A majority of the world’s cultivated macadamias are grown in Hawaii, but the crunchy, creamy nuts are in fact indigenous to Australia. In fact, as a recent study has shown, Hawaii’s macadamias appear to stem from a small group of trees, or possibly just a single tree, in Queensland—a stunning revelation, as far as nut-related discoveries go.
“It was a bit of a shock to see just how narrow the gene pool was from which the Hawaiian cultivars were developed,” Catherine Nock, a plant scientist at Australia’s Southern Cross University and first author of a new paper in Frontiers in Plant Science, tells Jennifer Nichols of ABC News. “They represent about 70 per cent of the trees that are grown in orchards around the world.”
Understanding diversity in plants is important to their conservation. Without much genetic variability, plant species are rendered more susceptible to factors like disease and climate change. So the team behind the recent study set out to map the genetic origins of the macadamia nut—a major crop in both Australia and Hawaii.
The researchers collected samples from both commercial macadamia orchards in Hawaii and wild macadamia trees in Australia, studying the plants’ genetic markers to establish relationships between them. To their surprise, the scientists found that most of the Hawaiian plants could be traced back to a single population of Macadamia trees on private land in Mooloo, a locality in Queensland’s Gympie Region. The genetic pool was so narrow that the team thinks the Hawaiian specimens may have originated from just one tree.
Based on historical records, we know that macadamia seeds were brought from Queensland to Hawaii twice in the late 19th century—first by one W.H. Purvis, then by R.A. Jordan, who planted the macadamias in Honolulu. Most of Hawaii’s initial macadamia seedlings, according to the researchers, likely come from Jordan’s sample.
“Historical records suggest that there was maybe six trees grown from that sample of nuts,” Craig Hardner, study co-author and horticultural scientist at the University of Queensland, tells Nichols. “Then when Hawaiians became aware of the delicacy of macadamias they thought that it was a good new crop to use for commercial production.”
In commercial settings, macadamias are reproduced by grafting, meaning that parts of different plants are joined together to create a new one. This process allows growers to create orchards of thousands from a select few individuals, according to Atlas Obscura’s Sabrina Imbler, which is why the genetic diversity of Hawaii’s macadamias is limited. The study authors found that genetic variation among wild macadamia trees in Australia is comparatively rich, which is good news, because wild samples could help “guide introduction of novel genetic diversity into selective breeding populations,” the study authors write.
There are signs, however, that the diversity of Australia’s macadamias is not as robust as it used to be. The researchers studied the DNA of three cultivated trees that were planted in Australia during the first decades of European settlement, among them an 1858 specimen that is believed to be the world’s first cultivated macadamia tree. These trees could not be traced back to any samples taken from the wild, suggesting that “there was some diversity at the time of European settlement that has been lost to commercial macadamia production systems,” Hardner says.
What’s more, due to factors like land clearing and development, three of Australia’s wild macadamia species are considered threatened, and one is endangered. So experts are keen to track down more centuries-old macadamia trees that harbor rare DNA—trees that are likely to exist on “little pockets ... on private land and even in people’s backyards and farming land,” Denise Bond, executive officer of Australia’s Macadamia Conservation Trust, tells Nichols.
“[E]very time we find a new population it's a potential stepping stone that joins up the other trees so that they can still be living and evolving as they would in the wild,” Bond adds. “If there's a network of them throughout the landscape they'll maintain the population dynamic that keeps them being a viable species.”