New research into our most iconic tree provides evolutionary insights

Researchers from the Royal Botanic Garden Sydney have used advanced genetic techniques to unlock the evolutionary history of a group of closely related eucalypts. Their findings not only challenge our current understanding of how Eucalyptus species form – but all plants and animals. 

Contrary to prior belief, new research shows that many evolutionary mechanisms, such as the merging of different lineages and the geographical separation of populations, can occur at the same time within the one species complex.

Ever since the publication of Darwin’s Origin of Species in 1859, evolutionary ecologists have been fascinated by the amount of different species on Earth and how they were formed. 

There are over 700 Eucalyptus species and many are either rare or only found in certain areas. As well as dominating our landscape, Eucalyptus trees are also a strong part of Australian art and culture.

From the iconic ‘Snugglepot and Cuddlepie’ children’s stories to Banjo Paterson’s ode to their subtle and strange perfume in his poem ‘The Wind’s Message’ - their inspiration and value is endless. 

Using the 'green ashes' to study evolution

The Eucalyptus species known as the ‘green ashes’ are a group of closely related eucalypts and include tall trees, medium trees and mallees (multi-stemmed plants that are usually less than 10 metres tall).

By comparing the highly genetically different group of ‘green ashes’ to the other species of Eucalyptus with more gene mixing, the team discovered that they were documenting the process through which new species of Eucalyptus form.

Technical Officer Biodiversity Genomics at the Royal Botanic Garden Sydney (and lead author of this recent research), Dr Susan Rutherford, said the green ashes are unique because they can grow in a range of habitats.

“The green ashes are found in fertile and nutrient poor soils from the coast to cold highland environments. The fact they have evolved to be so diverse and grow in different habitats makes them a perfect group for studying evolutionary processes,” Dr Rutherford said.
 
“To study their DNA and evolution, we needed to collect leaf material, so we embarked on an expedition to many beautiful and iconic places, such as Govetts Leap, Fitzroy Falls and Pulpit Rock.  
 
“The Blue Mountains is a hot spot for Eucalyptus diversity and the area was declared World Heritage partly because it is home to about 100 different species,” said Dr Rutherford.

Upon their return, the collected leaf material was given to the laboratory at the Royal Botanic Garden Sydney, where DNA was extracted from six green ash species and sent to Diversity Arrays Technology (DArT) Ltd in Canberra for analysis. 

The latest findings 
 

1. Potential discovery of a new species
The DNA results from a population of green ash mallees from the Greater Blue Mountains World Heritage Area came back highly differentiated and might be a new species of Eucalyptus.
 
“We are now looking to describe the potential new eucalypt, which involves going back to the site where we hope to catch the species in flower,” Dr Rutherford said.
 
“The plan is to grow seedlings at the Royal Botanic Garden Sydney to study their physical form and external structure, otherwise known as morphological analysis,” she said.

2. A eucalypt with a different story 
Following detailed genetic analyses, researchers also found that one of the green ashes, Eucalyptus cunninghamii, was genetically very different from all other Eucalyptus species sampled.
 
“This is interesting because the merging of genes is very common between most of the other green ash species,” Dr Rutherford said.  

“This suggests that E. cunninghamii is on a later stage on what is called the speciation continuum. Think of it as a timeline of complex processes by which different species have been formed,” she said.
 
By comparing the highly genetically different ‘green ashes’ to the other species of Eucalyptus with more gene mixing, the team discovered that they were documenting the process through which new species of Eucalyptus form.

3. Same, but different...

There was moderate movement of genetic materials between the rest of the sampled Eucalyptus species and an explanation for this finding could be what is called reticulate evolution. This process describes the origin of a species family or lineage (B) through the partial merging of two older lineages (A & C).
 
“Despite the ongoing mixing of genes between these species of Eucalyptus, remarkably, they seem to maintain unique structural differences,” Dr Rutherford said. 

What was previously considered problematic, Dr Rutherford’s findings show that many evolutionary mechanisms, such as the merging of different lineages and the geographical separation of populations, can occur at the same time within the one species complex.

This research was made possible by the generous financial support of the Dahl Trust.

Published journal article

Rutherford S, Rossetto M, Bragg JG, McPherson H, Benson D, Bonser SP and Wilson PG (2018) Speciation in the presence of gene flow: population genomics of closely related and diverging Eucalyptus species’. Heredity, DOI: 10.1038/s41437-018-0073-2