resistant Corylus mandshurica as an excellent wild hazel resource can provide an important resource for breeding of hazelnut, and the high-quality genome is also a key information to clarify the chromosome evolution among different genera of within Betulaceae.

Fig. 1. Corylus mandshurica, leaves, tender fruiting bracts and hazelnuts.

Recently, Horticulture Research published a research paper “The Corylus mandshurica genome provides insights into the evolution of the Betulaceae genome and hazelnut breeding” by Prof. Jian-Quan Liu’s team from Lanzhou University.

This study reported a high-quality chromosome-level reference genome for C. mandshurica. The high-quality genome presented here for hazelnut species helps us to clarify the biosynthesis of oleic acid in hazelnuts, investigate disease resistance in this species, and infer the evolutionary origin of the hazelnut genus. This genomic resource also provides a valuable foundation for improving our agronomic understanding of hazelnut and carrying out molecular breeding in the future, including cloning key genes that control hazelnut traits and identifying disease-resistance alleles.

Corylus mandshurica (2n = 2x = 22) was chosen for genome sequencing and assembly. To accurately assemble this highly heterozygous genome, Illumina, Nanopore and Hi-C technologies were adopted for sequencing, and a series of methods were performed for assembly. Approximately 367.77 Mb of genome sequence was oriented on 11 pseudochromosomes and its Contig N50 of 14.85 Mb. The transposable elements (TEs) accounted for about 68.74% of the genome, and identified 28,409 protein-coding genes.

Fig. 2. Genomic features, and evolutionary and comparative genomic analyses.

Comparative genome analyses were executed to reveal the karyotype evolutionary trajectories of the Betulaceae. And the results indicated that B. pendula karyotype correspond to the ancestral karyotype of the Betulaceae with 14 pair chromosomes, while the other diversity temperate forest species speciation by multiple large regions of chromosome segment rearrangements. Of which, C. mandshurica occupies the second place because of the occurrence of only 2 NCFs, 1 chromosome end-end merge and 1 RTA, and the loss of 3 satellite chromosomes. And the final chromosome number was reduced from 14 to 11 in C. mandshurica.

Fig. 3. Schematic representation of karyotype evolution in the five Betulaceae genomes from the ancestral Betulaceae karyotype and chromosome fusions/fissions during the karyotype evolution of C. mandshurica.

They also identified 764 oil biosynthesis related genes were identified in the C. mandshurica genome. Of these 96 were identified as participating directly in oil biosynthesis. The evolutionary analysis also found that ten genes under rapidly evolution or positively selection. And both SAD and FATA as the key enzymes in oleic acid biosynthesis were included, which may underlie the high oleic acid concentration in hazelnuts. Moreover, several gene families related to stress responses experienced rapid expansion in this hazelnut species, which may have increased the stress tolerance of this species. The reference genome presented here constitutes a valuable resource for molecular breeding and genetic improvement of the important agronomic properties of hazelnut.

Fig. 4. Metabolic pathways and numbers of genes associated with biosynthesis and accumulation of oleic acid and TAG in C. mandshurica.

Young researcher Yong-Zhi Yang is the corresponding author. PhD students Ying Li and Peng-Chuan Sun are first authors. This work was supported equally by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB31010300), the National Key Research and Development Program of China (2017YFC0505203), the National Natural Science Foundation of China (31590821 and 31900201).

Article access address: https://doi.org/10.1038/s41438-021-00495-1