The Juglandaceae family, more commonly know as the Walnut family, comprises approximately 60 known species, is home to several internationally significant nut crops like the Persian walnut (Juglans regia), pecan (Carya illinoinensis), and Chinese hickory (Carya cathayensis), along with valuable hardwood species such as the black walnut (Juglans nigra). The genus Carya, a part of this family, consists of about 20 species distributed worldwide, with an intriguing disjunctive distribution between East Asia (EA) and eastern North America (ENA). Notably, pecan and Chinese hickory represent this genus in ENA and EA, respectively, and they are the only two commercially cultivated nut trees in this group.

In recent years, the consumption of pecans and Chinese hickory has witnessed a significant surge due to their high nutritional value and associated health benefits. These nuts stand out due to their rich content of monounsaturated fatty acids, antioxidants, and an array of phytochemicals, particularly phenolic compounds. Furthermore, they are excellent sources of dietary fiber, protein, minerals, and B vitamins, including thiamine. Recent studies have shed light on the potential health advantages of including these nuts in one's diet, as they may reduce the risk of various diseases such as tumors, edema, hyperglycemia, and hyperlipidemia. As a result of these health benefits, there has been a considerable expansion in the cultivation of pecans and Chinese hickory.

In the United States, pecans yield over 130,000 tons annually, with a market value exceeding $600 million. Similarly, Chinese hickory production in China stood at nearly 30,000 tons annually until 2010, with a farm gate value of over $125 million per year. By 2017, annual production in China had exceeded 40,000 tons, according to the Chinese State Administration of Forestry and Grassland (unpublished data).

The pecan, native to the river bottomlands in the USA and Mexico, exhibits a broad range of climate adaptability, from 30° to 42°N latitude, with preferences for loamy, well-drained river bottom land. Despite being known and cherished among Native and colonial Americans as a delicacy, commercial pecan production in the USA did not commence until the 1880s. This period also marked the inception of pecan research activities by the US Department of Agriculture. The USDA's National Collection of Genetic Resources for Pecans and Hickories (NCGR-Carya) maintains more than 400 pecan cultivars from 25 US states and Mexico, with some of these cultivars being widely planted worldwide. In 2016, Mexico and the USA accounted for 93% of the world's pecan production, with other countries contributing the remaining 7%.

Chinese hickory, on the other hand, is a specialty crop in the Hangzhou region of Zhejiang province, China, where it has been cultivated for over 500 years, dating back to the Ming Dynasty. Both wild and domesticated Chinese hickories grow exclusively in moist valleys at the foothills of the Tianmu Mountains at elevations ranging from 500 to 1200 meters within the Zhejiang and Anhui provinces. This location provides them with full sun in sheltered positions. However, Chinese hickory is less adaptable to colder climates when compared to pecan, characterized by its smaller nuts, harder shells, and lower tolerance to various abiotic stresses such as heat, flooding, drought, and salinity. Consequently, the breeding of Chinese hickory has lagged behind pecan and has largely remained at domestication levels until recent years. Nevertheless, the species holds the unique characteristic of nucellar embryony (apomixis), which could be valuable for passing down disease-resistant traits during production.

Both pecan and Chinese hickory breeding programs share a common mission: to preserve, evaluate, and enhance genetic resources while developing superior cultivars with resistance to diseases, insects, and various abiotic stresses, alongside excellent nut quality. Several outstanding cultivars have been developed in both species, exhibiting desirable traits such as precocity, high yields, disease and stress resistance, and high nut and kernel quality. Conventional breeding methods typically require over 20 years to release a new cultivar due to extended periods of juvenility. However, advancements in modern biotechnologies, including genome sequence-based whole-genome-associated analysis and gene editing, offer the potential to expedite the selection process.

In a groundbreaking move to accelerate nut tree improvement in Carya, the whole-genome sequencing of reference genomes for pecan and Chinese hickory was initiated. This endeavor aimed to unlock the genetic secrets that could revolutionize breeding efforts and enhance the nutritional quality of these nuts. The project involved the sequencing of two Carya genotypes: the popular pecan cultivar "Pawnee" and a widely planted representative of Chinese hickory, "ZAFU-1." Additionally, 16 Carya species from both EA and ENA, as well as two Juglans species (used as out-groups), were re-sequenced. Employing a hybrid assembly strategy, the researchers successfully generated high-quality draft genomes for the Carya species. This comprehensive genomic analysis provides critical insights into the evolutionary history of these two genomes, their ability to adapt to biotic and abiotic stresses, and their capacity to accumulate essential nutrients like oils, polyphenols, amino acids, and B vitamins.

Read more: The genomes of pecan and Chinese hickory provide insights into Carya evolution and nut nutrition