Botanic Gardens Trust, Sydney, Australia

Eucalypts: adaptive variation versus vicariance

What drives speciation in eucalypts?

Susan Rutherford - PhD student University of New South Wales and Royal Botanic Gardens and Domain Trust

I began my PhD in July 2012 and am working on a joint project between the Royal Botanic Gardens and Domain Trust and the University of New South Wales. The objective of this study is to investigate speciation in a group of closely related eucalypts, known as the ‘green-leaved ashes’. The green ashes are a clade in the subgenus, Eucalyptus, which are characterised by glossy green leaves with prominent oil glands and brown seeds. They occupy a range of habitats in south-eastern Australia, with some species occurring as trees in tall, mountain forests on fertile soils and others as small trees or mallees on shallow soils on sandstone. The group includes the tallest flowering plant in the world, Eucalyptus regnans and the small mallee, E. cunninghamii (just over 1 m in height).

While previous studies (based primarily on morphology) suggest that the green ashes form a monophyletic group, there is some debate concerning differentiation among taxa and in particular the number of recognised species. As with many eucalypt species complexes, we have a limited understanding of their evolution and speciation mechanisms. Therefore, this project aims to address the following questions:

  1. what are the evolutionary relationships between these taxa and what are their patterns of divergence
  2. what drives speciation and adaptation in this group and
  3. what constitutes a species and how can a better understanding of adaptive evolution in these taxa contribute to theories of speciation?

The initial stages of this research focussed on collecting leaf material for DNA analysis from species of green ashes in order to construct a phylogeny of the group. Each recognised species has been sampled (aiming to collect from at least three individuals per taxa from multiple populations). The green ashes include 18 species, most of which are rare or localised. The majority of these species occur in the Sydney to Shoalhaven region, from the coast to the Upper Blue Mountains. A few species are found in north-western NSW and at the Queensland border, while others are located on the South Coast and Southern Tablelands, with some extending into Victoria. Co-occurring species in subgenus Eucalyptus (e.g. scribbly gums, stringy barks and peppermints) at each site were also sampled and are to be included in the phylogeny. Many of these species occur in some beautiful and interesting places, including exposed steep slopes and cliff edges at Pulpit Rock, Fitzroy Falls, Wadbilliga National Park and Kosciusko National Park; as well as mountain tops, such as Waratah Trig (north-western NSW), Mount Norman (Qld) and Mount Seldom Seen (Vic).

A number of people have assisted with the collecting, including Maurizio Rossetto, Peter Wilson, Doug Benson, Andrew Orme, Bob Coveny, Trevor Wilson, Dean Nicolle, Tracey Armstrong, Rusty Worsman, Aaron Smith, Danca Ciric, and Rebecca Reid. Andrew Orme has helped with specimen identifications, and mapping species distributions.

All samples and vouchers have been sorted and identified and hve been extracted for DNA. Samples will be sent to Diversity Arrays Technology (DArT) Pty Ltd in the ACT for sequencing. DArT Science and Conservation 43
is a method that simultaneously assays hundreds to thousands of markers across the genome. Carolyn Connelly, Margaret Heslewood, Marlien van der Merwe, Hannah McPherson and Juelian Siow have been providing advice and assistance with the extractions.

For the next stage of this project, a smaller number of species within the group in order to investigate their population genetics (also using DArT markers). It is hoped that a genome wide approach to these populations in combination with distributional, environmental, morphological and ecological data will provide insights into their adaptation and speciation mechanisms. Green ash species from the coast to Blue Mountains will be sampled. This area is environmentally heterogeneous and provides a unique opportunity for studying modes of speciation. Leaf material will be collected from up to 20 individuals per population and DNA extracted. This molecular data will be correlated with distributional, environmental and morphological data. Vouchers of each individual have been taken and will be used for taking morphologicalmeasurements.

Supervisors

Dr Maurizio Rossetto; Dr Peter Wilson; and Dr Stephen Bonser (Senior Lecturer, School of Biological, Earth and Environmental Sciences, University of New South Wales)

Waratah-Trig

Waratah Trig, north-western NSW, where E. codonocarpa occurs.

Peter-Wilson-and-Andrew-Orme
Peter Wilson and Andrew Orme sampling leaf material from E. laophila along the Glowworm Tunnel Road, Newnes Plateau.

Photos: S. Rutherford