The shift from hunting and gathering to cultivating crops and livestock was one of the most important developments in human history. But despite its significance, many questions still remain about how crops were first domesticated. One much debated question has been at what point during domestication is genetic diversity lost? Many modern crops have very low genetic diversity compared with their wild relatives. This can lead to increased susceptibility to pests and diseases. Is diversity lost early when plants are first brought into cultivation? Or is it a long, slow process over many thousands of years?
Most crops cultivated today were first grown many thousands of years ago, making it very difficult to examine these questions. However, New Zealand provides an ideal place to look at the early stages of domestication because it was one of the last places in the world to be settled. We know that Māori brought several crops with them when they arrived in New Zealand, but they also started to grow new plants that they found here, such as karaka. The initial cultivation of these New Zealand crop plants must have happened no more than 800 years ago, the date of Māori arrival.
The case of rengarenga
We have just published a paper that examined the domestication of one such plant – rengarenga (Arthropodium cirratum). The fleshy roots of rengarenga were eaten and used medicinally and the explorer William Colenso recorded it being grown around Māori villages at the end of the nineteenth century.
It can now be found from Northland to Marlborough, mostly in coastal regions. In the southern North Island and the South Island rengarenga is usually found near Māori archeological sites. This has led to the suggestion that Māori planted these southern populations of rengarenga.
We examined rengarenga’s DNA to see what it would tell us about its cultivation. We found that rengarenga from almost every different region within its presumed pre-human range, in the northern North Island, had a unique DNA variant (29 different DNA variants in total). But it was a different story for the plants derived from Māori plantings. We only found two DNA types within these plants. This shows that a large loss of genetic diversity has already happened, even in this very early stage of domestication.
Origins of cultivated rengarenga
We were also able to use our DNA results to determine the origin of the cultivated southern plants of rengarenga. It is likely that they came from the eastern Bay of Plenty and/or East Cape region. We aren’t sure why plants from this region were selected to be cultivated further south – maybe they had some favourable characteristics, such as bigger or tastier roots. Or maybe this was simply the area where it was first recognized that rengarenga was edible and able to be cultivated.
A declining taonga
There was one disappointing outcome from our study. During our fieldwork we noticed that rengarenga has disappeared from some areas where it has been recorded in the past. Although it is not currently considered endangered, this decline is concerning because unique genetic variants may be going extinct. Rengarenga is browsed by introduced mammals, slugs and snails. In many places, plants are now restricted to cliffs that are difficult for these predators to access.