Kōwhai’s global family tree

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.

Pelu at Lago Ranco

Pelu (Sophora cassioides) from Chile. This species is a close relative of New Zealand’s kōwhai species. Photograph by Dick Culbert

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).

Sophora map

Map of the distribution of Sophora sect. Edwardsia. New Zealand is the biodiversity hotspot with 8 species.

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.

Toromiro at the Royal Botanic Gardens, Melbourne

Toromiro (Sophora toromiro) from Rapanui has been extinct in the wild since the 1960s. Luckily it survived through cultivation in botanic gardens like the above tree in the Royal Botanic Gardens, Melbourne. Photograph by Raffi Kojian, courtesy of Gardenology.org

2 Responses

  1. John Grehan

    “This is quite an interesting paper, although perhaps not in the way anticipated by the authors. The paper takes the view (assuming I understand the viewpoint correctly) that the origin of Sophora species across various oceans is the result of chance dispersal and their ‘low’ (threshold not specified) molecular divergence indicated that this took place relatively recently. This context leads to the interpretation of seed dispersal as a mechanism for the establishment of oceanic island Sophora from one or more centers of origin (New Zealand?).

    The problem with the view that the distribution of Sophora is the result of random chance dispersal is that the distribution of sect. Edwardsia and its sistergroup S. tomentosa are anything but the kind of mixture one would expect. Instead there is a very precise pattern of allopatry between the two groups. This allopatry does not follow the latitudes and neither does ecology explain the boundary between the groups where the main breaks occur at Madagascar (tomentosa present) and Mascarenes (sect Edwardsia present), at the McPherson-Macleay Overlap (tomentosa present) and Lord Howe Island (Edwardsia present), and around the Society Islands (both but without sympatry).

    This very precise global allopatry is consistent with the expectations of vicariance of a widespread ancestor established on a former topography with sect. Edwardsia distributed over a region that is now the Pacific and tomentosa with a distribution range now broken by the Atlantic and Indian Ocean basins. In this context the dispersal ability or potential of Sophora seeds is less about explaining the origin of the allopatric distributions than it is an explanation of how each allopatric lineage has managed to survive and persist in their respective areas of distribution. The seeds may be understood as acting as a means of survival over regions that have experienced immense geographic and ecological disruption. The biogeography of Sophora, along with other examples of Australasia-central Pacific groups, is outlined in Michael Heads’ 2014 book “Biogeography of Australasia” (Cambridge University Press).”

    Reply
  2. Lea Robertson

    Fascinating. Thanks Lara.

    Reply

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