Toropapa are some of Aotearoa’s most intriguing plants. This genus of shrubs has been baffling botanists for over 100 years. Not only do they show extreme variation in leaf shape but some of the leaf forms bear an uncanny resemblance to unrelated plant species. Te Papa scientist Lara Shepherd recently led a team that examined the relationships of a selection of leaf forms – could these different forms actually be different species?
Toropapa is found only found in Aotearoa and five species are currently recognised. Three of these can appear to resemble completely unrelated species. This apparent mimicry has been suggested to be a defence against being eaten by browsing animals, such as moa, with tasty toropapa resembling plants that taste bad.
One of these species, Alseuosmia banksii, only occurs in Northland but has a huge variety of leaf shapes and sizes. Some look remarkably like other species such as ramarama, quintinia (tāwheowheo) and maire.
Here is a selection of leaf forms of Alseuosmia banksii:
We collected samples of Alseuosmia banksii from throughout its distribution in Northland, including representatives of all the leaf forms we found. At the time we were confident that there were multiple species currently lumped under the name A. banksii because we were seeing the same distinct forms in different locations.
A surprising result
Our DNA results told another tale. With a single possible exception, DNA couldn’t tell most of the Alseuosmia banksii forms apart. This result suggests that these different leaf forms are part of a single interbreeding species.
In fact, the main genetic differences were geographic – plants from northern Northland were more closely related to each other, regardless of leaf shape, than they were to plants of southern Northland.
So, plants with leaves that looked like ramarama in northern Northland were not closely related to similar-looking plants in southern Northland. Instead, they were genetically more similar to plants that grew nearby but which looked totally different.
Even more surprising was that Alseuosmia quercifolia was also not distinguished genetically from A. banksii, indicating that they may be a single species.
It is also possible that the diverse range of leaf forms in A. banksii is itself a defence against browsing animals. Plants that are difficult to identify may be at an advantage if browsers can’t recognise them.
Reference
Lara Shepherd, Andrew Townsend, Peter de Lange, Leon Perrie (2023) Genetic analysis of a genus of plant mimics (Alseuosmia A.Cunn; Alseuosmiaceae) reveals incongruence between morphology and phylogeny and possible mimetic polymorphism. Evolutionary Journal of the Linnean Society. Evolutionary Journal of the Linnean Society 2, 1-18. link to paper
For more photos of toropapa see our previous blog post, How many species of the confusing shrub toropapa are disguised in the bush?
