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.