Australian Wild Origins of Macadamia Domestication in Hawaii

Macadamia has an interesting history. Within the last century it journeyed from Australian rainforests through Hawaii to become a global nut crop. While many crops such as rice, wheat, apples and almonds were domesticated thousands of years ago, the earliest macadamia orchards were established just over 100 years ago. The preferred species for commercial production, Macadamia integrifolia, was first introduced to Hawaii in the late 19th century. Its expansion as a commercial crop occurred in Hawaii from the 1920s. Seedling orchards, grafting and breeding in Hawaii produced cultivars that now represent the majority of the trees grown in orchards worldwide.
 
According to historical records, macadamia was taken to Hawaii by W.H. Purvis between 1881 and 1885 with trees planted near Kukuihaele on the Big Island. This was followed by a second introduction by R.A. Jordan in 1892 that was grown in Honolulu on Oahu. There is limited information on the source of the Purvis introduction although three trees growing in Kapulena are thought to be from the original plantings. The Jordan introduction was reportedly M. integrifolia seed from Queensland, Australia, and is represented by an old tree still growing in Honolulu.
 
Macadamia is endemic to the lowland rainforests of subtropical eastern Australia and commercial varieties are only a few generations removed from their wild ancestors. Genetic research was undertaken to try to find out where the seed taken to Hawaii was collected. Dr. Catherine Nock from Southern Cross University, and Dr. Craig Hardner and colleagues from University of Queensland sequenced the chloroplast genome of Hawaiian macadamia varieties and trees from the early introductions as well as wild trees in Australia. The chloroplast genome is maternally inherited and traces the seedline, and has provided valuable insight into the origins of apple and citrus.
 
Most of the Hawaiian varieties were traced back to a site at Mooloo in southeast Queensland that is now thought to be the original source of the seed taken to Hawaii. The research, which was published in Frontiers in Plant Science, shows that the Hawaiian varieties were sourced from a narrow gene pool, and represent a small portion of the genetic diversity available in remnant wild populations.  The brief domestication history and persistence of many natural populations was helpful in pinpointing the wild origins of macadamia. A single chloroplast haplotype was shared by 24 of the 26 Hawaiian varieties and old trees in the study suggesting that they may have originated from seed collected from a single tree. These include some of the most widely grown cultivars and founders of current breeding programs: HAES 246, 294, 333, 344, 425, 508, 660, 741 and Honokaa Special. Only HAES 791 was different and it was most closely related to wild trees from the northernmost population at Mt Bauple.
 
The wild relatives of food crops are an irreplaceable resource for improved food security and production. Identifying the origins of domestication is important for the conservation of wild ancestors and can guide the introduction of untapped diversity through selective breeding. The introduction of novel diversity in wild plants to cultivated varieties has led to improvements in many crops including rice, barley, tomato, potato, banana and apple. There is great potential to improve productivity, pest and disease resistance and adaptation to climate variability in macadamia. Yield is impacted by a wide range of insects and microbial pathogens. We now know that genetic variation in Macadamia integrifolia is geographically structured and that most commercial varieties are from the northernmost part of the distribution. This information will be useful in the search for naturally resistant trees and the future development of new, improved varieties.
 
Since the establishment of commercial seedling orchards in Hawaii in the 1920s and development of reliable grafting techniques in the 1930s, the macadamia industry has undergone rapid global expansion. World macadamia production increased faster than that of any other tree nut over the past 10 years, according to INC data. Macadamias are valued at US$1.1 billion representing about 2% of the global trade in tree nuts in 2018/19 (kernel basis). Australia, South Africa, Kenya, and the United States are the major producers and the crop is also cultivated in China, South East Asia, South America, Malawi, and New Zealand. Future growth in global production is predicted following recent extensions in planting, particularly in China and South Africa.
 
Despite the growing success of cultivated macadamia, all four species in the genus Macadamia are listed as threatened. While high levels of natural genetic variation were found, there was evidence that some has been lost through land clearing from the 19th century. The world’s first cultivated macadamia tree may be one planted in 1858 by Walter Hill in the Brisbane Botanical Gardens that is still alive today. His records were sadly destroyed in a flood so there is no information on its origin. The ‘Walter Hill’ tree, another at University of California Berkeley and an old Brisbane backyard tree were closely related to each other but not to any of the wild populations included in the study. Brisbane was settled by Europeans in 1824 and much of the native vegetation was subsequently cleared for urban development. Dr. Nock and Dr. Hardner are working with the Macadamia Conservation Trust and a citizen science project, The Wild Macadamia Hunt, to find more old trees that may preserve the genetics of extinct wild populations.
 
Future research will extend to a larger diversity set including the other commercially important species M. tetraphylla to identify the origins of varieties developed in Australia and other parts of the world. An improved understanding of the distribution of macadamia diversity will help to prioritise wild populations and trees for conservation and inclusion in ex situ and breeding collections.