While macadamia is a native Australian plant, most of the world’s cultivated macadamia trees are varieties bred in Hawaii.
Dr Catherine Nock from Southern Cross University and Dr Craig Hardner from University of Queensland studied the structure of the chloroplast genome of Hawaiian varieties and mapped it back to trees in the wild in Australia.
The research, published in Frontiers in Plant Science, suggests that the global macadamia industry may have originated from seed collected from a single tree.
“It was a bit of a shock to see just how narrow the gene pool was from which the Hawaiian varieties were developed,” Dr Nock said.
Seed collected in Australia was taken to Hawaii in the 19th century.
Dr Hardner said that seeds from a tree, or perhaps a couple of trees, were taken from Gympie.
“That sample was the foundation of the Hawaiian macadamia industry which supplies around 70 percent of the world’s macadamia varieties,” said Dr Hardner, a horticultural science researcher.
The tree nut crop is native to the lowland rainforests of subtropical Australia (south east Queensland and northern New South Wales) and the varieties grown in orchards are only a few generations removed from their wild ancestors.
“Most of the germplasm in Hawaii and particularly that used extensively throughout the world for commercial production came from a single population, and possibly even a single tree, at Mooloo, north-west of Gympie,” said Dr Hardner.
“Understanding the genetic diversity of trees in the wild is important because macadamia is a relatively new crop compared to crops such as peaches, where many centuries of domestication have helped improve important traits."
“The potential to improve traits such as disease resistance and climate variability is substantial.”
A key finding of the research is that significant diversity of wild macadamia has been lost through land clearing since European development from the 19th century.
Although the macadamia nut was likely a component of the diet of Australia’s Indigenous people, the first recorded European contact with macadamia was in 1848.
“The world’s first cultivated macadamia tree was likely planted in 1858 by Walter Hill in the Brisbane Botanical Gardens, and it is still alive today,” Dr Hardner said.
The genetics of this tree and two others that date back to this era - one planted at the University of California in Berkeley in 1879 and one growing in Dr Hardner’s backyard in Yeronga - do not map back to any of the recent samples taken from the wild.
“This suggests that there was some diversity at the time of European settlement that has been lost to commercial macadamia production systems," he said.
“We could well find that some old macadamia trees growing in people’s backyards might also have this genetic diversity – like the tree in my backyard.”
He and Dr Nock are working with the Macadamia Conservation Trust, macadamia industry and other stakeholders to sample old trees for genetics that have been lost to macadamia production systems.
The macadamia industry is worth approximately $3 billion annually and has undergone rapid global expansion in the last 50 years.
Australia, South Africa, Kenya, and the United States are the largest 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.
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