The distribution of kōwhai and its close relatives throughout the Southern Hemisphere has intrigued scientists, including Charles Darwin, for over 150 years. Recent research led by Lara Shepherd has revealed more about the relationships among this iconic group of plants.
Sophora sect. Edwardsia is a group of 19 species of small trees or shrubs mostly found around the Southern Hemisphere oceans. New Zealand is the global hotspot with eight species of kōwhai.
A further eight species occur on islands in the Pacific – one species is found on Réunion Island in the Indian Ocean and two in Chile (one of which also occurs on Gough Island in the Atlantic Ocean).
Charles Darwin used Sophora sect. Edwardsia to support his idea that the similarity of New Zealand and South American plants resulted from seed dispersal across oceans, suggesting that Sophora seeds may have floated between New Zealand and Chile.
Until then, other scientists thought that the separate landmasses must have been connected by landbridges to explain the distributions of similar species.
Previous DNA research supported Darwin’s claim by showing that the species are very closely related and only separated from each other within the last few million years. However, that study was not able to determine a family tree for the group because the DNA sequences they examined were mostly identical among the species.
Our new study used DNA regions that were more variable, revealing 20 different DNA sequence variants. Interestingly, several of these DNA sequence variants are shared between New Zealand and South American Sophora species.
This suggests that there have been a number of seed dispersal events between New Zealand and South America after the species separated. Ocean currents are known to link the two regions and probably assisted this seed dispersal, with kōwhai seeds able to float and germinate after being in seawater.
In contrast, many of the Pacific and Indian Ocean island species had unique DNA variants. This suggests that the Sophora on each island or island group has been isolated for long enough that DNA differences have evolved.
Another finding from our study is that toromiro (Sophora toromiro) from Rapanui/Easter Island, which is extinct in the wild, appears more closely related to New Zealand kōwhai than the geographically closer South American species.