1 INTRODUCTION
Walnut (Juglans L.) is the most important and valuable genus in the woody plant family Juglandaceae. Walnuts are grown worldwide for their edible nuts and high-quality wood (Feng et al., 2018; Zhang et al., 2019). Chinese walnut (J. cathayensis Dode) is an ecologically important, wind pollinated, endemic species that grows between China’s tropical zone (<22°N) and the Qinling Mountains/Huai River line (~34°N) in hilly mid-elevation areas (Zhang et al., 2015; Bai et al., 2014; Dang et al., 2015). It is a diploid plant with 16 chromosomes (2n = 2x = 32) that belongs to the group of species called Asian butternuts (section Cardiocaryon ) that also includes Japanese walnut (J. ailantifolia ) and Manchurian walnut (J. mandshurica ) (Figure 1; Zhao et al., 2018; Bai et al., 2016; Zhao et al., 2014). Chinese walnut is less valuable as a commodity than its close relative, Juglans regia (Persian or common walnut; Han et al., 2016; Dang et al., 2016; Feng et al., 2018), but Chinese walnut has been evaluated in breeding programs for its resistance to lesion nematodes (Trouern-Trend et al., 2020). In addition, Chinese walnut has potential as a medicinal crop (Yu et al., 2011; Li et al., 2013; Li et al., 2008; Sun et al., 2012; Bi et al., 2016).
The population genetics, morphology, and diversity of J. cathayensis have been described (Manning et al., 1978; Aradhya et al., 2007; Hu et al., 2017; Zhang et al., 2019), but in general, interest in Chinese walnut is based on its potential as a tertiary germplasm pool for improvement of common walnut (Zhou et al., 2017). Chinese walnut expresses horticultural traits such as cluster bearing habit (6–13 fruits per terminal) that make it attractive to Persian walnut breeders, and disease tolerance/resistance as compared to J. regia (Xu et al., 2007; Zhou et al., 2017) that recommend it as a rootstock. Wild populations of Chinese walnut and cultivated orchards of Persian walnut (J. regia ) grow sympatrically (Dang et al., 2019) but hybridization between these two walnut species is reportedly rare (Shu et al., 2016).
A high-quality genome is an important genetic resource for the improvement of horticultural traits in perennial crops (Dong et al., 2019; Zhang et al., 2020a). The availability of high-throughput sequencing has accelerated the publication of the genomes of walnut (Juglans ) species and hybrids (J. regia × J. microcarpa ) (Zhu et al. 2019; Martínez-García et al., 2016; Stevens et al., 2018; Bai et al., 2018; Zhang et al., 2020b). A combination of long reads (Nanopore sequencing platform), Illumina and Hi-C auxiliary assembly can be used to produce a high-quality, chromosome-level genome (Choi et al., 2020; Suryamohan et al., 2020; Zhang et al., 2019). Despite its importance for understanding walnut evolution and its utility for breeding, functional gene mining, and disease resistance, genomic resources for J. cathayensis are minimal. For these reasons, we undertook the assembly of a chromosome-level, high-quality reference genome assembly for Chinese walnut.