SOME genomic detective work has led two researchers to trace the global macadamia industry back to Gympie, Queensland.
University of Queensland horticultural science researcher Dr Craig Hardner and Dr Catherine Nock from Southern Cross University say a tree, or a perhaps a couple trees, were originally taken from Gympie to Hawaii in the 19th century, giving birth to the global industry.
That industry today is worth about $3 billion and has undergone rapid global expansion in the past 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.
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The two researchers studied the structure of the chloroplast genome from the Hawaiian macadamia industry and mapped it back to trees in the wild.
"That sample was the foundation of the Hawaiian macadamia industry which supplies around 70 per cent of the world's macadamia varieties," Dr Hardner said.
"Most of the gemplasm in Hawaii and particularly the germplasm 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."
The tree nut crop is native to Queensland and northern New South Wales, and modern macadamia production systems are only a few generations removed.
"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," Dr Hardner said.
"The potential to improve traits such as disease resistance and climate variability is substantial."
A key finding of the research, which was published in Frontiers in Plant Science, 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.
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.
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."
Dr Hardner 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.
Future growth in global production is predicted following extensive plantings in China and South Africa.