Phylogeny of Astereae
Island archipelagos such as New Zealand provide an outstanding opportunity to study evolutionary processes that have an impact upon biodiversity. We are currently investigating evolutionary processes such as dispersal and colonisation, speciation, reproductive isolation, hybridisation and differential extinction. This research uses the tribe Astereae (Asteraceae) to examine these processes and will result in a predictive framework for conservation and biosecurity managers to more effectively manage rare and endangered plants and to prevent the incursion of exotic plant species.
The tribe Astereae comprises one of the largest and most diverse groups in New Zealand, with many of the species considered threatened or endangered. The New Zealand members of tribe Astereae fall into two major clades with South American genera emerging as sisters. One clade achieves its greatest level of diversity in Australia, while the other has radiated primarily in New Zealand. The species of Brachyscome, Lagenifera, and Vittadinia, although represented in New Zealand, are probably outliers from species radiations in Australia or South America that have recently dispersed to New Zealand.
The second clade is comprised of over 100 endemic species that are traditionally placed in five genera: Celmisia, Damnamenia, Olearia, Pachystegia, and Pleurophyllum . Olearia and Pachystegia are woody shrubs, whereas Celmisia, Damnamenia and Pleurophyllum are subshrubs or herbs that arise from a woody base. Many of the species are showy ornamental shrubs that are widely cultivated in New Zealand. A chromosome number of 2n = 108 is the most commonly reported for members of the New Zealand clade, but elevated numbers as high as c. 432 have been reported for Olearia albida .
With over 180 species in the genus Olearia and 38 of these endemic to New Zealand, Olearia is perhaps the most diverse member of the New Zealand clade. Species of Olearia are found in a wide range of habitats from the subantarctic islands to temperate and tropical forest margins in Australia and New Guinea. Recent evidence indicates the genus is polyphyletic and spread across as many as 19 distinct lineages. They are generally recognised by their shrubby growth habit and in most cases dense indumentum on the lower surface of the leaves. However, there is remarkable diversity in leaf hairs, phyllotaxis, leaf shape and margin pappus bristles and the position and arrangement of the inflorescences. The type for the genus, Olearia tomentosa, is an Australian species that is very distinct from the New Zealand clade. Furthermore the New Zealand species of Olearia appear to be paraphyletic, hence well–supported subclades in New Zealand will likely be recognised as distinct genera. Preliminary results suggest the macrocephalous olearias and the divaricating tree daisies form well–supported monophyletic groups.
Pachystegia is a small genus including three distinct species. However, the species taxonomy is not clearly resolved and further study may show that an additional three distinct variants are worthy of species rank. The species of Pachystegia are very showy medium–sized shrubs with distinctive thick leathery leaves. They are confined to one of the driest regions in New Zealand, the north–eastern part of the South Island. Commonly known as Marlborough rock daisies, they are widely cultivated and make choice horticultural plants.
The genus Celmisia includes about 60 endemic New Zealand species and five in Australia. In recent phylogenetic analyses they fall into two distinct groups: one comprised of subshrubs with hard woody stems and branches and the second herbs arising from simple to hard rootstocks. The leaf morphology is diverse, but they are usually tomentose on the lower surface, and the hairs are simple. The scapes usually bear a single showy capitulum with white ray florets and yellow disk florets. The species were formerly divided into six subgenera with eight sections.
The monotypic genus Damnamenia is based upon the distinctive subantarctic plant D. vernicosa . Damnamenia is allied to Celmisia and was formerly recognised as the sole member of Celmisia subgenus Ionopsis, but was recognised as a distinct genus based upon a suite of unique morphological characters such as its obtuse stamen appendages, the tapering collar on the stamen filament, disc style arms that are wide, and short, purple disc corollas.
The genus Pleurophyllum includes three species, P. speciosum, P. criniferum, and P. hookeri, that are restricted to the subantarctic islands of New Zealand and Australia. They are distinguished from most other members of the New Zealand clade by their coloured florets. The ray and disk florets, when present, are almost universally purple; only a small group of macrocephalous olearias share this feature.
Current projects place the New Zealand taxa within a global taxonomic framework and provide a predictive framework to classify these plants in a manner that mirrors their phylogenetic relationship. We are also comparing the evolution of species–rich genera in tribe Astereae in two different island archipelagos.