103 Polyploidy and Speciation in Melicytus

Key Idea: Polyploidy has been an important factor in the evolution of the Melicytus species in New Zealand. The Melicytus genus of plants found in New Zealand comprise 11 species of shrubs and small trees including mahoe. They are divaricating, meaning they have intertangled branches at wide angles to each other. They are found in a diverse range of habitats ranging from coastal regions to alpine areas. Polyploidy has played an important role of speciation in Melicytus, including in the most recently named M. drucei, a hybrid found only on Mt. Taranaki. M. drucei is unusual in that it is a stable triploid (3N). The other Melicytus species are diploids (2N) or tetraploids (4N).

M. alpinus growth form

Melissa Hutchinson

M. alpinus flowers

M. alpinus (above) is found on coastal and dry alpine areas of the southern North Island and the South Island. It is a dense shrub growing 1-2 m tall. The external branches look like porcupine quills (hence its common name, porcupine shrub). Most of the leaves are found within the canopy, an adaptation to reduce water loss in dry climates. The small weather resistant flowers (top right) are produced in spring and early summer. M. alpinus is a tetraploid (2N = 64)

John Barkla

M. flexuosus

M. flexuosus (left) can grow up to 5 m tall. It is restricted to a few regions south of Pureora in the North Island but widespread throughout the South Island. It has divaricating, near leafless branches with many lenticels (pores) for gas exchange through the stems. It produces small flowers, which sit under the branches. M. flexuosus is diploid (2N=32). f M. drucei (right) probably arose through a hybridisation event between M. flexuosus and M. alpinus. M. drucei, is found only in the Ahukawakawa Swamp and Pouakai Range within Egmont National Park (Taranaki). It is restricted by the Pouakai volcano, and the extent of its habitat is 2 ha. f Both parent species occur in the Central North Island but not within Egmont

National park. The hybridisation event would have occurred when the parent species had more widespread distributions. The limited distribution of M. drucei suggests its formation was a single hybridisation event. f M. drucei is unusual in that it is a stable triploid and produces viable offspring, so it is called a permanent odd triploid. M. drucei is most similar in appearance to M. flexuosus, but in height and flower type it resembles the M. alpinus group.

M. drucei, is a triploid (2N=48).

With thanks to Bruce Clarkson, Jeremy Wolfe, Melissa Hutchinson, John Barkla, and Rewi Elliot and the NZ Plant Conservation Network for photographs.

M. drucei foliage

1. Has M. drucei arisen through autopolyploidy or allopolyploidy?

2. Using M. drucei as an example, discuss the role of polyploidy in speciation of Melicytus:

3. What evidence is there to suggest that M. drucei arose from a single hybridisation event?