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Speciation and evolutionary history of the green ash eucalypts

We are investigating speciation mechanisms in a group of closely related eucalypts commonly known as the ‘green ashes’.

The green ashes are in subgenus Eucalyptus and 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 (up to 100 m tall), as well as the small mallee, E. cunninghamii (< 1 m tall).

While previous studies (based primarily on morphological data) indicated that the green ashes form a monophyletic group, there is much debate concerning the differentiation and divergence of taxa, and more specifically the number of recognised species. As with many eucalypt species complexes, understanding the evolutionary history of the green ashes is a major challenge.

Therefore, for our study we address the following questions:

  1. What are the evolutionary relationships between these species?
  2. What is the relationship between phenotypic plasticity and evolutionary history across the green ashes?
  3. What are the patterns of genetic variation within and between selected green ash species?
  4. What are the similarities and differences in ecological factors and functional traits across lineages?

Our genetic research (using DArTseq) suggested that relationships in the green ashes were not always consistent with classifications based on morphology, and there was evidence of hybridisation and gene flow between species. Results from a common garden experiment showed high seedling plasticity for many species in response to high and low nutrients. Environmental niches varied among species, with more widespread species having higher leaf-level plasticity than species with narrower distributions. Since most green ash species are rare, and others are commercially significant, a better understanding of their plasticity and genetic diversity is important for their conservation and management. 

Current research

We are using a multifaceted approach incorporating phylogenetics, population genetics, functional traits and environmental niche modelling to investigate the evolutionary origins of the green ashes. More specifically, we are using a range of datasets to improve our understanding of ecological specialisation of the green ashes, and to better define species boundaries in a group that has long been taxonomically challenging.

Some of our relevant publications:

  1. Rutherford S, Rossetto M, Bragg JG, McPherson H, Benson DH, Bonser SP, Wilson PG. Speciation in the presence of gene flow: population genomics of co-occurring Eucalyptus species along altitudinal and latitudinal gradients. Submitted to Heredity.
  2. Rutherford S, Bonser SP, Wilson PG, Rossetto M (2017) Seedling response to environmental variability: the relationship between phenotypic plasticity and evolutionary history in closely related Eucalyptus species. American Journal of Botany 104: 840–857.
  3. Rutherford S, Wilson PG, Rossetto M, Bonser SP (2016) Phylogenomics of the green ash eucalypts: a tale of reticulate evolution and misidentification. Australian Systematic Botany 28: 326–354.

One of the 'green ashes', Eucalyptus stricta